Copernicus Sentinel-3 Next Generation Topography Mission Requirements Document

ESA UNCLASSIFIED – For ESA Official Use Only estec European Space Research and Technology Centre Keplerlaan 1 2201 AZ Noordwijk The Netherlands T +31 (0)71 565 6565 F +31 (0)71 565 6040 www.esa.int Copernicus Sentinel-3 Next Generation Topography (S3NG-T) Mission Requirements Document (MRD) Prepared by Earth and Mission Science Division Reference ESA-EOPSM-S3NG-MRD-3821 Issue/Revision 1.0 Date of Issue 15th October 2025 Status Issued Document type Mission Requirements Document (MRD) Distribution ESA Unclassified – For Official Use ESA UNCLASSIFIED – For ESA Official Use Only Page 2/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Recommended Citation: ESA (2025). Copernicus Sentinel-3 Next Generation Topography (S3NG-T) Mission Requirements Document (MRD), Version 1.0, 15th October 2025. European Space Agency, Noordwijk, The Netherlands, ESA-EOPSM-S3NG-MRD-3821, 327 pp. DOI: https://doi.org/10.5281/zenodo.17454428 ESA UNCLASSIFIED – For ESA Official Use Only Page 3/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Approval Title: Copernicus Sentinel-3 Next Generation Topography (S3NG-T) Mission Requirements Document Issue Number: 1 Revision: 0 Date: 15/10/2025 Author: Alejandro Egido (EOP-SME, Mission Scientist Phase B1/B2/C/D), Craig Donlon (previously EOPSME, currently EOP-FA, Mission Scientist Phase 0/A) and members of the Mission Advisory Group (MAG); Remko Scharroo, Michaël Ablain, Lotfi Aouf, Sylvain Biancamaria, Bertrand Chapron, Luciana Fenoglio, Joana Fernandes, Jesús Gómez-Enri, Christine Gommenginger, Johny Johannesen, Rosemary Morrow, Estelle Obligis, Marcello Passaro, Louise Sandberg Sørensen, Andrew Saulter, Ernst Schrama, Clement Ubelmann, Rosemary Willatt. Approved By Signature Date of Approval Thorsten Fehr (EOP-SM) Head/Earth and Mission Science Division Pierrik Vuilleumier (EOP-PY) Project Manager S3NG-T Betlem Rosich Tell (EOP-GC) Head Copernicus Ground Segment and Data Management Division Pierre Potin (EOP-E) Head of Copernicus Space Office Pier Bargellini (EOP-PZ) Copernicus Space Segment Programme Manager Authorised By Signature Date of Authorisation Rune Floberghagen (EOP-S) Climate Action, Sustainability & Science Department ESA UNCLASSIFIED – For ESA Official Use Only Page 4/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 ESA UNCLASSIFIED – For ESA Official Use Only Page 5/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Change Log Title: Copernicus Sentinel-3 Next Generation Topography (S3NG-T) Mission Requirements Document Reason for change Issue. Revision Date Original document based on Phase-0 CDF activities. 0 1 02/03/2021 Draft version issued to S3NG-T MAG for review. 0 2 13/03/2021 Consolidation based on S3NG-T MAG formal review and discussions at MAG #1 meeting, updates from Phase A/B1 Study team, Phase A/B1 PreTEB feedback and harmonisation with draft System Requirements Document (SRD). 0 3 15/04/2021 Updated for Phase A/B1 KO based on Bidder clarifications, internal discussions and MAG inputs. 0 4 18/03/2022 Updated with corrections prior to PCR 0 4.1 14/06/2022 Updated with post PCR conclusions 0 4.2 27/09/2022 Further clarifications for Phase B1. 0 4.3 20/10/2022 Update based on Phase A/B1 outcomes, change in requirements agreed at MAG#09. 0 5 11/12/2024 Requirements consolidation with MAG post Phase A/B1. 0 6 22/04/2025 Requirements consolidation with MAG post Phase A/B1. 0 7 18/06/2025 Requirements consolidation for Phase B2/C/D ITT; Internal ESA EOP-P, EOP-G review; Document preparation for signature; first stable version. 1 0 15/10/2025 ESA UNCLASSIFIED – For ESA Official Use Only Page 6/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Change Record Issue: 0 Revision: 4.3 Reason for change Date Pages Paragraph(s) MRD-0590 revied by MAG to clarify nadir altimeter and swath altimeter characteristics for metric computations based on clarifications requested by OHB/CLS.MRD 12/06/22 Sec. 5.8.2 MRD-0960 corrected to SPE of 0.01deg (not 0.1deg) following discussion with F. Boy. 12/06/22 Sec. 6.1 MRD-1010 Note 10 added regarding ITU and Ka-band allocations with respect to AMR-C. Channel frequencies updated to be in line with ITU allocations based on inputs from Y. Soldo 12/06/22 Sec. 6.2 MRD-1040 and Table 6.2-1 Channel frequencies updated to be in line with ITU allocations based on inputs from Y. Soldo. 12/06/22 Sec. 6.2 Updated Sec. 6.2.1 with better background information and references for MRAD on-board and on-ground RFI detection on Study Manager Request. 27/07/2022 Sec. 6.2.1 Based on clarifications requested by Industry on 20/10/2022, MRD-0360 added “For the handover from S3 to S3NGT…” commensurate with MRD-0370 and clarified in Note 6 that this requirement is largely directed to the nadir altimetry part of the S3NG-T. Note 5 added in MRD-0370 to clarify the same issue for concepts other than nadir altimetry that might fly in alternative orbits. 20/10/2022 Sec 5.5.2 Based on clarifications requested by Industry on 20/10/2022, MRD-0620 and MRD-1650 updated with notes regarding the application of on-board processing and availability of full PRF data on ground for nadir altimeters 20/10/2022 Sec. 6.1 and Sec. 7.5.1.2 Based on clarifications requested by Industry on 20/10/2022, MRD-1255 added to clarify content of Level 1a nadir altimeter data. MRD-0650 Note 6 added to ensure full PRF data with no RMC processing is provided at L1a from nadir altimetry. 20/10/2022 Sec. 7.4.2 and Sec. 6.1 Based on clarifications requested by Industry on 20/10/2022, MRD-0900 clarified to 1 minute of raw data per orbit 20/10/2022 Sec 6.1 Corrected filename error for southern hemisphere sea ice mask in MRD-0070 22/10/2022 Sec. 5.1.3 ESA UNCLASSIFIED – For ESA Official Use Only Page 7/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Issue: 0 Revision: 5 Reason for change Date Pages Paragraph(s) Updated Chapter 2 with current programmatic information. 11/12/24 Chapter 2 Modified and updated text in Chapter 3, with updated references and SWOT inflight results. 11/12/24 Chapter 3 MRD-0010, Requirement deleted; moved to definition section. 11/12/24 Sec. 5.0 MRD-0020, requirement deleted, specification beyond S3NGT mission scope. This requirement assumes a L3 product type, which is out of the scope of this MRD. 11/12/24 Sec. 5.0 Update of Measurement Mask names: Marine and Land Mask à Marine and Coastal Mask Hydrology Target Mask à Inland Water Mask Sea Ice Edge Mask à Sea Ice Mask Update of measurements mask definitions and specifications, MRD-0030 – MRD-0080 11/12/24 Sec. 5.1 – 5.4 Deleted requirements; redundant: MRD-200 – MRD-260 11/12/24 Sec. 5.3 MRD-340, updated commissioning phase duration to 9 months. 11/12/24 Sec. 5.5 MRD-370: modified requirement on hand-over of mission to offset uncertainty values; deleted all notes related to tandem operation phase. 11/12/24 Sec. 5.5 MRD-530: introduction of Sentinel-3 orbit specification, for S3NG-T to follow Sentinel-3 orbit ground track. 11/12/24 Sec. 5.6 MRD-560, -570: deleted, not applicable based on MRD-530 11/12/24 Sec. 5.6 Section 5.7 moved to products section. Requirements MRD-470 – MRD-520 merged with MRD-1220. 11/12/24 Sec. 5.7 Section 5.8, paragraphs on orbit selection moved to Appendices. 11/12/24 Sec. 5.8 Section 5.9.2, paragraphs on ocean sampling moved to Appendices. 11/12/24 Sec. 5.9.2 Req. MRD-590, removed notes on SWOT and other orbit considerations. 11/12/24 Sec. 5.9.2 Sec. 6.1, removed text justifying Ka-band nadir altimeter as baseline continuity for the mission; MRD-610, modified to specify Ku-band nadir looking SAR altimeter as baseline continuity. MRD-620, requirement deleted; redundant with MRD-1650. 11/12/24 Sec. 6.1 MRD-860, deleted, too low level requirement. 11/12/24 Sec. 6.1 ESA UNCLASSIFIED – For ESA Official Use Only Page 8/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-870, rewording for clarity; notes 2 and 3 deleted as superfluous. 11/12/24 Sec. 6.1 MRD-890, -900, modified text for clarification; RAW measurements à complex altimeter echoes. Note on acquisition mask, moved to Appendix. 11/12/24 Sec. 6.1 MRD-940, -950, -990 (not applicable to SSO), -1000 ( redundant with MRD-0870): deleted requirements, overspecification at MRD level. 11/12/24 Sec. 6.1 MRD-1010 consolidated frequency bands after phase A/B1. 11/12/24 Sec. 6.2 Requirements MRD-1030, -1050, -1060, -1120, -1130, -1150, - 1150, -1170, and accompanying text, deleted, overspecification at MRD level; flowed down to MWR requirements in SSRD. 11/12/24 Requirements MRD-1190, requirement deleted, overspecification, not needed by users. 11/12/24 Sec. 6.2.1 Text moved to definition section 11/12/24 Sec. 7.1 Created MRD-1220 requirement, to specify latency and availability for all data products. 11/12/24 Sec. 7.1 MRD-1320, requirement deleted, L1b specification beyond the scope of MRD, will come at later phases as part of SRD and/or GSRD. 11/12/24 Sec. 7.4 Table 7.5-1, removed TSCE and TSWE from required data prodcuts, no heritage from S3. 11/12/24 Sec. 7.5 MRD-1445, requirement created to ensure consistency between data products and operational modes. 11/12/24 Sec. 7.5 MRD-1530, requirement deleted, multimission requirement beyond the Level 2 scope of this MRD. 11/12/24 Sec. 7.5.1 MRD-1540, change of specification for drift requirement. 11/12/24 Sec. 7.5.1 MRD-1610, requirement merged with MRD-1590 (reworded). 11/12/24 Sec. 7.5.1 MRD-1630, -1640, merged with MRD-1620 (reworded). 11/12/24 Sec. 7.5.1 MRD-1700, -1710, merged with MRD-1690 (reworded). 11/12/24 Sec. 7.5.1 MRD-1760, updated specification for inland waters based on phase A/B1 results, and latest research. 11/12/24 Sec. 7.5.1 MRD-1765, reworded, removed quatitative value due to difficulty for verification. 11/12/24 Sec. 7.5.2 MRD-1790, merged with MRD-1820 11/12/24 Sec. 7.5.3 MRD-1800, requirement deleted; overspecification at MRD level. 11/12/24 Sec. 7.5.3 ESA UNCLASSIFIED – For ESA Official Use Only Page 9/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1810, requirement deleted. Duplicate of MRD-1850. 11/12/24 Sec. 7.5.3 MRD-1820, reworded, changed latency to NRT6H. 11/12/24 Sec. 7.5.3 MRD-1830, reworded, consolidated. 11/12/24 Sec. 7.5.3 MRD-1840, requirement deleted, merged with MRD-1465. 11/12/24 Sec. 7.5.3 MRD-1850, requirement reworded, consolidated. 11/12/24 Sec. 7.5.3 MRD-1880 to MRD-1920, and MRD-1950, moved to MRAD section as secondary mission objectives; 11/12/24 Sec. 7.5.3 MRD-1930, -1940, requirement deleted, no heritage nor traceability for requirements for the S3NGT mission. 11/12/24 Sec. 7.5.3 MRD-1960, -1970 consolidated based on MAG#09 discussions. 11/12/24 Sec. 7.5.3 MRD-1980, -1990, -2000, -2031, -2032, -2033, changed specification from goal to secondary mission objective. 11/12/24 Sec. 7.5.4 MRD-2030, requirement deleted, not a requirement. 11/12/24 Sec. 7.5.4 MRD-2031 to MRD-2034, created, from MRD-1880 to MRD1920, and MRD-1950, as secondary mission objectives. 11/12/24 Sec. 7.5.4 MRD-2090, requirement merged with MRD-2100. 11/12/24 Sec. 7.6.1 MRD-2150, -2160, -2170, -2180, requirements deleted, not mission requirements. 11/12/24 Sec. 8.1 MRD-2250, requirement deleted, not mission requirement. 11/12/24 Sec. 7.6.1 Issue: 0 Revision: 6 Reason for change Date Pages Paragraph(s) Modified and updated text. 22/04/25 Chapter 3 MRD-0030, modified to include large lakes. 09/04/25 Sec. 5.1.1 MRD-0070, update of Sea Ice Mask. MRD-0075, created to specify re-evaluation of SIM. 09/04/25 Sec. 5.1.3 MRD-0170, deleted requirement, not baseline continuity for S3, no traceability for this requirement. 09/04/25 MRD-0370, -380, removed TBC from requirement, confirmation of values through science study. 09/04/25 MRD-385, created to specify mission offset uncertainties for the key geophysical parameters over ocean. 09/04/25 ESA UNCLASSIFIED – For ESA Official Use Only Page 16/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 5.5.1 Commissioning Phase Requirements ......................................................................................... 97 5.5.2 Tandem Inter-calibration Phase Requirements ......................................................................... 97 5.5.3 Nominal Operations Phase Requirements ............................................................................... 102 5.6 ORBIT REQUIREMENTS ........................................................................................................................... 103 5.7 COVERAGE, REVISIT AND SAMPLING REQUIREMENTS ........................................................................... 106 5.7.1 Sampling and Coverage Requirements .................................................................................... 106 5.7.2 Ocean Sampling constraints ...................................................................................................... 106 5.7.3 Hydrology Sampling Constraints ............................................................................................... 110 5.7.4 Cryosphere Sampling Constraints ............................................................................................. 110 6 LEVEL 1 OBSERVATION REQUIREMENTS .................................................................................... 111 6.1 ALTIMETER INSTRUMENT REQUIREMENTS ............................................................................................. 111 6.2 MICROWAVE RADIOMETER REQUIREMENTS .......................................................................................... 125 6.2.1 Radio Frequency Interference (RFI) Mitigation Requirements. ............................................ 132 6.3 GEODESY REQUIREMENTS ..................................................................................................................... 134 7 DATA PRODUCT REQUIREMENTS .................................................................................................. 136 7.1 PRODUCT DELIVERY TIMELINESS AND AVAILABILITY REQUIREMENTS .................................................. 136 7.2 LEVEL 0 DATA PRODUCT REQUIREMENTS ............................................................................................. 137 7.3 LEVEL 1 DATA PRODUCT REQUIREMENTS ............................................................................................. 137 7.3.1 Level 1a Data Products ............................................................................................................... 137 7.3.2 Level 1b Data Products ............................................................................................................... 138 7.4 LEVEL 2 DATA PRODUCT REQUIREMENTS ............................................................................................. 140 7.4.1 Ocean Product Requirements .................................................................................................... 144 7.4.1.1 Sea Surface Height Requirements ........................................................................................................ 145 7.4.1.2 Sea State Requirements ......................................................................................................................... 150 7.4.1.3 Wind Speed at 10m over the ocean (U10) Requirements ................................................................. 154 7.4.1.4 Specific requirements for the Coastal Ocean ...................................................................................... 155 7.4.2 Inland Water Product Requirements ......................................................................................... 155 7.4.3 Cryosphere Product Requirements ........................................................................................... 156 7.4.3.1 Sea ice parameters .................................................................................................................................. 156 7.4.3.2 Ice sheet parameters ............................................................................................................................... 161 7.4.4 MRAD Product Requirements .................................................................................................... 162 7.4.4.1 Atmospheric products .............................................................................................................................. 162 7.4.4.2 Other Level 2 MRAD Products ............................................................................................................... 163 7.5 USER SERVICE REQUIREMENTS ............................................................................................................. 167 7.5.1 Reprocessing Requirements ...................................................................................................... 168 7.6 CALIBRATION AND VALIDATION REQUIREMENTS .................................................................................... 169 7.7 OPERATIONAL PERFORMANCE MONITORING ACTIVITIES ....................................................................... 170 7.8 USER SOFTWARE REQUIREMENTS ......................................................................................................... 170 8 REFERENCES ...................................................................................................................................... 171 APPENDIX I DEFINITION OF TERMS ..................................................................................................... 210 APPENDIX II MAJOR POLICIES AND EARTH OBSERVATION APPLICATIONS SUPPORTED BY THE S3NG-T MISSION ................................................................................................................................ 223 APPENDIX III EUROPEAN COMMISSION COPERNICUS USER NEEDS FOR TOPOGRAPHY MEASUREMENTS TO SUPPORT COPERNICUS .................................................................................... 230 III.1 COPERNICUS USER NEEDS FOR POLICY IMPLEMENTATION .................................................................... 230 III.1.1 Marine environment ..................................................................................................................... 230 III.1.2 Coastal management .................................................................................................................. 232 III.1.3 Fisheries and aquaculture .......................................................................................................... 234 III.1.4 Maritime Spatial Planning ........................................................................................................... 237 III.1.5 Arctic policy and polar areas ...................................................................................................... 238 ESA UNCLASSIFIED – For ESA Official Use Only Page 17/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 III.1.6 Inland Water ................................................................................................................................. 241 III.1.7 Emergency management ............................................................................................................ 245 III.1.8 Adaptation to Climate Change ................................................................................................... 248 III.1.9 Pollution at sea ............................................................................................................................. 250 III.1.10 Maritime transport, navigation and safety ................................................................................ 252 III.2 CMEMS REQUIREMENTS FOR FUTURE SATELLITE OBSERVATIONS .................................................... 255 III.3 C3S REQUIREMENTS FOR CLIMATE SERIES ............................................................................................ 258 III.4 EUROPEAN COMMISSION COPERNICUS LEVEL 2 PRODUCT AND PERFORMANCE NEEDS FOR S3NG-T IN THE 2030-2050 TIMEFRAME ............................................................................................................................ 260 APPENDIX IV S3NG-T REQUIREMENTS TRACEABILITY MATRIX. ................................................. 268 APPENDIX V TECHNICAL SUMMARY OF THE SENTINEL-3 TOPOGRAPHY MISSION ............... 289 V.1 SENTINEL-3 SAR RADAR ALTIMETER (SRAL) ...................................................................................... 293 V.2 SENTINEL-3 MICROWAVE RADIOMETER (MWR) .................................................................................... 294 APPENDIX VI SYNERGIES AND INTERNATIONAL CONTEXT ......................................................... 297 VI.1 ALTIMETER MISSIONS ............................................................................................................................. 297 VI.2 MISSIONS RELEVANT TO OCEAN WAVE MEASUREMENTS ..................................................................... 298 APPENDIX VII S3NG-T PRELIMINARY SYSTEM CONCEPTS AND CHARATERISTICS ................. 302 VII.1 S3NG-T SYSTEM CHARACTERISTICS ..................................................................................................... 302 VII.2 S3NG-T MEASUREMENT APPROACHES .................................................................................................. 304 APPENDIX VIII ALTIMETER ORBIT CONSIDERATIONS FOR MEASURING TIDES ......................... 306 APPENDIX IX ALTIMETER LEVEL 2 PERFORMANCE SPECIFICATION ......................................... 308 APPENDIX X FIDUCIAL REFERENCE MEASUREMENTS ................................................................ 313 X.1 CALIBRATION AND VALIDATION OF ALTIMETRY MISSIONS ...................................................................... 313 X.2 THE IMPORTANCE OF UNCERTAINTY BUDGETS...................................................................................... 317 APPENDIX XI RAW DATA ACQUISITION MASK EXAMPLE .............................................................. 319 APPENDIX XII LIST OF ACRONYMS ..................................................................................................... 323 ESA UNCLASSIFIED – For ESA Official Use Only Page 18/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 1 INTRODUCTION This document is the formal Mission Requirements Document (MRD) for the Sentinel-3 Next Generation Topography (S3NG-T) Mission. The S3NG-T Copernicus Extension Mission provides enhanced continuity of the Copernicus Sentinel-3 radar altimetry component. It is part of the evolution of the current Copernicus Space Component (CSC) capabilities described in the CSC Long Term Scenario (ESA, 2025) to address the User Requirements expressed by the European Commission (EC). The S3NG-T Mission Requirements Document (MRD) was an input to the European Space Agency (ESA) preparatory phase (Phase A/B1) study activities started in 2021, and to the implementation phase (Phase B2/C/D), to be kicked off tentatively Q1 2026. It is managed by the S3NG-T Mission Scientist according to the ESA Quality Management System (QMS) procedure for Mission Requirements Management (QMSPR-MMAN-2050-EOP), and Procedure for Mission Implementation and Operations (QMS-PR-MMAN-2070-EOP). 1.1 Document and Requirement Conventions 1.1.1 Terms The term “To Be Confirmed” (TBC) will be used in combination with the numerical definition of some performance parameters, the final value of which may be changed by the Agency as a result of the Definition Study engineering work. The term “To Be Determined” (TBD) will be used for the numerical definition of a parameter at a later stage. Engineering assumptions shall be made in consultation with the Agency for interim numerical definitions. Requirements marked as To Be Defined (TBD) or To Be Confirmed (TBC) indicate open issues and will be confirmed by the Mission Advisory Group (MAG) or by ESA in the course of Phase A/B1 or future Mission Phases B2/C/D/E1. The terms “shall” and “will” denote mandatory requirements. The term “goal” denotes a desirable extension to a requirement even though a commitment to such performance cannot be confirmed or verified. The terms “should” and “may” denote requirements whose implementation shall be discussed between the Contractor and the Agency. The term “Note” denotes additional information providing useful background information to a requirement. Following the advice of the European Commission (ESA-EOPSM-EXG-MOM-3811 December 2020), definitions for the meaning of “Continuity” and “Enhanced Continuity” in terms of NG missions were agreed using three ‘levels’ as follows: 1. Baseline continuity: This is the minimum definition for an NG-Mission. Baseline continuity products guarantee the continuity of Level 2 products with the same coverage, revisit, performance, sampling, delivery timeliness of existing ESA UNCLASSIFIED – For ESA Official Use Only Page 19/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 topography parameters (e.g. SSH, Hs, Sigma0, U10 etc.). Baseline continuity is derived directly from in-flight performance. 2. Enhanced continuity: Level 2 products that meet baseline continuity plus enhanced revisit and effective resolution (i.e. wavelength) and/or coverage, and/or performance, and/or timeliness of existing topography parameters (e.g. SSH, Hs, Sigma0, U10, etc.) to address Copernicus User Needs. 3. New products: a new Level 2 product to address Copernicus User Needs providing considerable enhancement over the baseline continuity (e.g. (Directional) Wave Spectra, sea surface height gradients and river gradients. Total Surface Current Velocity measurements are considered out of scope by the European Commission). Following this agreement, requirements for the S3NG-T are set using the following generic form: Requirement (the verb ‘shall’): baseline continuity derived from in-flight performance of the Sentinel-3 topography mission as articulated in Table 2.1.3.1. Enhanced (the term ‘goal’): specifies enhanced continuity or new products to address European Commission User Needs where goal may include a variety of product and performance aspects. 1.1.2 Requirement Numbering Within this MRD, requirements are identified by a unique alphanumeric code with the following format: MRD-DDDD The digits DDDD are requirements numbers. The sequence of these numbers may contain gaps and is independent for each combination of MRD-DDDD. Some requirements are supported by explanatory comments, which are in a different style. Assumptions are identified by a unique alphanumeric code with the following format: ASM-DDD where DDD follows an identical form to requirements numbering. Mission assumptions provide information on elements that are reasonably assumed to be available but outside the scope of this MRD. Within this MRD, European Commission User Needs are identified by a unique alphanumeric code with the following format: S3NG-T-UN-DDD The digits DDD are requirements numbers. The sequence of these numbers may contain gaps and is independent for each combination of S3NG-T-UN-DDD. Some requirements are supported by explanatory comments, which are in a different style. Within this MRD, European Commission Product Specification Needs are identified by a unique alphanumeric code with the following format: S3NG-T-UN-PDD ESA UNCLASSIFIED – For ESA Official Use Only Page 20/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The digits DD are requirements numbers. The sequence of these numbers may contain gaps and is independent for each combination of S3NG-T-PR-PDD. Some requirements are supported by explanatory comments, which are in a different style. Within this MRD, Mission Objectives are split into: Primary Objectives (PRI-OBJ-XX) that are mandatory for the success of the mission and Secondary Objectives (SEC-OBJ-XX) that shall not drive the system design. Paragraphs without such annotation provide information. 1.1.3 Requirements and Guidelines Compliance with the requirements specified within this document is necessary for the success of the mission. Harmonisation of the definition of these requirements and the technical design of the mission is necessary to ensure that the mission, as eventually implemented, will be capable of compliance with the requirements. Traceability to the requirements specified in the MRD shall be provided via suitable flow-down to lower-level specifications. Verification of MRD requirements will be achieved through verification of flowed-down requirements. The requirements in the MRD include the use of Notes that provide guidance or limitations. Unless otherwise stated, all quantities in this MRD are specified as Total Standard Uncertainty (TSU) 1-sigma zero mean based on a Gaussian (normal) probability distribution. ESA UNCLASSIFIED – For ESA Official Use Only Page 21/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 2 BACKGROUND AND JUSTIFICATION This chapter provides background information supporting the definition and justification of the MRD requirements. Please note that the information reflects the status at the time of writing (July 2025) and may not be fully up to date or consistent with the latest developments in the Copernicus Programme or the S3 NGT mission design. Copernicus [http://www.copernicus.eu/] is a European system for monitoring the Earth in support of European policy. It includes Earth Observation satellites (notably the Sentinel series developed by ESA), ground-based measurements and, services to processes data to provide users with reliable and up-to-date information through a set of Copernicus operational services related to environmental and security issues. These include: • Copernicus Marine Environmental Monitoring Service (CMEMS [http://marine.copernicus.eu]), • Copernicus Land Monitoring Service (CLMS [http://land.copernicus.eu/]), • Copernicus Atmospheric Monitoring Service (CAMS [https://atmosphere.copernicus.eu/]), • Copernicus Security service (CMS, BS, SEA) [https://www.copernicus.eu/en/copernicus-services/security], • Copernicus Climate Change Service (C3S) [http://climate.copernicus.eu] and, • Copernicus emergency Service (EMS) [http://emergency.copernicus.eu]. Copernicus services provide critical information to support a wide range of applications, including environment protection, management of urban areas, regional and local planning, agriculture, forestry, fisheries, health, transport, climate change, sustainable development, civil protection and tourism. Copernicus satellite missions are designed to provide ‘upstream’ inputs to all Copernicus Services as systematic measurements of Earth’s oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. The core users and consumers of Copernicus services are policymakers and public authorities that need information to develop environmental legislation and policies or to take critical decisions in the event of an emergency, such as a natural disaster or a humanitarian crisis. The Copernicus programme is coordinated and managed by the European Commission. The development of the observation infrastructure is performed under the aegis of the European Space Agency (ESA) for the space component and of the European Environment Agency (EEA) and the Member States for a separate, but important, for the in-situ measurement component. As set out in the ESA Program Board for Earth Observation (PBEO) paper (ESA/PBEO(2017)31 Paris, 5 September 2017), the Copernicus Space Component (CSC) has been established as the largest and most proficient Earth Observation infrastructure in the world. With seven high-performance satellites in orbit and more than 200,000 registered Sentinel data users on the ESA/EC Copernicus data portal as well as numerous sophisticated operational services, the system has evolved at a rapid pace. In order to preserve the momentum in fulfilling user needs, the future evolution of the CSC needs to be initiated now in close cooperation with all stakeholders. ESA UNCLASSIFIED – For ESA Official Use Only Page 22/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 According to the Copernicus Regulation (EU Regulation No. 377/2014 dated 3 April 2014), ESA has been mandated by the EU Council and European Parliament to define “the overall system architecture for the Copernicus space component and its evolution on the basis of user requirements, coordinated by the Commission”. Following this rationale, ESA, in close interaction with the EC, EUMETSAT and Member States, has identified key components of a Long-Term Scenario. Evolution in the Copernicus Space Component (CSC) is taking place to meet priority user needs not addressed by the existing infrastructure, and to reinforce Copernicus services by providing new capability in the thematic domains of CO2 monitoring, polar monitoring, and agriculture/forestry monitoring. This evolution, embodied by six Copernicus Sentinel Expansion Missions, is synergetic with the enhanced continuity of services being targeted by the parallel development of the Next Generation of Sentinels. In the context of Satellite Altimetry, it is recognised that this capability is a fundamental tool for the European Copernicus services providing measurements over the global ocean and, increasingly in the coastal zones and inland waters. Based on sustained investments started in the 1980's, Europe has gained a leading role in this domain via a long series of missions (ERS-1/-2 (e.g. Francis, 1984), TOPEX/Poseidon (Fu et al., 1994)), the Jason series (e.g. Lambin et al., 2010; Vaze et al., 2010), Envisat Radar Altimeter 2 (e.g. Zelli, 1999) CryoSat-2 (Wingham et al., 2006, Drinkwater et al, 2006) and, as part of the Copernicus system, Sentinel-3 (Donlon et al., 2011, 2016) and Sentinel-6 (Donlon et al., 2021). Currently, the topographic system is built around three observation components: sun-synchronous orbit, mid-inclination tidal-free orbit and polar orbit. A gap of sun-synchronous orbit data was experienced after the loss of Envisat in 2012 (compensated by the extensive use of CryoSat-2 over the oceans). Europe, through the Copernicus Programme, is guaranteeing the medium/long-term continuity of both dense ground track sampling (currently sun-synchronous) and midinclination altimetry reference orbits via, respectively, Sentinel-3 (initiated in the Global Monitoring for Environment and Security (GMES) era, e.g. Aguirre et al., 2009) and Sentinel-6, the latter in cooperation with the US. The Copernicus polaR Ice and Snow Topography ALtimeter mission (CRISTAL), (e.g. Kern et al., 2020), one of the Copernicus Sentinel Expansion Mission for the evolution of the current CSC, will guarantee continuity of the unique measurements of the icesurface elevation change, successfully initiated by the CryoSat-2 mission. The aim of CRISTAL is to obtain high-resolution sea ice thickness and land ice elevation measurements and includes the capability to determine the properties of snow cover on ice so as to serve Copernicus’ operational products and services of direct relevance to the polar zones. The Sentinel-3 Next Generation Topography (S3NG-T) mission address the need for a timely extension of the current Sentinel-3 capability in terms of stability and continuity, while improving coverage, temporal revisit time, and quality of products and services. ESA UNCLASSIFIED – For ESA Official Use Only Page 23/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 2.1 Copernicus Services The Copernicus services transform this wealth of satellite and in situ data into valueadded information by processing and analysing the data together with numerical forecasting and prediction models. Datasets stretching back for years and decades are made comparable and searchable, thus ensuring the monitoring of changes; patterns are examined and used to create better forecasts, for example, of the ocean and the atmosphere. Maps are created from imagery, features and anomalies are identified and statistical information is extracted. These value-adding activities are streamlined through six thematic streams of Copernicus services: The Copernicus Marine Service (CMEMS http://marine.copernicus.eu) leads activities in this domain. It provides regular and systematic reference information on the physical and biogeochemical state, variability and dynamics of the ocean and marine ecosystems for the global ocean and the European regional seas. The provision of data on sea surface height, sea level rise, ocean circulation, winds and waves over the ocean and sea ice derived from satellite altimetry are all fundamental inputs to CMEMS activities. The most important satellite-based observation is sea surface height (SSH) from altimetry. The SSH is an integral of the ocean interior properties and is a strong constraint for inferring the 4D ocean circulation through data assimilation. CMEMS user needs clearly articulate that continuity of the present Copernicus satellite observing system should be first guaranteed as this is mandatory for maintaining the CMEMS service. This holds, in particular for the Sentinel 6 altimeter reference mission and the twin-satellite constellation of Sentinel 3 mission. However, CMEMS capability is evolving to higher resolution forecasting and prediction models and a fundamental user need is to constrain CMEMS models with new topography observations. Today the ocean is very under sampled even when using the Copernicus and all contributing nadir-pointing altimetry missions including Jason-2/3, ESA CryoSat-2, ISRO/CNES AltiKa, NSOAS HY-2A altimeters. Multiple nadir altimeters (at least 4 altimeters) are required (this was required 10 years ago) to adequately represent ocean eddies and associated currents in models. Much higher space/time resolution (≤50 km, ≤5 days (CMEMS, 2017)) will be needed in the post 2025 time period. The enhanced continuity of Sentinel-3 altimetry is fundamental to the future evolution of CMEMS that will critically rely on the enhanced sampling characteristics of S3NG-T. Copernicus Global Land Monitoring component of the Land Service (CGLMS, http://land.copernicus.eu/) leads activities in this domain. It provides geographical information on land cover to a broad range of users in the field of environmental terrestrial applications. This includes land use, land cover characteristics and changes, vegetation state, water cycle and earth surface energy variables and the terrestrial cryosphere. The Water Surface Elevation (WSE) is defined as the height of surface of continental water bodies in meters above the geoid. As a secondary objective for the mission, Sentinel-3 altimetry provides estimates of WSE for water bodies located along the satellite's ground tracks as defined at https://www.altimetry-hydro.eu/. The quality of the measurements depends on the size of the water body and on the terrain and vegetation characteristics surrounding targets. Measurements of river and lake WSE derived from ESA UNCLASSIFIED – For ESA Official Use Only Page 24/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T are a fundamental input to CGLMS’s component of the Land Service and are an essential element of S3NG-T. The Copernicus Climate Change Service (C3S, http://climate.copernicus.eu) leads activities in the domain of climate change. It supports society by providing authoritative information about the past, present and future climate in Europe and the rest of the World. The C3S mission is to support adaptation and mitigation policies of the European Union by providing consistent and authoritative information about climate change. C3S users include scientists, consultants, planners and policy makers, the media and the public. C3S offers free and open access to climate data and tools based on the best available science. It maintains an active dialog with users and endeavours to help them meet their goals in dealing with the impacts of climate change. C3S relies on climate research carried out within the World Climate Research Programme (WCRP) and responds to user requirements defined by the Global Climate Observing System (GCOS). It provides an important resource to the Global Framework for Climate Services (GFCS). Changes in sea level rise, ocean circulation wind and waves, sea ice and ice sheets and river and lake WSE derived from satellite topography are all fundamental inputs to C3S activities. The S3NG-T mission will provide enhanced continuity of these topography measurements in the 2030-2050 timeframe. The Copernicus service for Security (https://www.copernicus.eu/en/copernicus-services/security) leads activities in this domain. The European Boarder and coast Guard Agency (FRONTEX, https://frontex.europa.eu/) provides the border surveillance component of the Copernicus Security Service to support the EU’s external border surveillance information exchange framework (EUROSUR) by providing near real time data over land and at sea around the EU’s borders. In the area of maritime surveillance, the European Maritime Safety Agency (EMSA, http://www.emsa.europa.eu/copernicus.html) operates the maritime surveillance component of the Copernicus Security Service. Copernicus Sentinel 1, 2 and 3 together with additional satellite data are combined with other sources of maritime information to monitor maritime areas of interest The European Satellite Centre (EU SatCen) provides Support to External Action (SEA) within the Copernicus Security Service. SEA assists the EU in its operations, providing decision makers with geo-information on remote, difficult to access areas, where security issues are at stake. Sentinel-3 satellite altimetry provides measurements for a variety of applications relevant to the Copernicus Security applications including sea level rise, ocean circulation, wind and waves over the ocean, sea ice parameters and monitoring of the Arctic regions. S3NG-T must enhance continuity of these measurements and provide enhanced sampling in support of Copernicus Security Services. Copernicus Emergency Management Service (CEMS, http://emergency.copernicus.eu/) leads activities in this domain. It provides all actors involved in the management of natural disasters, man-made emergency situations, and humanitarian crises with timely and accurate geo-spatial information derived from satellite remote sensing and completed by available in situ or open data sources. The Copernicus EMS consists of a mapping component and an early warning component. ESA UNCLASSIFIED – For ESA Official Use Only Page 25/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The mapping component of the service (Copernicus EMS - Mapping) has a worldwide coverage based on satellite imagery implemented by the European Commission DG Joint Research Centre (JRC). Copernicus EMS - Mapping can support all phases of the emergency management cycle: preparedness, prevention, disaster risk reduction, emergency response and recovery. The early warning component of the Copernicus EMS includes the European Flood Awareness System (EFAS, which provides overviews on ongoing and forecasted floods in Europe up to 10 days in advance) and the European Drought Observatory (EDO, which provides drought-relevant information and early-warnings for Europe. Global Flood Awareness System (GloFAS), Global Wildfire Information System (GWIS) and Global Drought Observatory (GDO) are used at global level. Measurements of storm surge and river and lake WSE derived from S3NG-T are a fundamental input to CEMS and are an essential element of S3NG-T. The Copernicus Atmospheric Monitoring Service (CAMS, https://atmosphere.copernicus.eu/) leads activities in this domain. It provides consistent and quality-controlled information related to air pollution and health, solar energy, greenhouse gases and climate forcing, everywhere in the world. CAMS is implemented by the European Centre for Medium-Range Weather Forecasts (ECMWF). S3NG-T Measurements of atmospheric properties from microwave radiometer measurements, wind speed over the ocean and ocean waves are important inputs to CAMS – particularly for oceanatmosphere coupling of models that are growing in maturity. 2.2 The Copernicus 2.0 Long-Term-Scenario for a Topographic Ocean and Ice Measurement Family. As set out in the ESA Program Board for Earth Observation (PBEO) paper (ESA/PBEO(2017)31 Paris, 5 September 2017), the Copernicus Space Component (CSC) has been established as the largest and most proficient Earth Observation infrastructure in the world. However, the future evolution of the CSC needs to be initiated now, in close cooperation with all stakeholders, to remain fulfilling evolving user needs. The intense use and increased awareness for the potential of Copernicus have generated great expectations for an evolved Copernicus system. There is now a large set of concrete needs and requirements for the future configuration of the CSC. User and observation requirements have been identified, structured and prioritized in a continuous reflection process led by the EC. Results from EU policy analyses, consultation of Services and Member States, as well as various workshops, gap analyses, studies and task forces now provide the rationale for a ‘Long-Term Scenario’ (LTS, ESA, 2025). Two distinct sets of expectations have emerged from the user consultation process: 1. Stability and continuity of the system, while increasing the quantity and quality of CSC products and services, lead to one set of requirements. These requirements are addressed by an Extension of the current Sentinel 1 to 6 satellite capability by providing enhanced continuity of baseline Copernicus observations. 2. Emerging and urgent needs for new types of observations constitute a second distinct set of requirements. They are addressed by a timely Expansion of the ESA UNCLASSIFIED – For ESA Official Use Only Page 32/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 • CMEMS(2017): Copernicus Marine Environmental Monitoring Service (CMEMS) requirements for the Evolution of the Copernicus Satellite Component (CMEMS, 2017). • C3S(2020):C3S in the document “EC User Requirements For The Sentinel-NG Topography Family, Inputs To ESA Ad-Hoc Expert Group” (C3S, 2020) In Appendix III, these inputs have been used to derive numbered high-level European Commission Copernicus User Needs at system level and Product Needs. Section 3 discusses Level 2 Products and Performance in a system context and, based on European Commission User Needs, provides background and justification for S3NG-T mission requirements. Appendix IV provides traceability between high-level European Commission Copernicus User Needs and S3NG-T Mission Requirements. 2.4 Copernicus Sentinel-3 Mission and In-Flight Performance The Copernicus Sentinel-3 mission, part of the first generation of Copernicus satellites designed to ensure the long-term collection and operational delivery of high-quality measurements to Copernicus ocean, land, and atmospheric services. Sentinel-3 measurements are used both in their own right to monitor ocean, inland waters, sea ice and land ice parameters and as input to data assimilation systems that constrain global and regional numerical prediction models. Mission Requirements are set out in the Sentinel-3 Mission Requirements Traceability Document (MRTD, Donlon, 2011). A full description of the Sentinel-3 Mission is provided in Donlon et al. (2016) and a technical summary of the mission is provided in Appendix V for reference. Assessing the uncertainties of different altimeter parameters and geophysical corrections is not straightforward. Several methods can be used to estimate these quantities that yield different results depending on the temporal and spatial scales assessed. Raynal and Labroue (2021) provide a detailed assessment of global uncertainties as part of the Sentinel-3 Mission Performance Centre (MPC) activities. They define the following spatial/temporal uncertainties: Sub mesoscales and mesoscales uncertainty: • White noise: this uncertainty is uncorrelated on time and is related to the instrumental measurements (altimeter). • Short-time temporal uncertainty (< 10 days): this includes all the uncorrelated and correlated uncertainty in time for time scales < 10 days. It is important to define these uncertainties for oceanographic applications in conjunction with mesoscale or sub-mesoscale studies. Long scale uncertainties: • Medium temporal uncertainty (2 months – 1 year): these include all correlated temporal uncertainties at medium scales e.g. periodic signals (annual, semiannual...). The description of these uncertainties is useful for applications requiring long time series (e.g. climate reanalyses). ESA UNCLASSIFIED – For ESA Official Use Only Page 33/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 • Long-term uncertainties (> 1 year): these include inter-annual and long term stability (drift) uncertainty. These are the most important for climate applications as they directly impact the global mean sea level trend. In flight performance of the Sentinel-3A and Sentinel-3B altimetry mission is regularly reported in the scientific literature through validation activities and applications. In addition, the Sentinel-3 Mission Performance Centre (MPC) provides annual and cyclic reports available at https://sentinel.esa.int/web/sentinel/user-guides/sentinel-3-altimetry. This material is used to specify in-flight performance of Sentinel-3 that in turn, defines S3NG-T baseline continuity requirements used throughout this document. Performance requirements for the Sentinel-3 topography mission are provided in Table 2.4-1 together with Mission Requirements set out in Donlon (2011) and set the baseline continuity performance for the S3NG-T mission. ESA UNCLASSIFIED – For ESA Official Use Only Table 2.4-1. Copernicus Sentinel-3 topography mission performance budget (Donlon, 2011) and Sentinel-6 mission performance budget (EUMETSAT, 2018). All values are specified for measurements integrated over 1s, for Significant Wave Height (Hs) of 2m and for σ0 of 8dB. The specified altimeter random error assumes perfect Brown or Haynes model echoes. NRT3H is Near Real Time performance within 3 hours of acquisition, STC is short time critical within 48 hours of data acquisition for Sentinel-3 and 36 hours for Sentinel-6, NTC is non-time critical within 30 days of acquisition for Sentinel-3 and 60 days for Sentinel-6. Quantity Sentinel-3 Requirements (from Donlon, 2011 and S3 SRD v4.0) Sentinel-3 NTC In flight Performance (S3A and S3B) Sentinel-3 In flight Performance Sources Sentinel-6 Requirements (from EUMETSAT, 2018)2 (NRT3H/STC/NTC) Requirement (NRT3H/STC/NTC) Goal (NRT3H/STC/NTC) Ku-band Instrument range noise (cm) ≤1.3 ±0.35 ±3.45 (compared to PFAC FRM) Mertikas et al. (2020) NTC over PFAC FRM transponder • S3A: ±0.20 ±3.45 (FRM uncertainty) • S3B: ±0.35 ±3.45 (FRM uncertainty) Long-term average (4.7 years) S3A SAR over PFAC transponder from S3MPC.CLS.APR.006: • ±1.22 (range bias: 0.64) Long-term average (2.6 years) S3B SAR over PFAC transponder from S3MPC.CLS.APR.006 • ±1.38 (range bias:-0.79) Average (22 months ) S3A SAR over the ocean from Raynal and Labroue (2021): • 1.25 for wavelength 0.71.0 km (following the spectral approach of Zanife et al. 2003) • 1.2 cm for >50 km >10 days for wavelength 700 m to 7 km (computed from standard deviation of the range elementary measurements) 0.8(a) 0.5 2 For reference, performance requirements for the Sentinel-6 mission are also provided as a guide to the performance form current state of the art nadir pointing altimeter instruments (Poseidon-4) incorporating digital technology elements. ESA UNCLASSIFIED – For ESA Official Use Only Page 35/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Ionospheric path delay (cm) ≤0.7 ≤0.4 (compared to GIM) Long-term average (4.7 years) S3A dual-frequency from S3MPC.CLS.APR.006: • ≤0.32 compared to GIM Long-term average (2.6 years) S3B dual-frequency from S3MPC.CLS.APR.006: • ≤0.31(3) compared to GIM Average (22 months ) S3A SAR over the ocean from Raynal and Labroue (2021): • 1 cm (computed as the standard deviation after high pass filtering of 1Hz to remove the geophysical signal). Note • <0.1 cm filtered to 300 km (computed as the standard deviation after high pass filtering of 1Hz to remove the geophysical signal) 0.5(b) 0.3 Sea state bias (cm) ≤2.0(4) 2.0 (difficult to assess) Average (22 months) S3A SAR over the ocean from Raynal and Labroue (2021): • SSB variability estimated to be 0.3 (computed as the standard deviation after high pass filtering of 1Hz to remove the geophysical signal) 2.0 1.0(e) Dry tropospheric path delay (cm) ≤0.7 <0.1 <0.1 Average (22 months ) S3A SAR over the ocean from Raynal and Labroue (2021) 0.8/0.7/0.7 0.5 Wet tropospheric path delay (cm) ≤1.4 <0.7 ±1.0 Frery et al. (2020) comparison to J3 crossovers: • S3A:0.55 ±0.78 (PLRM 3 parameter algorithm against J3) • S3B:0.56 ±0.82 (PLRM 3 parameter algorithm against J3) 2019 S3A NTC from S3MPC.CLS.APR.006: • 0.04±1.36 for SAR 5 parameter algorithm against model 2019 S3B NTC from S3MPC.CLS.APR.006: • 0.08±1.38 for SAR 5 parameter algorithm bias against model 1.2/1.2/1.0 0.8 3 Sentinel-3B C-band range is in average 8.8 cm shorter than Sentinel-3A one: shorter C-band range implies a less negative ionosphere correction, which is consistent with the results here. 4 Chelton (1994) considered 1% SWH for SSB uncertainty that corresponds to a 2 cm SSB uncertainty level for 2m Hs. This is considered maximum global RMS uncertainty over the open ocean. ESA UNCLASSIFIED – For ESA Official Use Only Page 36/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Average (22 months) S3A SAR over the ocean from Raynal and Labroue (2021): • 0.12 Altimeter range noise over ocean RSS with allocations above (cm) ≤2.9 2.64/2.61/2.53 1.49 RMS radial orbit (cm) ≤10/4/3(5) <0.7 (POE) ROE Radial S3A (S3B) from CNES and ESOC GMV-GMESPOD-RSR-0015: • <1.4 (<1.3) ±0.5 (±0.6) MOE Radial S3A (S3B) from CNES and ESOC GMV-GMESPOD-RSR-0015: • <0.75 (<0.75) ±0.13 (±0.14) POE Radial S3A (S3B) from CNES and ESOC GMV-GMESPOD-RSR-0015: • <0.65 (<0.57) ±0.09 (±0.07) 5.0/2.0/1.5 3.0/1.5/1.0 Total RSS sea surface height (SSH) (cm) ≤10.4/5/4.2 <1.0 ±3.19 (PFAC FRM) Mertikas et al. (2020) NTC over PFAC FRM ocean infrastructure: • S3A: ±0.62 ±3.19 (FRM uncertainty) S3B: ±0.75 ± 3.19 (FRM uncertainty) Average (22 months) S3A SAR over the ocean from Raynal and Labroue (2021): • 1.33 for wavelength 0.71.0 km (following the white noise spectral approach of Zanife et al.2003) 5.65/3.29/2.94 3.53/2.12/1.80 Hs (dynamic range 0.5-20 m) ≤20 cm or 4% < 10 2019: S3A (S3B) bias against ECMWF WAM model (standard deviation of difference, SDD) from S3MPC.CLS.APR.006: • ≤0.02 (≤0.02), SDD ≤0.27 (≤0.35) for Hs ≤12 m 2020: S3A (S3B) bias against ECMWF WAM model from MPC team(ref TBC): • ≤0.04 (≤0.06), SDD ≤0.25 (≤0.26) for Hs ≤12 m 15 cm ±5%(c) 10 cm ±5%(c) 5 S3 GSRD (S3-RS-ESA-SY-0091) GSR-PDGS-PRO-080, GSR-PDGS-PRO-081. See also notes in MRD-300 regarding current performance of Sentinel-6 POD. ESA UNCLASSIFIED – For ESA Official Use Only Page 37/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 2019: S3A (S3B) Bias against in situ data from S3MPC.CLS.APR.006 • <0.02 (≤0.02) SDD <0.35 (≤0.35) for Hs ≤12 m 2020: S3A (S3B) bias against in situ data from MPC team (ref TBC): • <0.12 (0.14), SDD <0.30 (0.31) for Hs ≤ 12 m Average (22 months) S3A SAR over the ocean from Raynal and Labroue (2021): • 9.7 for wavelength 0.71.0 km (spectral analysis method) • 8.9 for wavelength 0.71.0 km (standard deviation of Hs measurements) • 7.3 uncertainty (geographically correlated) for large scales > 50 km and < 10 days. Wind speed (U) (dynamic range 3 to 20 m/s) ≤2 <0.2 2019: S3A(S3B) SAR difference to in situ from S3MPC.CLS.APR.006: • ≤0.1 (-0.15), SDD ≤1.37 (≤1.38) for U2m ≤ 26 m 2020: S3A (S3B) SAR difference to in situ from MPC team (ref TBC): • ≤ -0.10 (≤ -0.15), SDD ≤1.47 (1.57) for U2m ≤ 26 m 2019: S3A (S3B) SAR difference to ECMWF WAM model from S3MPC.CLS.APR.006: • ≤0.14 (≤0.03), SDD ≤1.06 (≤1.05) for U2m ≤ 26 m 2020: S3A (S3B) SAR difference to ECMWF WAM model from MPC team (ref TBC): • ≤0.14 (≤0.03), SDD ≤1.06 (≤1.05) for U2m ≤ 26 m Average (22 months) S3A SAR over the ocean from Raynal and Labroue (2021) • 0.16 standard deviation of 1Hz after high-pass filter to account for Hs noise. 1.5(k) 1.0 Sigma0 (-10dB - +50 dB)(d) ≤ ±1, long term drift of ≤0.3dB over mission lifetime. <0.2 ~0.3 dB stability for 2019. 2019 S3A and S3B from S3MPC.CLS.APR.006: • 1.8 Standard deviation 2016-2018 • 11.0 dB mean 2016-2018 (S3A and S3B) Average (22 months) S3A SAR over the ocean from Raynal and Labroue (2021): 0.3(j) 0.3 ESA UNCLASSIFIED – For ESA Official Use Only Page 38/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 • 0.04 dB for wavelength 0.71.0 km (following the spectral approach of Zanife et al. (2003) • 0.03 dB for wavelength 700 m to 7 km (computed from standard deviation of the Sigma0 elementary measurements) • 0.14 uncertainty (geographically correlated) for large scales > 50 km and < 10 days, bias depends on the radial velocity to first order. Surface Water Elevation (m) Secondary Objective: no specification Large Rivers (≥3 km):≤0.24 m Kittle et al (2021) 0.32 m using Sentinel-3A and Sentinel-3B datasets over the Zambezi river basin where in situ gauging stations are available. Haliki and Niedzielski (2022) obtained <0.22m (0.12 – 0.44m) uncertainty using Sentinel-3A over Polish rivers. Fenoglio et al (2022) in press 18 cm for rivers over 500m width and crossing angles ≥60°. N/A N/A a. After ground processing, averaged over 1 second, for 2-meter wave height. b. Derived from Kuand C-band range difference, averaged over 200 km. c. Valid for the range of 0.5 to 8 m Hs. d. S3 SRD states: Total absolute accuracy after cross-calibration with other altimeter missions [RA-PE-020]. The long-term drift error of the sigma-0 internal calibration shall be less than 0.3dB during the mission lifetime [RA-PE-030 a] e. Could also be expressed as 1% of Hs, to be reached at the end of the commissioning phase. j. After cross-calibration with other altimeter missions. k. For the range of 0.5 to 8 m Hs. ESA UNCLASSIFIED – For ESA Official Use Only 3 SCIENTIFIC JUSTIFICATION OF S3NG-T MISSION MEASUREMENT REQUIREMENTS This chapter provides background information supporting the definition and justification of the MRD requirements. Please note that the information reflects the status at the time of writing (July 2025) and may not be fully up to date or consistent with the latest developments in the Copernicus Programme or the S3 NGT mission design. Satellite altimetry provides an essential data set for the Copernicus services (e.g. Le Traon et al, 2019) providing measurements over the global ocean and, increasingly in the coastal zones and inland waters. Microwave radiometers supporting nadir-pointing radar altimeter payloads are also extensively used to monitor atmospheric characteristics in the troposphere (e.g. Varma et al., 2020, Quartly et al., 2019a,b). Altimeter-derived applications span many topics, from fundamental science to operational oceanography, and a wide range of scales from regional studies to global systems and from short-term weather forecast to long-term sea level monitoring. The importance of satellite altimetry cannot be overstated in terms of the impact on operational oceanography (e.g. Munk, 2002) and climate science (e.g. IPCC, 2014, 2019). Measurements are used in a variety of applications to enable quasi-global estimates of sea level rise (e.g. Cazenave et al., 2018, Veng and Andersen, 2020), ocean sea state (e.g. Ardhuin et al., 2019, Ribal, and Young, 2019, Dodet et al., 2020), large-scale ocean and mesoscale circulation, (~30-300 km and ~5-90 day) (e.g. Chelton et al., 2007), wind speed over the ocean (e.g. Abdalla, 2012, Bushair and Gairola, 2019), estimates of sea ice thickness and volume (e.g. Laxon et al., 2003; Laxon et al., 2013; Tilling et al., 2018), mapping of land ice topography, elevation changes and mass balance (e.g. Sandberg Sørensen et al, 2018, Schröder et al, 2019; Helm et al, 2014; Otosaka et al, 2023), geodesy applications (e.g. Bloßfeld et al., 2020), ionospheric mapping (e.g. Ray, 2020) and river and lake heights estimation (e.g. Creteaux et al, 2017, Emery et al., 2018, Gao et al., 2019, Roohi et al., 2019) amongst others. In Copernicus (EU, 2014), measurements are used for operational ocean monitoring/forecasting and derivation of geostrophic ocean currents by the Copernicus Marine Environment Monitoring Service (CMEMS, e.g. LeTraon et al., 2015, 2019), wave forecasting and climatology (e.g. Campos et al., 2020, Bidlot et al., 2017, Cooper, and Forristall,1997), climate monitoring/prediction by the Copernicus Climate Change Service (C3S, Buontempo et al., 2020), numerical weather prediction (e.g. Campos et al., 2020), the study of ocean tides (e.g. Carrere et al., 2020) and gravity field mapping (Sandwell et al., 2019). Other diverse applications include sea-floor mapping (Smith and Sandwell, 1997), investigation of ocean wave-current interaction (Quilfen and Chapron, 2019), dual-frequency radar altimeter inputs to computation of rain rates (Quartly et al, 1999a, 2000), computation of integrated water vapour and cloud liquid water content (e.g., Lázaro et al (2019), computation of ocean/atmosphere gas fluxes (e.g. Frew et al., 2007, Goddijn-Murphy et al, 2013), monitoring ship traffic (Tournadre, 2014), estimating extreme waves (e.g., Alvez and Young, 2004, Hanafin et al., 2012), tracking icebergs (Tounadre et al., 2008). In addition, altimetry increasingly contributes to our understanding of the hydrological cycle by monitor variations of rivers, lakes, reservoirs and flooded regions (e.g. Crétaux et al, 2017, Emery et al., 2018, Papa et al., 2022). Escudier et al. (2017) give an extensive overview of the research and operational applications of altimetry. ESA UNCLASSIFIED – For ESA Official Use Only Page 40/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Given the considerable range of applications, a sustained Sentinel-3 altimetry constellation capability is required to address operational science and societal needs. 3.1 S3NG-T and Copernicus Marine Monitoring and Prediction Altimetry has revolutionized the understanding of large-scale ocean dynamics, i.e. the large-scale forces (pressure gradient and wind stress) that set the ocean in motion. Satellite altimetry is the most mature technique for mapping balanced geostrophic motions of the upper ocean and has led to breakthroughs in our understanding of the dynamics of large-scale (larger than roughly 200 km wavelength) oceanic circulation with, to date, unequalled views of eddy kinetic energy on a global scale. Today, a mixture of operational data in low inclination 66° orbits are available (e.g. from the Sentinel-6 satellites, Jason series, complemented by polar orbit satellites including Sentinel-3, CryoSat-2, AltiKa and HY2 series). Satellite altimeters measure Sea Surface Height (SSH), that is a combination of the marine geoid (Bruinsma et al. 2013) and an ocean component including a temporal mean sea surface height, and time-variable Sea Surface Height Anomalies (SSHA) relative to this mean. The main balance of forces at the mesoscale and larger scales (~10 km and more) is between the Coriolis force and the pressure gradient associated with SSHA (e.g. Ducet et al., 2000; Penven et al., 2014, provides estimates of ocean current anomalies in the upper water column that are in geostrophic balance), the smaller scales (< 10 km) are dominated by ageostrophic sub-mesoscales (McWilliams 2016) with small signatures in altimetric topographic height that are not yet resolved by satellite altimetry techniques. Other unbalanced motions due to tides, internal tides and internal waves also have a signature in altimetric topographic height. Altimetric observations, particularly from missions on non-sun-synchronous orbits, provide measurements of the open-ocean and coastal tides and the phase-locked internal tides (Ray and Zaron, 2016), helping us to improve our global tidal models to better predict the ocean tides. However, nonphase locked, non-predictable internal tides and their energy cascade also have an imprint on altimetric sea surface topography over scales < 200 km (e.g. Zaron, 2017), and these are not in geostrophic balance. This mixture of balanced geostrophic flow and unbalanced SSH signals allows us to observe the different dynamics and their interactions with altimetric observations. The challenge is in separating the unbalanced component in order to calculate geostrophic currents or higher-order fields (geostrophic vorticity, strain, etc). There are difficulties when computing estimates of ocean surface geostrophic currents from altimetric SSH due to the breakdown of the geostrophic approximation towards the equator, in shallow waters, near the coast (Mulero et al., 2022; 2024), at the marginal ice zone, within sea ice regions, due to uncorrected tides or internal tides, and generally at wavelengths below ~200 km that are unresolved by today’s altimetry constellation (e.g. Dibarboure et al., 2014). Altimetry is particularly important for operational models. Le Traon et al. (2017) emphasise that altimeter sea level observations are the only remotely sensed information that reflects the ocean state far below the surface because temperature and salinity variations at all depths contribute to variations in SSH. Sea level from satellite altimetry is an integral of the density structure of the ocean interior and provides a strong constraint on the 4D ocean state estimation.Because of the critical reliance on altimetry, the development of operational oceanography and operational altimetry ESA UNCLASSIFIED – For ESA Official Use Only Page 41/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 occurred simultaneously. The global data assimilation experiment (GODAE) was set up in 1997 after the successes of the TOPEX and ERS missions, and the main GODAE demonstration was phased with the Jason-1 and ENVISAT altimeter missions. Since then, the GODAE community (now OceanPredict, https://oceanpredict.org/) has been maintaining strong links with satellite altimetry communities and major progress has been made in developing and optimising the use of satellite altimeter data for operational oceanography. CMEMS modelling and data assimilation systems thus highly depend on the status of the altimeter constellation. Both OSEs (Observing System Evaluations) and OSSEs (Observing System Simulation Experiments) demonstrate the major contribution of altimetry (see Le Traon et al., 2019). At least four radar altimeters are required to observe the mesoscale currents. A long-term series of a high-accuracy altimeter system (Jason/Sentinel-6 satellites) is needed to serve as a reference for the other altimeter missions and for the monitoring of climate signals notably for C3S. The corresponding improvement of products will answer European policy needs (SWD, 2019) and will impact the different areas of benefits of CMEMS: • maritime security and safety, including search and rescue activities; • maritime transport through improved surface currents; • pollution monitoring and offshore operations; • seasonal and weather forecasting through improved ocean/wave/atmosphere coupling and representation of surface layers; • coastal zone monitoring and forecasting; • riverine influence in the coastal environments; • fish egg and larvae drift modelling; • ocean productivity (e.g. at ocean fronts). CMEMS user needs for satellite altimetry (CMEMS, 2017) are founded on the future capability of ocean modelling systems. As CMEMS intends to evolve around 2025 to monitor and forecast the ocean at finer scale and improve the monitoring of the coastal zone, model resolution will increase by a factor of at least three (e.g., global 1/36° and regional 1/108°). Furthermore, more advanced data assimilation methods will be available. These models will be used to force downstream coastal models with a potential resolution of a few hundred meters and/or ensemble prediction approaches in the next 10-15 years. To support such high-resolution model systems, observations are required to constrain the model solutions at the appropriate time and space scales. Since observations are likely to be limited (particularly in situ observations of full ocean depth) more advanced data assimilation and ensemble forecast methods will be required that provide probabilistic forecast capability that statistically represents the uncertainties in the relatively unconstrained forecasts. Outputs should be able to characterize, at fine scale, the upper-ocean dynamics to improve knowledge of ocean state and forecasts of ocean dynamics required as boundary conditions for nested very-high resolution regional and coastal models. The coverage, revisit and sampling characteristics of the present satellite altimeter system (in an operational sense as opposed to ‘one-off’ scientific research missions) includes Copernicus Sentinel-3A/C; Copernicus Sentinel-3B/D; Copernicus Sentinel- ESA UNCLASSIFIED – For ESA Official Use Only Page 48/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Figure 3.1.1-2 (top, left) Altimeter Hs measured by four altimeters (Jason-1, Jason-2, ERS-2, and Envisat) on 13 and (top, right) 14 Feb 2011. Such exceptional values were emphasized by Bancroft (2011) issue of MWL: “Altimetry data from 1100 UTC on the 14th [. . .] reveal seas as high as 66 ft (20.1 m), the highest the author has seen in this type of imagery.” The black square in the left (right) panel indicates the location of the most extreme sea states measured during these two days by the Envisat (Jason-2) altimeter, respectively. (middle, left) Focus on the altimeter (black) Hs values estimated along the Envisat and (middle, right) Jason-2 tracks indicated by the squares boxes In the top panel above, and computed from the WW3 model forced by ECMWF (red), NCEP (green), and NCEP+10% (blue) winds. A running average has been applied to the altimeter data (~5-km resolution) to better match the resolution of the WW3 model (0.5°). (bottom, left) Wind speed from different sources interpolated on the same Envisat and (bottom, right) Jason-2 altimeter tracks. For both panels, black (green) lines give the altimeter (NCEP) wind speed. For the left (right) panel, the dashed red line gives the ASCAT scatterometer (Jason-2 radiometer) wind speed. On the left panel, the blue line gives the Oceansat-2 wind speed. All estimates have been computed at the spatial resolution of the NCEP fields. The dashed blue lines show the storm force (V ≥ 24.5 m s-1) and hurricane-force (V ≥ 32.7 m s-1) wind thresholds. A running average was again applied to the altimeter data to better match the resolution of the other data sources (~25 km). (from Hanafin et al., 2012). ESA UNCLASSIFIED – For ESA Official Use Only Page 49/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 3.1.3 Challenges in the coastal zone In Europe, approximately 40% of the population lives within 50 km from the coast. Coastal zones are densely populated, exhibit high rates of inhabitant’s growth and urbanization, concentrate economic assets and critical infrastructures, support green and blue economy and, therefore experience huge socio-economic and environmental changes (Geraldini et al, 2021). The Organization for Economic Co-operation and Development (OECD) note that by 2030, the ‘Blue Economy’ could outperform the growth of the global economy, both in terms of added value and employment. In the coming decade marine energy, marine biotechnology, coastal tourism, transport and food production sectors could offer unprecedented development and investment opportunities (EC, 2019). A variety of International and European Union Directives relate to the coastal ocean and depend directly on the availability of ocean topography measurements. Figure 3.2-1. The most requested parameters for Coastal Ocean parameter linked to value by users and directives gathered from eighteen Institutions and Authorities in Italy (from User Needs Analysis for the Definition of Operational Coastal Services, Geraldini, 2021) From the European Space Agency perspective, it is important to maintain a close connection to the user community and be fully aware of priorities and user needs. As an example, Geraldini et al (2021) recently provide an assessment for the importance of ocean currents, sea level, and waves for coastal applications linking their application to Maritime security (coordinated by EMSA) and the directives above for Italy. The methodology is applied to Italian institutional users, but it is scalable to the European level. Figure 3.2-1 shows that the most important variable derived from satellite altimetry, in terms of user need to address EC directives in the coastal region of Italy, is sea state (waves). Ocean currents, sea level and flooding are also highly ranked. The results provide a clear overview of the coastal user requirements, highlighting the common need of integrated information for coastal zone management (Geraldini, 2021). This example may not represent the user needs of all countries. ESA UNCLASSIFIED – For ESA Official Use Only Page 50/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Coastal sea level, as observed at the shores of exposed coastlines, is strongly influenced by small scale and highly variable ocean currents and eddies, tides, and wave-induced processes (wave setup, wave runup, infra-gravity waves). Moreover, waves play an important role in sea level at the coast either directly (it has been demonstrated in CMEMS coupled wave/ocean system considering the role of waveinduced stress in improving the sea level estimate in severe storm conditions), or indirectly through their influence on the wind stress, storm surge (Marcos et al., 2019) and wave setup and runup. Therefore, coastal flood risk assessment and forecasting needs to use a wider range of wave parameters than basic significant wave height. Dodet et al. (2019) note limitations in our understanding of coastal sea-level variability due not only the lack of satellite information at the coast and the scarce frequency of co-located wave measurements in time, but also the lack of information on the wave period and direction. Given the tremendous social, economic and biological value of coastal zones, an enhanced core monitoring of coastal zones is critical for a wide range of applications (coastal zone management, climate adaptation, coastal modelling, aquaculture, navigation and shipping, marine renewable energy, fisheries, oil spill management, search and rescue, disaster and emergency management, and academia), to respond to various policies (Marine Strategies Framework Directive (MSFD), Water Framework Directive (WFD), Bathing Water Directive (BWD), Marine Spatial Planning directive, Flood Directive, Integrated Coastal Zone Management (ICZM), Bathing Water, Common Fisheries Policy) and for resilience to climate change as expressed in the Copernicus user needs Staff Working Document. 3.2 S3NG-T and Copernicus Wave and Wind Monitoring and Prediction Since an altimeter fundamentally measures the surface roughness characteristics (via the normalised radar backscatter) and then determines the radar range from these measurements, all satellite altimeters (nadir pointing or otherwise) inherently measure ocean waves. The radar backscatter return depends on the sea state through the mean square slope (mss) and is typically interpreted in terms of the surface wind speed through the correlation between mean square slope and surface wind (e.g. Cox and Munk, 1954).The normalised radar backscatter coefficient (Sigma0) is used to correct sea surface height measurements (via sea state bias), and derive estimates of the wind speed over the ocean at 10m height, U10. Wave height (Hs) and wave directional spectra, E( j , l ), are of great importance for Marine Operations and Marine Emergency response (search and rescue, oil and other pollution) and fundamental to track Stokes velocities that control the dispersion and transport of marine Plastic Debris. Pollution of the seas from plastics and microplastics is one of the three major areas of the Strategy for Plastics (https://eurlex.europa.eu/eli/dir/2019/904/oj) adopted by the Commission in 2018 and enacted into European Law on 2 July 2019; most of the proposed Actions are directly or indirectly related to marine litter, including its international dimension. In reference to the set of “maritime” requirements for the S3NG-T, the European Maritime Safety Agency (EMSA) compiled information on new/additional type of observational products that are needed ESA UNCLASSIFIED – For ESA Official Use Only Page 51/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 for search and rescue operations, sea surface pollution and vessel navigation/routing purposes. Provision of wave data is a highly important component of Copernicus’ mission to deliver both safety and socio-economic benefits to European citizens. Over the last decade the role of waves processes in influencing boundary exchanges between the ocean, the atmosphere, the cryosphere and land has had increased focus in terms of understanding and properly simulating the wider ‘earth system’, leading to a broader requirement to observe these phenomena to further develop atmosphere and ocean components of Copernicus systems. Within Copernicus, satellite derived wave information is used extensively. Within CMEMS, they are used by CMEMS Wave Thematic Assembly Centre (Wave TAC) and Monitoring and Forecast Centres (MFCs) and similarly used by analogues in National Meteorological Services and commercial forecast organisations. For marine operations downstream users have a fundamental requirement to use waves forecasts as part of any marine operational planning and execution. Typically, this is based on CMEMS services or via third parties exploiting Copernicus data. Similarly, design and renewable resource assessment projects have a primary requirement for consistent long-term datasets. Multi-parameter requirement is in search and rescue and oil spill response activities (Christensen et al, 2018) for which EMSA are responsible. Recent research into these particle tracking problems has illustrated the need to better understand the two-way exchanges between atmosphere, wave and ocean and to better utilise the wave spectrum, for example in calculation of Stokes Drift effects (Rohrs et al, 2012). The need to continue to extend the geographic coverage of observations to higher latitudes, in order to correctly project further long-term loss of Arctic sea-ice and to stimulate research into processes driving sea ice growth/break-up/melting at ocean-iceatmosphere interfaces, will also be important. This is critical as EMSA (and CMEMS) need access to the wind sea directional wave spectrum to estimate the Stokes drift and thereafter perform drift forecasting modelling for search and rescue, sea surface pollution (extremely important for modelling marine litter circulation) and vessel navigation/routing purposes. Users tend to use products produced by a wave model and parameters derived from a directional wave spectrum including significant height and wave period, in their decision-making systems. Therefore, continuity and expansion are the key improvements, particularly if data are extended into the polar and coastal zones. Addressing coverage and continuity of basic Hs wave parameters ignores the growing number of more sophisticated users, such as heavy lift and pipelay marine contractors, coastal engineers and marine architects who have a clear need of wave period, direction and spectral wave information in addition to significant wave height (e.g. Lai and Slyozkin, 2014). All of these parameters can be derived from a well observed or modelled representation of the wave spectrum. Additionally, there is a need to determine swells waves for the whole of the European sea basin that can in part be resolved by Satellite altimetry (e.g. Moreau et al, 2018). Well-calibrated and validated satellite data provide potentially more consistent baseline measurements of wave conditions over multiple decades than is achievable with current in-situ wave measurement networks, which use a variety of platform types and instrumentation with varying standards of data processing and quality control (e.g. ESA UNCLASSIFIED – For ESA Official Use Only Page 52/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Durrant et al., 2010; Gemmrich et al., 2011; Timmermans et al., 2020). The importance of satellite data as a component of the observations-model mix used to underpin Copernicus and member states metocean services will only increase as new frontier regions, previously unobserved using in-situ methods, begin to be exploited (for example the Arctic). Utilising wave information to improve altimetry products is essential and well demonstrated for nadir altimetry. At high resolution, it becomes extremely challenging to separate SSH from sea state since the sea state defines the scattering surface to which a radar instrument responds and from which SSH is computed. The so-called Sea State Bias (SSB) (e.g. Tran et al, 2010a, 2010b, 2006, Quartly et al. 2019) ‘correction’ is used to adjust the retrieved SSH to compensate for this effect although this remains one of the largest uncertainties in the SSH measurement. The ranging error due to ocean surface waves (the SSB) remains the largest source of uncertainty in altimeter sea surface height and sea level estimation (typically 3-5% of significant wave height). Sea state bias continues to be estimated through empirical mission-specific look-up tables based on significant wave height and wind speed, even though research indicates the critical need for new observations of wave period and directional wave spectra to finally make progress with this tenacious problem. As radar altimeters gain better sampling characteristics through unfocussed and fully focused SAR techniques (Egido & Smith, 2017) the impact of sea state becomes more challenging depending on the radar technology and geometry used. This is already apparent in moderate resolution nadir pointing altimeters such as Sentinel-3 (e.g. Boy et al., 2016, Moreau et al., 2018 amongst others) where swell systems may be detected when aligned orthogonally to the flight path of the altimeter. Availability and quality of 2D wave spectra, wave period and wave direction information; extended coverage towards high latitudes (impact of waves on sea ice) and in coastal and shallow waters; higher temporal sampling; and observations co-located with atmospheric and ocean boundary layer parameters are all areas that will lead to an improved CMEMS service. In particular, extending the spectral domain over which wave data can be assimilated to shorter swells and maturing wind-seas should yield significant improvements in wave model predictive capabilities. Data assimilation techniques based on the use of the China France Oceanography Satellite (CFOSAT) SWIM wave directional spectra data (e.g. Hauser et al., 2020) reveal improved performance compared to traditional nadir only Hs measurements (e.g. Wang et al, 2021). CFOSAT wave products are included in the CMEMS product catalogue. Spectral and directional spectral sea state parameters are one of the most important measurements to support future evolution of CMEMS wave forecasting models, CMEMS ocean atmosphere coupled models and C3S coupled systems. This is because the exchange of heat, gas, and momentum occurs via the wave action at the sea surface. In additional, sustained global sea state observations for climate applications are clearly evident in the C3S user needs. In reality, individual ocean surface waves are relatively small scale (order 10-100m) transitory phenomena, so the information used to describe waves more often focuses on statistics that describe the ‘sea-state’; i.e. representing a population of waves sampled over a given time interval or spatial area. The most complete description of sea-state is provided via two-dimensional (frequency-direction) energy spectra, and it is the prediction of evolution of these spectra in space and time that is used as the basis ESA UNCLASSIFIED – For ESA Official Use Only Page 53/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 for state-of-the-art ocean wave forecast models. As a result, there is a critical demand for high quality measurement of ocean wave spectra in addition to derived parameters more normally communicated to users of wave data, such as significant wave height. However, for wave spectra there is a significant gap in observing capabilities. In-situ measurements, which are generally made at a point location, capture enough information to measure wave spectral statistics in frequency space, but will not have enough fidelity to fully represent the directional distribution of wave energy in complex cases (Ardhuin et al., 2019). Observations of wave spectra from satellites, made using a Synthetic Aperture Radar instruments such as those onboard Copernicus Sentinel-1, achieve a better two-dimensional measurement of spectral energy, but only robustly for a limited set of low frequencies (<0.08Hz, wavelengths >220m; Ardhuin et al., 2019) and with some dependency on the instrument track relative to wave direction. New capability to measure directional wave spectra from space was demonstrated recently for the first time with the Surface Waves Investigation and Monitoring (SWIM) instrument carried by CFOSAT. First results show great promise for measuring spectral properties of ocean waves in the wavelength range [70–500 m] and were shown to produce positive and significant impact in wave models when used in combined assimilation with Sentinel-1 (Hauser et al., 2020; Aouf et al., 2019). Recent work of Altiparmaki et al (2022) demonstrated new techniques that exploit fully focused SAR processing of nadir altimeter data to provide 2D wave spectra estimates. This is a promising development for Sentinel-3 next generation topography. Consolidating spaceborne observing capability to fill these observation gaps is needed not only to provide decision makers with near real-time observations of sea-state at high detail, but also to improve the evidence base used by models in deriving parameterisations of spectral evolution, better constrain such models through data assimilation at regional and global scales, and increase the evidence base of observed spectra that can be used in structural design studies. From a satellite observation perspective, this will involve new generation of satellites that can provide sustained measurements over the extended range of ocean wavelengths including significantly shorter wavelengths representative of high frequency wind-sea waves. Whilst there are considerable benefits in extending the capabilities of satellites to measure the full wave spectrum, any future developments need to be mindful that, whilst general coverage of the global oceans by satellite instruments is good overall, revisit times to specific locations by individual missions (order of a week or more) remain relatively long compared to storm development timescales (order 1-2 days) and assimilation windows (order 6 hours). The issue of revisit time and coverage may be resolved through one or a combination of increased swath, level of orbit, or number of operational missions, but should also be considered in light of the application. For example, where long-period swells which cross ocean basins over timescales of days, are assimilated in global or ocean-scale wave models, the important requirement is that the swell field is sampled regularly along its track rather than repeat sampling a given point. A similar criterion is the primary requirement for measuring significant wave height in high and mid-latitude storm tracks. Conversely, when considering smaller systems such as tropical cyclones or short fetch coastal seas, revisit frequency becomes a limiting factor. Current revisit times, even with several operational missions flying, make it impossible, from satellites alone, to derive reliable statistics on extreme ESA UNCLASSIFIED – For ESA Official Use Only Page 54/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 wave parameters and associated impact on sea level (Marcos et al., 2019; Jiang, 2020). In addition, the quality of wave data measured in shallow tidal waters and within a few kilometres of the coast still needs improvement. Although recent efforts in improving the processing of altimeter echoes in the coastal zone (for example through projects such as ESA COASTALT and ESA Climate Change Initiative for Sea State) have increased the number of significant wave height observations that can be correctly retrieved in the coastal zone, (Schlembach et al., 2020), implementing this in an operational context still remains a challenge. Additionally, both the revisit and data quality issues restrict the utility of satellite observations being used in real-time forecasting for regional applications in shallow shelf seas and, therefore, the impact of these data in Copernicus Marine Environment Monitoring Service (CMEMS) regional products. The generic European Commission user needs demonstrate a potential tension between products to improve quality and extend the range of wave spectra and derived parameters retrieved versus maintaining/increasing coverage of the most commonly used significant wave height parameter. However, the clear fact is that no single or dual constellation of instruments can achieve the revisit times needed for data intensive applications such as regional wave model data assimilation. For example, a requirement provided from ECMWF is a revisit of 6 hours for a 75km x 75km grid box. The time window in this requirement is difficult to relax, since this represents a standard window for numerical weather prediction, but the required size of the revisited area will vary according to application, e.g. it is governed by the fetch areas in the region being modelled. Even with some relaxation of the spatial requirement this type of need can only be met by taking a multi-mission view. As a result, the requirement for a next generation instrument should be to optimise sampling and revisit times, at least maintaining the present contributions made by Copernicus instruments to the collective dataset. An enhanced sampling might be achieved through one, or a combination, of increased revisit time or use of multiple altimeter instruments enhancing the Sentinel-3 baseline. There is a clear need for a next generation instrument to continue to provide directional wave spectral information and, further, to extend the range of wavelengths that can be robustly sampled to at least the limits of the SWIM instrument on CFOSAT (approximately 70 m) and, preferably, closer to the target set for the ESA EE9 SKIM concept of £30 m (Ardhuin et al., 2019). The difference between these wavelengths, when considered in wave period and associated wind speed terms are: for 70 m, approximately 7 seconds and for fully developed seas at a wind speed of 12 m/s; versus, for 30m, approximately 3.5 seconds for fully developed seas at a wind speed of 6 m/s This represents a substantial improvement in the range of sea-states that could be properly sampled by an instrument, leading not only to better estimation of the full wave spectrum but enabling use of the observations in applications requiring good specification of wind-sea, such as particle tracking, and making the observations more relevant to next generation modelling systems that will seek to directly represent the interactions between atmosphere, waves and ocean surface circulation. The shorter wavelengths are particularly important where the instrument observes regional seas with constrained fetch lengths. In terms of resolution, whilst the existing sampling ESA UNCLASSIFIED – For ESA Official Use Only Page 55/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 frequency of Sentinel-1 provides a target baseline, some relaxation may be preferable to widen the range of wavelengths that can be robustly observed. Any improvements in the available spectral data may also require review and update of Copernicus downstream products; for example, the addition of wind-sea components to swell partitions derived and distributed by CMEMS or, potentially, providing a ‘First 5’ spectral product that can be matched to the in-situ data equivalent. For coverage, maintaining the strategy adopted by CryoSat-2 and Sentinel-3 to measure high latitudes will be increasingly important in view of the likely further diminishment of sea-ice coverage and future exploitation of the Arctic region. Similarly targeting robust retrievals of wave data at the coastline is needed. ECMWF have expressed a target proximity to the coast of 2 km, whilst a more relaxed target of 4 km could deliver a similarly high-quality coastal boundary condition and potentially reduce the difficulties in quality control involved where coastal infrastructure (e.g. major ports or coastal refinery complexes) extends from the land into the sea. However, rapid changes in sea state are known to occur within the final 1-2 km from land, invoking energetic small-scale processes (interactions with currents and shallow bathymetry) that dominate ocean-land interactions in the coastal zone. Concerning the stability of the trend, the requirements for sea state stability should use the requirement for trend stability for regional sea level as a reference. The need on wave height trend accuracy for mean values and extremes is thus under 1 mm/year for coastal areas. Finally, the range of sea-states that can be robustly sampled by the instrument needs to be considered. Existing altimetry carries the risk of under-sampling extreme seas in short lived storms, such as tropical cyclones, so calibration is limited for very high and phenomenal sea-states. Nevertheless, there is a strong requirement from a design perspective to limit uncertainty in the measurement of phenomenal sea-states; for example, 1-100 year return period significant wave height values in the northern North Sea of 17m or greater (Santo et al., 2016). Therefore, a target range of observation from 0.1m to over 20m significant wave height has been proposed by ECMWF. It is proposed that the objective for trends on Hs should apply to both the mean value and extreme values (up to percentile 99 and 100 year return period), given the relevance of the extreme coastal wave events for the coastal habitats and infrastructures. Based on this discussion a set of target requirements are provided in Table 3.2-1. Table 3.2-1. Wave variable requirements for next generation Copernicus missions, based on User Needs provided by the European Commission. Variable Timeliness Precision and quoted accuracy Along-track sampling Repeat Cycle Spatial coverage Significant wave height 95% of data within 3-hours 0.01m 5% or 0.1m 1-20Hz** along track 2 days Global*, to within 4km of coast 2D wave spectra 95% of data within 3-hours Spectra: 10% wavelength; 5 degrees direction; 10% energy 20x20km 2 days Global ESA UNCLASSIFIED – For ESA Official Use Only Page 56/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 * Wave information is required to monitor coastal changes in waves due to interaction with bathymetry, setup, runup, breaking, refraction, diffraction, often happening in last hundreds of meters from coast. ** As global wave models evolve to finer resolution, a spatial resolution of 4-5 km can be expected by 2030. At this point a new standard is required when using along track Hs with better sampling than 1 Hz. 5Hz along track has been shown to better describe wave-currents interactions at the sea surface (lessons from CFOSAT mission). Note that Sentinel-3 data can already provide up to 80 Hz sampling along track in Level 1a products. 3.3 S3NG-T and Copernicus Hydrology Monitoring and Prediction Inland water products from Copernicus are relatively new. They include inland water, wetness, snow and land ice products. Water levels are computed for lakes larger than 50 ha and intersections of major river networks. Lake water quality products and lake surface water temperature are produced according to the Global Lakes and Wetlands Database6 (GLWD that provides water masks/classification for lakes and wetland) and the Water Framework Directive7. At pan-European level, Copernicus produces water and wetness products (permanent water, temporary water, permanent wetness and temporary wetness), a water and wetness probability index and water bodies’ areas covered by inland water along the year. Copernicus at this stage does not provide regular information on rivers dynamics and especially on river runoff and overall hydrological dynamics, nor on under water systems. NG-TC should help to leverage substantially this gap within the Copernicus services. Moreover, the portfolio of Copernicus for hydrology and water is new and the user community is not yet fully identified. Major recognised users are the river basin authorities and agencies, regional and local authorities in charge of drinkable water, wastewaters, agencies in charge of agriculture, emergency services and hydro-meteorological administrations. It also includes private actors of the hydro-energy sector or the water transport. Communities working along the coastline where inland waters connect with the seas are also relevant. At global level, water Essential Climate Variables are produced to report to the International Global Climate Observing System program (GCOS). Considering GCOS requirements from https://gcos.wmo.int/en/essential-climate-variables/rivers/ecv-requirements Water Surface Elevation (WSE) Level is requested at ≤3 cm for Lakes, and ≤10 cm for rivers with trends of WSE levels ≤10 mm/decade. Snow cover has a strong influence on the Earth's radiation and energy balance. Changes in snow extent tend to amplify climate fluctuations. This phenomenon needs to be well identified for the prediction of water balance, streamflow and river runoff in hydrological models used for water resource management, climate modelling and arctic/sub-arctic area monitoring. The key cryosphere parameters monitored within Copernicus are the area snow extent, snow water equivalent and lake ice extent. Satellite radar altimetry currently provides an important source of Water Surface elevation (WSE) observations at ‘virtual stations’ where the altimeter ground track crosses the river centre line (Créteaux et al, 2017, Tarpanelli et al. 2020). The Sentinel3 and Sentinel-6 missions provide, as a secondary mission objective, WSE estimates for a variety of targets and the use of such type of satellite measurements is growing 6 https://www.worldwildlife.org/pages/global-lakes-and-wetlands-database 7 Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, OJ L 327, 22.12.2000, p. 1-73. ESA UNCLASSIFIED – For ESA Official Use Only Page 57/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 fast within the hydrology community. Sentinel-6 currently accommodates 31,805 targets of which 8,655 define rivers, 21,666 define lakes, 1,484 define reservoirs with no targets defined yet for glaciers. Sentinel-3A and B satellites include ~12900 virtual stations as of March 24th, 2021 (see also www.altimetry-hydro.eu). Sentinel-3 can be used to provide spatiotemporal characterization of wetlands floodplains and their connectivity to rivers and clear seasonal patterns can be seen where the satellite ground tracks cross these areas (e.g. Kittel et al., 2021). Altimeter WSE data have also been used to calibrate and refine hydrological models (e.g., Domeneghetti et al., 2014; Dubey et al., 2015; Finsen et al., 2014; Schneider et al., 2018). Although not straightforward, rating curves establish a functional law between altimetric water height and in situ discharge observations for each river virtual station (e.g. Tarpanelli et al. 2020). Rating curves rely on knowledge of simultaneous discharge and water elevation. Rating curve might not be stable in time due to the impact of high-flow flood conditions modifying the channel form. Furthermore, in situ measurements from a river gauge are rarely located at the same location as a satellite altimeter virtual station, requiring careful consideration of the different conditions at the virtual station and the physical station. Satellite altimetry and WSE processing techniques have steadily evolved over the last 25 years to provide a technique that is capable of resolving the height of river and lake WSE above a reference datum to an accuracy of multi decimetres at best over rivers, uncertainty could be much lower over very big lakes (e.g. Frappart et al., 2008; Medina et al., 2008; Biancamaria et al., 2017; Tarpanelli et all., 2020; Vu et al., 2018; Villadsen et al., 2016). This has important implications for successful monitoring of wetlands and floodplains with small fluctuations in WSE level (e.g., Dettmering et al., 2016). Jiang et al. (2020) evaluate a number of nadir altimeter retrackers for WSE using in situ measurements and for 26 Virtual Stations in plain areas achieved a root mean square error (RMSE) ranging from 0.12 m to 0.9 m. For 24 other virtual stations, the RMSE was over 1 m. In a following paper, Jiang et al. (2023) showed that Sentinel-3 Near real Time (NRT) products provide similar RMSE than Non Time Critical (NTC) products, when compared to in situ data at 25 locations. A recent literature review over 20182022 concerning altimetry over inland has been performed by Kossieris et al. (2024). They find that Sentinel-3 was used in almost half of the reviewed papers and is therefore the most used altimeter mission in the articles published between 2018-2022. Median RMSE between Sentinel-3 and in situ data on WSE ranges between a little bit more than 0.1 m to 1.4m. For some lakes, RMSE lower than 0.1m can even been obtained. This RMSE are provided for WSE anomalies (i.e. the temporal mean is removed from in situ and Sentinel-3 WSE time series), as in situ gages and Sentinel-3 tracks are not colocalized. The ability to measure river WSE depends on river width, the river reach orientation to the satellite ground track, the surrounding topographic relief, land cover (and especially the number of other water targets near the river), and features of the satellite altimeter. These aspects complicate received nadir-altimeter waveform shapes (multiple peaks) and the impact of snagging on off nadir targets. Another approach that could be applied is the fully focused SAR, which can obtain an along-track resolution of 0.5 m (Egido and Smith, 2017). This smaller footprint could differentiate different targets along-track but still be subject to across-track heterogeneous surfaces. Technically, this issue can be potentially solved by acrosstrack interferometry if future missions can operate in SARIn mode. This has been ESA UNCLASSIFIED – For ESA Official Use Only Page 64/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 • it is compliant with the resolution of regional/global hydrology models, which need observation for calibration, validation, assessing the missing physical processes and even for assimilation, • many big basins (Amazon, Congo, Niger, Ganges/Brahmaputra, Mekong, etc.) are transboundary basins, where downstream countries do not have any information on the state of the resources upstream and therefore have fewer possibilities for forecasting floods or droughts, • reservoirs (and even lakes) that have the most influence on river networks and associated populations living in the watershed have volume change dynamics in line with these sampling characteristics, • it will provide key components for studying water budget at the watershed scale and/or at global scale over long time periods over a decade or even longer including the time series of past, present and future altimetry missions. By addressing these aspects in the S3NG-T, Copernicus will be able to deliver new hydrology information to address European Policy and directives as requested in the SWD (2019). 3.4 S3NG-T and Copernicus Cryosphere Monitoring and Prediction The Earth’s cryosphere plays a critical role in our planet’s radiation and sea level budgets, and several Essential Climate Variables (ECVs) are associated to the cryosphere (https://gcos.wmo.int/en/publications/gcos-implementation-plan2022). Loss of Arctic sea ice is exacerbating planetary warming owing to the ice-albedo feedback (e.g. Budyko, 1969; Serreze and Francis, 2006; Screen and Simmonds 2010), and loss of land ice is the principal source of global sea level rise (Chen et al. 2013; Horwath et al; 2022). Moreover, the high latitude seas provide fundamental ecosystem services (including fisheries management and other resources), sustains numerous indigenous communities, and due to sea-ice loss are emerging as a key area for economic exploitation. But the fragile ecosystems are subject to pressures from a growing number of maritime and commercial activities. Sentinel-3 reaches 81.5° N and S of the equator covering sea ice and ice sheet regions. The following user needs stem from the need to maintain continuity of the current Sentinel-3 mission capability. • Maintain ocean topography measurements from the open ocean and into the polar seas. This will contribute to the observation system for global observation of mean sea level, mesoscale and sub-mesoscale currents, wind speed and significant wave height in the Polar regions. These measurements are fundamental inputs to operational oceanography and marine forecasting services in the polar oceans. • To support applications related to coastal and inland waters. Observation of water level at Arctic coasts as well as of rivers and lakes is a key quantity in hydrological research. Rivers and lakes not only supply freshwater for human use including agriculture but also maintain natural processes and ecosystems. The monitoring of global river discharge and its long-term trend contributes to the evaluation of global freshwater flux, which is critical for understanding the ESA UNCLASSIFIED – For ESA Official Use Only Page 65/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 mechanisms of global climate change. Moreover, the frozen rivers and lakes are important navigation routes in the Arctic regions, which encountered dramatic changes in the context of global warming. Their observation could help forecasting their evolutions and organizing alternative modes of transport. • In synergy with other satellite missions, measure and monitor variability of Arctic and Southern Ocean sea-ice thickness. Seasonal sea ice cycles are important for both human activities and biological habitats. The seasonal to inter-annual variability of sea ice is a sensitive climate indicator; it is also essential for long term planning of any kind of activity in the Polar Regions. Knowledge of snow depth over sea ice is necessary to derive improved accuracy in measurements of sea-ice thickness and is also a valuable input for coupled atmosphere-ice-ocean forecast models. On shorter timescales, measurements of sea-ice thickness and information about Arctic Ocean sea state are also essential support to maritime operations and ship routing over polar oceans. • In synergy with other satellite missions, measure and monitor the surface elevation and elevation changes of polar glaciers, ice caps and the Antarctic and Greenland ice sheets. The ice sheets of Antarctica and Greenland store a significant proportion of global freshwater volume and are important for climate change, the water cycle and contributions to sea level. Monitoring grounding line migration and elevation changes of floating and grounded ice sheet margins is important to identify and track emerging instabilities, which can negatively impact the stability of the ice sheets, leading to ice mass loss and ultimately result in accelerated future sea-level rise. The Arctic Ocean is changing dramatically responding to significant global atmospheric warming by pan-Arctic sea-ice retreat and thinning (e.g. Meier et al., 2014; Werner et al., 2016). The rise in Arctic near-surface air temperatures has been more than twice as large as the global average in recent decade (e.g. Serreze and Francis, 2006) that is called ‘Arctic amplification’ (Kellog, 1975) although the underlying causes of Arctic amplification remain uncertain (e.g. Screen and Simmonds, 2010). Arctic amplification is the outcome of many complex and interrelated feedback mechanisms and processes (e.g. longwave radiation flux changes caused by e.g. changes in cloud type and cover, ice albedo feedbacks, changes in heat fluxes between the atmosphere and ocean due to changes in sea ice extent). It is expected that the Artic Amplification will become stronger in the near future (ref: https://doi.org/10.1016/j.gloplacha.2011.03.004). Measurements of geophysical and societal change provide the evidence to underpin the establishment, implementation and monitoring of policy, policy decisions and their impact, not just in Europe, but across the world. In the Arctic region, several extreme concerns have been recently raised by the International Panel for Climate Change (IPCC, 2018): • Warming greater than the global annual average is being experienced in many land regions and seasons, including two to three times higher in the Arctic with warming generally higher over land than over the ocean. • Climate related risks for natural and human systems are higher for global warming of 1.5°C than at present. These risks depend on the magnitude and rate of warming, geographic location, levels of development and vulnerability, and on the choices and implementation of adaptation and mitigation options. ESA UNCLASSIFIED – For ESA Official Use Only Page 66/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 • There is high confidence that the probability of a sea-ice-free Arctic Ocean during summer is substantially lower at global warming of 1.5°C when compared to 2°C. With 1.5°C of global warming, one sea ice-free Arctic summer is projected per century. This likelihood is increased to at least one per decade with 2°C global warming. Effects of a temperature overshoot are reversible for Arctic sea ice cover on decadal time scales. • Populations at disproportionately higher risk of adverse consequences of global warming of 1.5°C and beyond include disadvantaged and vulnerable populations, some indigenous peoples, and local communities dependent on agricultural or coastal livelihoods. Regions at disproportionately higher risk include Arctic ecosystems, dryland regions, small-island developing states, and least developed countries. Poverty and disadvantages are expected to increase in some populations as global warming increases; limiting global warming to 1.5°C, compared with 2°C, could reduce the number of people both exposed to climate-related risks and susceptible to poverty by up to several hundred million by 2050. • Limiting global warming to 1.5°C compared to 2ºC is projected to reduce increases in ocean temperature as well as associated increases in ocean acidity and decreases in ocean oxygen levels. Consequently, limiting global warming to 1.5°C is projected to reduce risks to marine biodiversity, fisheries, and ecosystems, and their functions and services to humans, as illustrated by recent changes to Arctic sea ice and warm water coral reef ecosystems. These changes may lead to dramatic consequences as discussed by Stephen (2018) who describes the societal impacts of a rapidly changing Arctic. Climate change and globalisation are the dominant drivers of societal impacts in the Arctic. As the climate changes, access to the Arctic is improving and, through globalisation and new economic development, a rapid transformation of the environmental and geo-political environment of the region is in progress. While there is a strong desire for sustainable development of the fragile Arctic environment at both the national and international level, significant societal impacts are inevitable. The societal impacts of a rapidly changing Arctic are thus complex, uncertain and ambiguous. In response, the European Commission and the High Representative of the Union for Foreign Affairs and Security Policy issued to the European Parliament and the Council, on 27 April 2016, a joint communication that proposed "An integrated European Union policy for the Arctic". The communication highlights the strategic, environmental and socio-economic importance of the Arctic region, including the Arctic Ocean and adjacent seas. The Arctic’s fragile environment is also a direct and key indicator of the climate change, which requires specific mitigation and adaptation actions, as agreed at the COP-21 held in Paris in December 2015. To this end, the "integrated EU Arctic policy" has identified and is addressing three priority areas: 1. Climate Change and Safeguarding the Arctic Environment (livelihoods of indigenous peoples, Arctic environment). 2. Sustainable Development in and around the Arctic (exploitation of natural resources e.g. fish, minerals, oil and gas), “Blue economy”, safe and reliable navigation (e.g. the Arctic Northern Sea Route). ESA UNCLASSIFIED – For ESA Official Use Only Page 67/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 3. International Cooperation on Arctic Issues (scientific research, EU and bilateral cooperation projects, fisheries management/ecosystems protection, commercial fishing). Continuously monitoring the vast and harsh Arctic environment in such a changing world with Earth observation, navigation and communication satellites (considering the sparse population and the lack of transport links) is considered essential to the successful implementation and effective management of the Integrated EU Arctic Policy. The existing Copernicus programme already offers operational thematic services in the fields of atmosphere monitoring, marine environment monitoring, land monitoring, climate change, emergency management and security. For example, the CMEMS Arctic – Monitoring Forecasting Centre (ARC MFC) provides accurate forecast and reanalysis products for sea ice, ocean, biology and surface waves in the whole Arctic. The system is based on a numerical ocean model assimilating in situ and satellite data. The Copernicus Atmospheric Service (CAMS) provides information products about atmospheric composition and solar radiation. Several products are of interest for the Arctic region including: monitoring and assessing the impact of emissions from fires at high latitudes (Canada, Siberia) and transport of the corresponding plumes of gases and aerosol affecting atmospheric composition in the Arctic region and monitoring and forecasting of the ozone layer, including Arctic “mini-holes” events. The Copernicus Climate Change Service (C3S) is developing new approaches to provide highresolution regional climate-quality reanalysis over the Arctic and production of sea-ice and ice sheet Essential Climate Variables. In addition, Economic Sectoral analyses of Arctic shipping addresses the impact of climate change on ship routing issues. A long-term programme to monitor the Earth’s polar ice, ocean and snow topography is important to both operational and scientific communities with interests in the Arctic and Antarctic. Europe has a direct interest in the Arctic due to its proximity. Changes in the Arctic environment affect strategic areas including politics, economics (e.g. exploitation of natural resources including minerals, oil and gas, fish) and security. It also has an indirect interest in the Antarctic due to the Antarctic Treaty, which permits international access in support of science. Besides economic impacts of Antarctic and Arctic changes (Whiteman et al, 2013), Europe’s interest in both Polar Regions is due to their influence on patterns and variability in global climate change, thermohaline circulation and the planetary energy balance. Last but not least, changes in the Arctic system have potential impacts on European weather, with consequences for extreme events (Francis, 2017). "An integrated European Union policy for the Arctic" (https://ec.europa.eu/environment/efe/news/integrated-eu-policy-arctic-2016-12-08_en) emphasises the strategic, environmental and socio-economic importance of the Arctic region, including the Arctic Ocean and adjacent seas. Continuously monitoring the vast and inhospitable Arctic environment with satellites (considering the sparse population and the lack of transport links) is considered essential. Following this, several guiding documents have been prepared in a European Commission-led user consultation process: Polar Expert Group (PEG) User Requirements for a Copernicus Polar Mission Phase-I report (12th June 2017), Duchossois et al. (2018a), hereafter referred to as PEG-1 report, and the Phase 2 report on Users’ requirements (31st July 2017), Duchossois et al. (2018b), hereafter referred to as PEG-2 report. ESA UNCLASSIFIED – For ESA Official Use Only Page 68/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 3.4.1 S3NG-T and Copernicus Sea Ice Monitoring and Prediction Floating ice parameters are listed as the top priority for the polar mission user requirements by a collective of polar experts (Duchossois et al. 2018a,b). These parameters are selected considering the availability of existing Copernicus mission products and services of direct relevance to the Artic as well as their needs for improvement (e.g. in terms of spatial resolution, accuracy, repeat coverage, length of time series, etc) and the current level of technical and/or scientific maturity for some candidate parameters. Sea ice represents a group of variables, notably: concentration, area and extent, fast ice, thin, firstand multiyear ice, floe size, motion/drift, deformation, age, thickness, volume, snow cover/depth, freeze-up/melt time and melt pond coverage. Sea ice is thus a name for a number of variables. A critical aspect is the role of ocean waves that modify the sea ice parameters (concentration, fast ice, floe size, motion/drift, deformation, thickness, melt/freeze times). Other floating ice quantities include iceberg locations, drift and volume change as well as ice shelf thickness and extent. Sea ice parameters from CMEMS are widely used for navigation, maritime security, coastal and marine environment, marine resources and weather and climate applications. With retreating sea ice due to global warming, we expect ice free conditions over vast areas of the Arctic Ocean. This suggests that we are no longer in a fetch limited Arctic Ocean and we will likely encounter more fully developed sea conditions. This has a huge impact for navigation and maritime safety and on the freeze/thaw regime in sea ice thickness. We can see a variety of shipping routes that will take advantage of open water conditions using routes that may even traverse the Arctic Ocean directly if sea ice conditions are thin enough. Climate models suggest that this is something to be considered in the 2030-2050 timeframe [Kim, et al., 2023]. As a secondary mission objective, Sentinel-3 provides measurements over sea ice derived from SRAL measurements along the ground track in SAR mode. Estimating Arctic-wide sea ice thickness on monthly time-scales was made routinely possible (e.g. Laxon et al. 2003, Tilling et al., 2018) with the advent of high-latitude satellite altimeters using altimeter range measurements to estimate sea ice freeboard (the height of the sea ice surface relative to local sea level). ESA UNCLASSIFIED – For ESA Official Use Only Page 69/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Figure 3.4.1-1 Typical elevation probability density function and spatial scales of sea ice and leads. (b) Side-profile view of sea ice (vertically exaggerated) showing the snow and sea ice layers, freeboard and thickness relations, and typical bulk densities. (from Armitage and Kwok, 2019). Under the assumption that a sea ice floe is in hydrostatic equilibrium with the surrounding ocean, and assuming bulk values for the densities of the ice and ocean, and the density and thickness of the snow cover, freeboard can be converted to sea ice thickness. Lawrence et al (2019) derive radar freeboards from the Sentinel-3 and CryoSat-2 satellites are therefore expected to be consistent, and will provide essential continuity of the high-latitude radar altimeter time series into the 2030s. Based on 6months of gridded data (Nov 2017 – April 2018), Lawrence et al (2021) report a mean radar freeboard for Sentinel-3A of 0.1 ±0.015 m. Mean freeboard differences between Sentinel-3A and CryoSat-2 are stable with monthly values between 0.9 cm and 1.1 cm ±0.07 cm. The standard deviation on the mean difference, which reflects the spatial variability of Sentinel-3A and CryoSat-2 differences is also stable across all months at 6.4 ±0.4 cm. The grid specifications are 1.5° longitude by 0.5° latitude grid, corresponding to grid cell dimensions of approximately 80 - 55 km at 60° latitude and 30 - 55 km at 81° latitude. A 2 cm intermission bias on sea-level anomaly, and a 1 cm offset between S3A and CS2 freeboard is attributed to different re-tracker configurations necessary for S3A when retracking leads and floes, Lawrence et al (2019). The increase in sampling resolution afforded by the combination of CryoSat-2, Sentinel-3A and Sentinel-3B data using orbit ground track files averaged over 30 days of data, shows significant improvements when using data from all three satellites. For a grid of resolution 1.5° longitude x 0.5° latitude, the addition of S3A and S3B data to the existing CS2 dataset can reduce the uncertainty on monthly gridded radar freeboard by a quarter on average (Lawrence et al., 2019). Sentinel-3 allows observations up to 81.5° latitude at ten-day frequency (only achievable with a month of CS2 data at the same resolution) allowing sub-synoptic changes both in sea ice volume evolution and ESA UNCLASSIFIED – For ESA Official Use Only Page 70/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 ocean circulation to be resolved. Vandemark et al. (2004) model the relationship between low-incidence Ka-band Sigma0 measurements to wind speed (via its relation to the ocean mean surface capillary wave slope variance), modified for low wind speeds to allow specular reflection. See Armitage and Ridout (2015) for details of the low-level Ka-band AltiKa processing over sea ice and Armitage and Kwok (2019) and Kacimi et al (2025) for an assessment of swath altimeter capability and performance expectations of over sea ice. InSAR swath altimeter retrievals in the ice-covered ocean share more similarities with retrievals over inland river/lake surfaces than the open ocean: namely, mixed surface types within the swath (sea ice floes/leads vs. vegetation/river), mixed scattering behaviours from the different surfaces (bright and specular leads/rivers vs. lower backscatter sea ice floes/vegetation), and variable geometry and size of the different surfaces (see Armitage and Kwok, 2019). This means that, as for Hydrology Targets, either a performant on-board processing scheme is used or raw radar measurements with minimal on-board processing are available on-ground for further processing to retrieve sea ice parameters. 3.4.2 S3NG-T and Copernicus Ice Sheet Monitoring and Prediction Sentinel-3 acquires data to support a range of secondary objective glaciological applications including monitoring ice sheet elevation and elevation change. Ice sheet parameters are listed in the requirements by a collective of polar experts (Duchossois et al. 2018a,b). These parameters are selected considering the availability of existing Copernicus mission products and services of direct relevance to the Polar Regions as well as their needs for improvement (e.g. in terms of spatial resolution, accuracy, repeat coverage, length of time series, etc) and the current level of technical and/or scientific maturity for some candidate parameters. Complex surface topography over ice sheets (especially margins) complicates the echo return which diverges from its classical shape (Ray et al., 2015). This difference can range from a slight distortion of the theoretical waveform shape, to multiple superimposed reflections from distinct surfaces within the doppler beam footprint. Working with complex waveforms is one of the major challenges associated with processing radar data over regions of complex topography. Table 3.4.2-1 Noise level and slope-related component (in degrees) of the measurement precision (from Schröder et al., 2019) ESA UNCLASSIFIED – For ESA Official Use Only Page 71/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Radar altimetry measurements over the ice sheets need to be adjusted for the effects of the ice sheet surface slope, which typically ranges from 0.1° to 1.5° in Antarctica, introducing a 1.4 to 20.9 km lateral shift in the point of closest approach (Brenner et al., 1983; Levinsen et al., 2016; Remy et al., 1989, , https://doi.org/10.1016/j.rse.2006.02.026, https://doi.org/10.5194/tc-16-2225-2022) or, equivalently, a 1.2 to 274.2 m error in the estimated elevation if the measurement was assumed to be originating from nadir. Schröder et al., 2019 studied the measurement precision of Seasat, Geosat, ERS-1/2, Envisat and CryoSat-2 data over Antarctica by evaluating intra-mission crossovers between ascending and descending profiles, with a time difference of less than 31 days. They find that the precision is clearly a function of surface slope (Table 3.4.2-1). McMillan et al (2019) show that over low-slope regions of the ice sheet interior, Sentinel-3 SRAL achieves both an accuracy and a precision of ~10 cm, with ~98% of the data validated being within 50 cm of co-located airborne measurements. Across the steeper and more complex topography of the ice sheet margin, the accuracy decreases, although analysis at two coastal sites with densely surveyed airborne campaigns shows that ~60 %–85%of validated data are still within 1 m of co-located airborne elevation measurements. Figure 3.4.2-1 shows the single cycle crossover differences as a function of slope for Sentinel-3 cycle 12 (McMillan et al, 2019). Figure 3.4.2-1: Elevation differences at orbital cross-overs for cycle 12 of the Sentinel-3A mission and comparison to the gradient of the surface slope. McMillan et al., 2019 ESA UNCLASSIFIED – For ESA Official Use Only Page 72/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Through analysis in three regions of the Antarctic Ice Sheet, McMillan et al (2019) demonstrate the early promise of Sentinel-3 SAR altimetry as a platform for long-term, operational monitoring of ice sheets elevation and elevation change. Sentinel-3A delayDoppler retrievals mapping ice sheet elevation change resolve known signals of ice dynamic imbalance and to detect evidence of subglacial lake drainage activity. A recent study confirms the value of Sentinel-3 data to extent the long time series of ice sheet volume and mass change estimates (Simonsen et al., 2021). McMillan et al (2020) note that the quartet of Sentinel-3 satellite altimeters will provide a continuous record of ice sheet elevation change. To ensure consistency of measurement between each of the four satellites, rigorous in-flight intercomparison is required. To facilitate this, Sentinel-3B was initially flown in a unique tandem formation with Sentinel-3A, enabling near instantaneous, co-located measurements to be acquired. evolution. They find that (1) there is no significant difference between S3A and S3B instrument precision, (2) that there is no significant difference between the accuracy of S3A and S3B elevation measurements, and (3) that both instruments resolve near-identical echoes of the ice sheet surface, even over complex, non-linear coastal terrain (both instruments operate with statistically equivalent accuracy and precision, even over complex ice margin terrain). Both Sentinel-3A and Sentinel-3B satellites can be used interchangeably to monitor ongoing ice sheet evolution; effectively doubling the spatial coverage of measurements available, now that Sentinel3B has moved to its nominal orbit. Satellite altimetry is a the key dataset used to monitor the elevation changes of the ice sheets. Users who aim at constructing longer time series of elevation changes require information on the waveform parameters. Especially in the Arctic, the temperature has been increasing (Serreze & Barry, 2011 ) and extreme melt event are becoming more frequent (ref). It is essential to know how the surface characteristics are changing over time and how they might affect the height retrieval from the retracker. The occurrence of surface melt is known to complicate the interpretation of elevation and elevation change, and changes in waveform characteristics can be linked to changes in the penetration of the radar signal into snow (Khvorostovsky, 2012; Slater et al., 2019; Rutishauser et al., 2023). Therefore, it is essential that waveform parameters such as the leading edge width (LeW), trailing edge slope (TeS) and especially Sigma0 are provided to the user. And it is important that the method used to derive those parameters are explicitly described in the documentation. “closed loop” and “open-loop” tracking. 3.5 S3NG-T and Copernicus Climate Monitoring and Prediction As the ocean warms it expands and sea level rises. This rise is further increased by the melting of ice on land, which then flows into the sea. Sea level has increased throughout the altimeter record, but recently sea level has risen at a higher rate due partly to increased melting of ice sheets in Greenland and Antarctica. In 2019, the global mean sea level reached its highest value since the beginning of the highprecision altimetry record (January 1993). ESA UNCLASSIFIED – For ESA Official Use Only Page 73/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 As noted by the WCRP (2017) “Coastal sea level rise is among the most severe societal consequences of anthropogenic climate change. Contemporary global mean sea level rise will continue over many centuries because of anthropogenic climate warming, with the detailed pace and final amount of rise depending substantially on future greenhouse gas emissions”. Over the coming decades, regional sea level changes and variability will significantly deviate from global mean values. The detailed sea level change along coastlines can therefore potentially be far more substantial than the global mean rise and will depend on many processes involving the ocean, the atmosphere, the geosphere and the cryosphere. Societal concerns about sea level rise originate from the potential impact of regional and coastal sea level change and associated changes in extremes on coastlines around the world, including potential shoreline recession, loss of coastal infrastructure, natural resources and biodiversity, and in the worst case, displacement of communities and migration of environmental refugees. Local sea level rise and extreme events can have significant impacts on coastal zones. On subsiding coasts, the impacts of resulting sea level rise are already demonstrable in some coastal cities and deltas. However, there is a lack of evidence to attribute rising climate-induced sea level to coastal impacts (Cramer, W., et al., 2014, Section 3.3). By the end of the 21st century, it is very likely that a large fraction of the world’s coasts will be affected by climate-induced sea level rise (Church et al., 2013). WCRP (2017): WCRP Grand Challenge: Regional Sea Level Change and Coastal Impacts Science and Implementation Plan Detlef Stammer, Robert Nichols, and Roderik van de Wal (co-chairs) and The GC Sea Level Steering Team Version 1.0 February 2017 From the WCRP plan: Available coastal sea level data can be used to assess spatial structure of variability, although resolution is limited. Until systems like SWOT are flown, developments in coastal along-track altimetry retrievals (e.g., ESA Sea Level CCI efforts, Cazenave et al., 2022) present the best prospects. The production of long-term consistent quasi-global records of sea level change and its components is possible only through high-quality continuous observations and proper uncertainties attached. To ensure this continuity, the core instruments of the current observing systems consisting of nadir satellite altimetry (TOPEX/Poseidon, Jason-1 -2 and -3, Sentinel-3A –3B and 6MF), satellite gravimetry (GRACE and GRACE-FO), ocean autonomous temperature and salinity profilers (e.g., Argo network) and tide gauges, etc., need to be maintained and replaced before their end of life. To ensure the long-term stability and accuracy, intercalibration of new core instruments against older ones needs to be taken into account in instrumental design. For observations based on multiinstruments such as satellite altimetry, the continuity of a highly accurate and stable reference mission such as the Topex/Jason series is essential to calibrate less accurate missions and achieve higher spatio-temporal resolutions. Better sea level records close to the coast would involve improvements in 1) the geophysical corrections (in particular: the wet tropospheric correction using ESA UNCLASSIFIED – For ESA Official Use Only Page 80/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 highest possible resolution, but do not specify a requirement for the MSS or the dynamic topography field or the marine gravity field. SWOT data allows mapping 2D MSS slopes, revealing 2D features down to 8 km in wavelength, (Yu et al., 2024), not visible with 30 years of past 1D conventional altimetry. SWOT's MSS will be a starting basis for S3NG-T; the longer time series of 2D observations should better isolate the ocean and geoid components of the MSS. 3.7.2 Sea level rise The present day estimated sea level rise is according to R.S. Nerem at the university of Colorado 3.3±0.3 mm/yr with an acceleration, estimated by different groups to be of 0.083±0.025 mm/yr2, (Nerem et al., 2018), or 0.11±0.05 mm/yr2, (Guerou et al., 2023). This notable disparity between the two estimates of sea level rise acceleration stems from challenges in interpreting TOPEX data and the significant uncertainties associated with it. The long-term user requirement is 0.1 mm/yr ([5,95]% confidence level) over a decade according to (Meyssignac et al., 2023). Measuring sea level rise with a satellite altimeter puts significant constraints on the stability of the altimeter instrument including the required corrections for refraction of the radar signal and the accuracy of orbit determination which is intrinsically coupled to maintenance of the geodetic reference system. A recommended approach is to rely on SLR, DORIS, and GNSS, since maintenance of an International Terrestrial Reference System: ITRF, see also (Altamimi et al, 2016) is essential if we want to provide continuity on observing sea level rise consistently over several decades. The International Laser Ranging Service (ILRS) mechanism is necessary to tie different networks together. International Earth Rotation and Reference Systems Service (IERS) activities (global time and reference frame standards, Earth Orientation Parameter and International Celestial Reference System standards) are essential. A long-term stable microwave radiometer is required for the wet tropospheric correction (e.g. AMR-C on Sentinel-6) and a dual frequency altimeter is required for reducing the ionospheric path delay along the altimeter to sea surface. To understand the sea level budget ( ΔVolume (altimetry) = ΔMass (gravimetry) + ΔDensity (T/S profiles) we need external information on the monthly temporal gravity field, and an ARGO system of oceanic profiling floats. Both are required to understand the heat and mass terms that make up sea level rise as we see it from the altimeter instrument. Of fundamental importance is also a tide gauge network – this is a reference dataset and it must be maintained, finally we need an “Agreement” on a glacial isostatic adjustment models (currently there is no agreed reference GIA model) 3.7.3 Tides A next generation topography constellation offers an opportunity to independently observe ocean tides. The white paper from (Arbic et al 2015) gives information on barotropic and internal tides. For barotropic tides, the M2 rms is 5 to 7 mm between 66N and 66S for deep ocean tides. On the European shelf tides are more like 3 to 5 cm ESA UNCLASSIFIED – For ESA Official Use Only Page 81/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 rms, but individual difference can be larger (e.g. around the UK coast). Arctic tides are noisier: 4-6 cm rms, Antarctic tides: 3-4 cm rms. Altimetry data is assimilated in various global models (FES, GOT, TPXO, EOT) which all meet high accuracy; the models can be confronted with network of independent deep-sea tide gauges. The recent SWOT swath observations on a non sun-synchronous orbit are already providing improved barotropic tides in coastal, regional and high-latitude regions (Hart-Davis et al., 2024), and global improvements are expected after many years of SWOT observations. For internal tides the surface features are smaller but can reach20 cm. near bathymetric gradients. Global models forced by the atmosphere and tides could be efficient for improving corrections for the non-phaselocked internal tides (e.g. HYCOM model, Shriver et al, 2013). While the GRACE mission requirements are stringent, inaccuracies in tide models will show up in the residuals but this can be largely resolved over time (hot spots are already improved). Tides beneath ice shelves beneath flexing ice shelves is a different topic, for details see (Padman et al, 2008). It should be remarked that the bathymetry is a big concern for coastal regions ocean tides. Cancet et al. (2018, 2020) show great improvements in areas where we have good bathymetry ESA UNCLASSIFIED – For ESA Official Use Only Page 82/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 4 S3NG-T MISSION AIMS AND OBJECTIVES 4.1 S3NG-T Mission Aim Considering the User needs expressed by the European Commission and concisely articulated in the previous sections, the aim of the Copernicus Next Generation Sentinel-3 Topography (S3NG-T) Mission is: To ensure continuity of Sentinel-3 in flight performance topography capability in the 2030-2050 timeframe. 4.2 S3NG-T Objectives Mission requirements are then derived from mission Objectives. The primary objectives of the S3NG-T mission are to: PRI-OBJ-1. Guarantee continuity of Sentinel-3 topography measurements8 for the period 2030 to 2050 with performance at least equivalent to Sentinel-3 in-flight performance as defined in Table 2.4-1 (‘baseline mission’). PRI-OBJ-2. Respond to evolving user requirements and improve sampling, coverage and revisit of the Copernicus Next Generation Topography Constellation (S3NG-T and Sentinel-6NG) to ≤50 km and ≤5 days (CMEMS, 2017) in support of Copernicus User Needs. PRI-OBJ-3. Enhance sampling coverage, revisit and performance for Hydrology Water Surface Elevation measurements in support of Copernicus Services. PRI-OBJ-4. Respond to evolving user requirements and enhance topography Level 2 product measurement performance. The secondary objectives9 of the S3NG-T mission are to: SEC-OBJ-1. Provide directional wave spectrum products that address evolving Copernicus user needs. SEC-OBJ-2. Provide new products10 that address evolving Copernicus user needs. 8 See definition of Topography Parameters. 9 Secondary objectives shall not drive the system design. Examples of new products would include directional wave spectrum and topography gradients. 10 Examples of new products with respect to those provided by Sentinel-3 today include sea surface height gradients and river reach averaged gradients, river width and water area. Total Surface Current Velocity measurements are considered out of scope by the European Commission. ESA UNCLASSIFIED – For ESA Official Use Only Page 83/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 5 S3NG-T MISSION REQUIREMENTS MRD-0010 Requirement deleted. MRD-0020 Requirement deleted. 5.1 Definition of Measurement Masks 5.1.1 Marine and Coastal Mask In the coastal zone and over rapidly changing atmospheric moisture structures including cloud structures, precipitation and land-sea breeze (amongst other phenomena) lead to more complex atmospheric temperature, pressure and moisture structures that are significantly different from open ocean conditions. Their influence may persist for several hundred kilometers offshore. Furthermore, due to the input of freshwater from rivers and estuaries, bio-geo-chemical compounds and pollutants are more ubiquitous in this region. Natural compounds from biological decay form long-chain organic surfactants on the ocean surface in calm conditions that, together with pollutants on the ocean surface, inhibiting the growth of capillary waves (to which the radar altimeter responds) and modifying the surface emissivity (to which the microwave radiometer responds). Furthermore, these physical, bio-geo-chemical and atmospheric aspects and characteristics present a very complex sea surface topography structure and lead to specific measurement challenges compared to the open ocean. Different Level 2 data processing techniques and algorithms are used in different regions (e.g. the open ocean, more complex coastal regions, within estuaries, etc.) and data are across a transition region that includes both land and water. A clear definition of the actual coastline and the transition zone is required. This is the purpose of the Marine and Coastal Mask. Level 1 products are anticipated as continuous half-orbit or full orbit products regardless of the surface type. Global Self-consistent, Hierarchical, High-resolution Geography Database (GSHHG, Wessel and Smith, 1996) is a high-resolution geography data set, amalgamated from two databases: World Vector Shorelines (WVS) and CIA World Data Bank II (WDBII). The former is the basis for shorelines while the latter is the basis for lakes. The WDBII source also provides political borders and rivers. GSHHG data have undergone extensive processing and should be free of internal inconsistencies such as erratic points and crossing segments. The shorelines are constructed entirely from hierarchically arranged closed polygons. GSHHG combines the older GSHHS shoreline database with WDBII rivers and borders, available in either ESRI shapefile format or in a native binary format. Geography data are provided in five resolutions: crude(c), low(l), intermediate(i), high(h), and full(f). Shorelines are organized into four levels: boundary between land and ocean (L1), boundary between lake and land (L2), boundary between island-in-lake and lake (L3), and boundary between pond-in-island and island (L4). Datasets are in WGS84 geographic (simple latitudes and longitudes; decimal degrees). The full Resolution data set is provided at a resolution of 0.04 km. ESA UNCLASSIFIED – For ESA Official Use Only Page 84/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0030 The S3NG-T Marine and Coastal Mask (MCM) shall be established according to the following specification: § The Coastal Region is defined as 80 km on the ocean side and 20 km on the land side from the Shoreline Boundary (i.e. the Coastal Region is nominally 100 km in extent). § All inland seas (including large lakes). § Shoreline boundary and definition of estuaries with an estuary entrance of ≥ 2 km for rivers articulated in the Inland Water Mask. § Open Ocean defined as all areas of open ocean water, but not part of the Coastal Region, Inland Seas and Estuaries. § Polar Oceans including areas seasonally occupied by sea ice. Note 1: The Shoreline Boundary separating land from ocean is defined by the Hierarchical, full-resolution (0.04 km) Shoreline (GSHHS) high-resolution dataset (Wessel and Smith, 1996) available from https://www.ngdc.noaa.gov/mgg/shorelines/gshhs.html). Note 2: This mask is used to define the coastline and as input to different Level 2 processing schemes within the ground segment. Note 3: Level 1 products are anticipated as continuous half-orbit or full orbit products regardless of the surface type but will depend on the ground segment concept. Note 4: Small islands were a challenge in previous masks for Sentinel-3 and these can be addressed using other satellite data (e.g. Sentinel-2) to validate or improve on the coastal mask definition as required. Note 5: S3NG-T-UN-022 requires a definition of the coastal zone as 24 nautical miles (44,448 km) offshore. A definition of 80 km allows a margin for an error in the coastal boundary definition. This is consistent with NG-T-UN-029 that requires the coastal zone definition to include a 12 nautical mile offshore limit. 5.1.2 Inland Water Mask For the S3NG-T mission, a hydrology target mask will be defined to prioritise hydrology measurements. Figure 5.1.2-1 shows a Copernicus Sentinel-1 image of the Amazon River highlighting the spatial characteristics of a very large river and smaller tributaries. In this case a method to determine which hydrology targets can/should be acquired must be used based on the characteristics of targets to be observed. Allen and Pavelsky (2018), then completed and improved by Altenau et al. (2021), used 30 years of Landsat images to create a global dataset of river widths (initially named Global River Widths from Landsat, GRWL, database and now known as SWOT River Database, SWORD) at mean annual discharge, for rivers wider than 30 m. This dataset is used to map river reaches larger than a given threshold (e.g. Figure 5.1.2.2(a) for 80 m threshold or Figure 5.1.2.2(b) for 150 m threshold). The total amount of river reaches that could potentially be observed depend on a given minimum observed river width threshold. Increasing the threshold decreases the number of observed river reaches. Also, with a large threshold, gaps appear in some rivers due to ephemeral variability. It should be reiterated that the GRWL database corresponds to river width when the ESA UNCLASSIFIED – For ESA Official Use Only Page 85/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 discharge reaches its averaged value. This means that during low flows, some river reaches do not flow, while during high flows, more river reaches will be wider than the threshold. Figure 5.1.2-1. Copernicus Sentinel-1 dual-polarisation image of the Amazon River acquired on 3 March 2019. The Javari River, or Yavari River, is visible as a thinner blue line weaving through the tropical rainforest. Figure 5.1.2-3 (a) shows the percentage of the total river lengths over Europe for which the width exceeds a given threshold. This plot shows some remarkable thresholds in mean river width variations for European rivers. If the minimum observed width is equal to 150 m, 35% of the total river lengths will be observed. This percentage increases to 40% if the minimum observed width is 100 m. For lakes, the same approach can be followed using the CIRCA-2015 database (Sheng et al., 2016). This database contains all lakes of the globe with an area greater than 60 m x 60 m. Figure 5.1.2-3(b) shows the percentage of the total area of French lakes with an area greater than a given threshold. Contrary to rivers, the shape is quite linear and it is difficult to extract significant thresholds. Figure 5.1.2-2(a). Rivers wider than 80 m in the GRWL database (Allen and Pavelsky, 2018). Image courtesy of S. Munier (CNRM) Figure 5.1.2-2(b) Rivers wider than 150 m in the GRWL database (Allen and Pavelsky, 2018). Image courtesy of S. Munier (CNRM) ESA UNCLASSIFIED – For ESA Official Use Only Page 86/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The primary hydrology related objective for S3NG-T is to sample water elevation of rivers greater than or equal to 100 m width and lakes/reservoirs with areas above 250 m x 250 m, i.e. 0.0625 km2 to resolve the physical phenomena. These objectives are in concordance with the recommendations from the COSPAR (2018) report and will expand considerably the water level product from the Copernicus Global Land and Copernicus Climate Change Services. It will thus contribute to the monitoring of lake water level, lake water extent and river water level Essential Variables (EV) identified by the Group on Earth Observations (GEO) and to the Lake Water Level/Extent Essential Climate Variables (ECV) defined by the Global Climate Observing System (GCOS). They also contribute to the estimation of two main EV and ECV which characterise water fluxes on continents: lake volume change and river discharges. S3NG-T will contribute to monitoring the achievement of the United Nations Sustainable Development Goals 6 (SDG 6; Reyers et al, 2017) and the European Union Water Framework Directive (Directive 2000/60/EC). Figure 5.1.2-3(a). Percentage of total river lengths observed over Europe versus the minimum river width observed (from the GRWL database; Allen and Pavelsky, 2018). Image courtesy of S. Munier (CNRM) Figure 5.1.2-3(b). Percentage of total observed lake area over France versus the minimum lake area observed (from the CIRCA-2015 database, Sheng et al., 2016). Image courtesy of S. Munier (CNRM) River and lake targets have considerable seasonal variation in terms of horizontal extent and WSE. This must be accommodated in the Inland Water Mask. River and Lake monitoring is currently a secondary mission objective for Sentinel-3 and Sentinel-6. For Sentinel-3, hydrology ‘virtual stations’ are located at exact locations in the Sentinel-3 orbit ground track and will, but the time of the S3NG-T first launch, constitute a significant unbroken time series record of altimeter hydrology measurements. Following discussions with the European Commission, and in the interest of maximising the opportunities to measure and considerably more Hydrology targets compared to that of Sentinel-3, a target of ≥90% continuity for Sentinel-3 Hydrology targets maintained in the OLTC database (rivers, lakes and reservoirs) is defined. ESA UNCLASSIFIED – For ESA Official Use Only Page 87/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0040 An Inland Water Mask (IWM) shall be established according to the following specifications: § River width: open, non-vegetated water ≥ 0.1 km including seasonal variations § River length: ≥ 10 km § Open water bodies (non-vegetated water e.g. lakes reservoirs, wetlands etc) larger than 250 m x 250 m, i.e. 0.0625 km2,(goal: 100 m x100 m). § Estuaries with an estuary entrance of larger than 2km for rivers articulated in the Inland Water Mask. Note 1: The S3NG-T Inland Water Mask specifies Inland Waters for the S3NG-T mission. Note 2: The purpose of the Inland Water Mask is to minimise the downlink of data by selecting pre-defined hydrology targets. Note 3: It is expected that some water bodies smaller than the requirement definition could be measured by S3NG-T. Note 4: Deleted. Note 5: River and lake seasonal variations should be accommodated in the Inland Water Mask; linked to MRD-0050. MRD-0045 For baseline continuity, ≥90% of the hydrology targets (i.e. river, lakes and reservoirs virtual stations) included in the Sentinel-3 Open Loop Tracking Command (OLTC) shall be included (see note 1) as defined in S3NGT-OLTC-SIZING.tgz Note 1: Reference to the Sentinel-3 OLTC in this requirement does not imply the choice of any altimeter implementation solution. Its purpose is to recognise that measurements at the virtual station targets that are maintained by the Sentinel-3 Mission will be continued by S3NG-T. Location of 57449 S3A OLTC virtual stations as of March 2022 as specified in S3NGT-OLTCSIZING.tgz Location of 57093 S3A OLTC virtual stations as of March 2022 as specified in S3NGT-OLTCSIZING.tgz ESA UNCLASSIFIED – For ESA Official Use Only Page 88/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Note 2: For sizing purposes ESA has created a global kml map of gridded river density based on Yamazaki et al (2019), “MERIT Hydro: A high-resolution global hydrography map based on latest topography datasets”, the JRC Global Surface Water Layers (Pekel et al, 2016) and ESA WorldCover (ESA, 2021). KML files are available on request from ESA in Open_Water_Density_PPP_Datasets.zip. (left) Yamazaki et al (2019), “MERIT Hydro river width (right) ESA 25km river density map provided for sizing purposes. MRD-0050 The Inland Water Mask shall be re-evaluated and uploaded to the satellite up to 4 times per year to account for seasonal variations in elevation and in horizontal extent Note 1: This is necessary since river spatial extent and position may change seasonally. Note 2: Depending on the concept, a more extensive and elaborate approach compared to the Sentinel-3 OLTC approach may be required (e.g. larger onboard database or regular updates of the database in-flight several times per year to address seasonal variation) Note 3: Deleted MRD-0060 The Inland Water Mask shall be delivered to users as an open product. Note 1: This provides a convenient way to engage the user community in the operational management of the Inland Water Mask ESA UNCLASSIFIED – For ESA Official Use Only Page 89/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 5.1.3 Sea Ice Mask For the S3NG-T, the area where sea ice can be expected is defined in a Sea Ice Mask (SIM). It is defined by the median ice extent over the period 1979-2010, based on sea ice climatology. The sea ice record to be used for the definition of the SIM is not specified to allow alignment with the operational framework of the Sentinel-3 and/or CRISTAL missions at the time of launch of Sentinel-3 NG Topo. MRD-0070 The S3NG-T Sea Ice Mask (SIM) shall be defined as the median sea-ice extent climatology over the period 1979-2010 (TBC). Note 1: This specification is updated to allow the use of NSIDC or OSI-SAF sea ice record to allow alignment with the current Sentinel-3 MPC and/or CRISTAL at the time of launch of S3NG-Topo. Note 2: The following files serve as a starting point for budgeting purposes: Northern Hemisphere: ftp://osisaf.met.no/reprocessed/ice/conc/osi450/1979/02/ice_conc_nh_ease2-250_cdr-v2p0_197902271200.nc Southern Hemisphre: ftp://osisaf.met.no/reprocessed/ice/conc/osi450/2014/09/ice_conc_sh_ease2-250_cdr-v2p0_201409191200.nc MRD-0075 The Sea Ice Mask (SIM) shall be re-evaluated and uploaded to the satellite once per month to account for monthly variations in sea ice extent. 5.1.4 Land Ice Mask The baseline continuity definition is to cover Land Ice targets that fall on the ground track of Sentinel-3A and Sentinel-3B. As a starting point, for Greenland, surface type information from BedMachine Greenland version 3 (Morlighem et al., 2017) is used. The mask is freely available and can be accessed directly via the NASA National Snow and Ice Data Center (NSIDC). The BedMachine Greenland version 3 mask is based upon the MEaSUREs Greenland Ice Mapping Project (GIMP) Land Ice and Ocean Classification Mask, Version 1, supplemented with grounding line information (the boundary between grounded ice and floating ice tongues) based upon unpublished InSAR data (courtesy of E. Rignot and J. Mouginot). The surface type mask is shown in Figure 5.1.4-1. For Antarctica, surface type information from BedMachine Antarctica version 2 (Morlighem, 2020) is used. The mask is freely available and can be accessed directly via the NASA National Snow and Ice Data Center. The BedMachine Antarctica version 3 mask uses Grounding Lines derived using interferometric SAR data (Rignot et al., 2011). In each case, a buffer of 50 km is required to account for fluctuations in the glaciated area. The Land Ice Mask may need to be extended to include larger regions of interest depending on the final concept selected. ESA UNCLASSIFIED – For ESA Official Use Only Page 96/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 reference) is already less than 0.1 mm/s using Galileo only and the dynamic model without the use of the Galileo High Accuracy Service (HAS) (Darugna et al, 2022). Galileo GNSS-based onboard orbit determination can now reach a similar performance as the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) navigation system (Montenbruck et al, 2022). 5.4 Mission Lifetime Requirements The CSC LTS (ESA, 2025) defines the baseline operation lifetime for the current and next generation Copernicus Programme. MRD-0310 The S3NG-T shall guarantee 20 years of continuity of the Copernicus Sentinel-3 Topography mission. Note 1: This requirement implies a series of satellites. If recurrent satellites are foreseen, these are assumed to extend the operational mission duration. Note 2: It is assumed that Copernicus 1.0 assets (i.e. Sentinel-3, Sentinel-6) will cover the period up to 2030. Note 3: It is strongly advised to transition to S3NG-T satellites before the demise of Copernicus 1.0 assets to guarantee tandem phase operations and maintain stability of the Sentinel-3 time series that is required by Copernicus Services. Note 4: A topography reference mission capability is assumed to guarantee baseline performance that is at least equivalent to Sentinel-6 (Table 2.4-1) covering the 2030-2050 time period. MRD-0320 The minimum design lifetime of each S3NG-T satellite shall be ≥7.5 years. Note 1: The nominal operational lifetime of all future Sentinels is assumed to be 10 years (with the exception of Sentinel-6-NG operating in a higher orbit) as described in the ESA CSC-4 Long-term Scenario. Note 2: This requirement includes a commissioning period of ≤9 months. MRD-0330 Consumables (e.g. propellant) of each S3NG-T satellite shall include margins allowing an extension of the lifetime of the corresponding satellite by ≥5 years. Note 1: The nominal operational lifetime of all future Sentinels is assumed to be 10 years (with the exception of Sentinel-6-NG operating in a higher orbit) as described in the ESA CSC-4 Long-term Scenario. ESA UNCLASSIFIED – For ESA Official Use Only Page 97/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 5.5 Mission Phase Requirements 5.5.1 Commissioning Phase Requirements Following the launch of a new satellite a Commissioning phase is implemented to bring a satellite into full operational state. During this phase, the majority of groundprocessing algorithms will be brought on-line. Uncertainties will be derived following dedicated calibration and validated activities to establish compliance to requirements. It is acknowledged that estimates of instrument drift will require a longer period that could extend over the mission lifetime. The approach depends on statistical analysis and by comparison with external in situ Fiducial Reference Measurements (FRM) or other satellite missions. Calibration/verification outputs will be compared with the mission requirements and the theoretical error budget specification. Typically, the commissioning phase is six months long but may be extended to accommodate Tandem phase requirements with requirements tailored to specific missions. At the end of satellite commissioning activities, the satellite will be fully characterised and calibrated with a complete update of a Satellite System Characterisation and Calibration Data Base (SSCDB). Based on these activities the pre-launch mission performance uncertainty budget can be updated ready for operational use. MRD-0340 The S3NG-T satellite and payload complement shall be fully commissioned in ≤ 9 months. Note 1: This requirement is independent of any additional requirements for tandem inter-calibration activities. MRD-0350 At the end of satellite commissioning activities, the pre-launch mission performance uncertainty budget shall be updated and delivered to users for operational use. Note 1: The pre-launch performance estimates will be validated in flight. Note 2: User access to the mission performance uncertainty budget is requested. L1 and L2 products will include uncertainty estimates. Note 3: Establishing meaningful trends of uncertainties may require several years of in-flight data collection. 5.5.2 Tandem Inter-calibration Phase Requirements Properly characterising differences between satellite instruments either as part of a series or in constellation is critical to the success of GCOS ECV activities and in turn the activities of the Copernicus Climate change Service. Recognizing that, even though satellites could be of an identical design, it is expected that differences in performance of payload instruments will exist due to subtle tolerances of materials, manufacture and pre-flight characterisation. Furthermore, as demonstrated by the very successful Jason series, Copernicus Sentinel-3 and Sentinel-6 tandem flights (Donlon et al., 2016; ESA UNCLASSIFIED – For ESA Official Use Only Page 98/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 2019), there are enormous benefits to conducting a Tandem flight in terms of in-flight calibrations, calibration and validation activities that will significantly enhance the early application of data from new satellites and the overall mission robustness. Noting the discussion above, it is essential that relative discrepancies between satellites are properly characterised for CDR construction and the success of the entire Copernicus topography constellation. The main challenge is to reduce uncertainties when comparing data from different satellite missions that form a time series. When data are obtained from two satellites at different times but at the same location (i.e. in Tandem flight), there is significant correlated uncertainty: • Uncertainty due to ocean geophysical space and time variability that complicates inter-comparison, especially in regions dominated by mesoscale structure (1-10 days, <10-50 km), which are particularly lucrative to understand inter-satellite bias; • Uncertainty due to atmospheric space and time variability. Both issues introduce uncertainty to the direct inter-calibration of S3NG-T instruments. However, if a new satellite is added to a mission series resulting in more than one satellite on-orbit at the same time, by flying these satellites in tandem separated by ~30-60 seconds on the same ground track, the correlation between these uncertainties is maximized so that for all practical purposes they can be ignored: the difference between two satellite measurements is solely due to instrumental aspects. Thus, when appropriate, information learned from the commissioning and calibration of one satellite may be transferred to a second satellite with confidence. In addition, End Of Life (EOL) estimates can be established by comparing the performance of the ‘new’ satellite with the ‘old’ satellite to improve the long-term stability of the climate data records derived from Sentinel-3 and S3NG-T. For all satellites contributing to GCOS ECV (such as Sentinel-3 see Prandi et al., 2021), and particularly those that constitute a series of follow-on satellites like Sentinel-3, the optimal duration for a tandem flight is 12 months as requested by GCOS (2016) and required by the European Integrated Policy for the Arctic (climate pillar). 12-months is considered mandatory where significant technology changes have been implemented (e.g. change from Jason class Low Resolution Mode (LRM) altimetry to Sentinel-6 Synthetic Aperture Radar (SAR) or change from nadir-only altimetry to swath altimetry). A 9-month tandem calibration phase is considered sufficient (Ablain et al, 2019) to understand the technical differences between old and new technologies. However, for a satellite system of identical design, observing both the north and south hemisphere (and therefore a holistic view of seasonal aspects) a 6-month or less tandem flight duration may be sufficient. This excludes the additional time required for the satellite to drift towards the tandem position and towards a nominal phasing position after a tandem phase. Should the orbit choice of S3NG-T be incompatible with a tandem flight together with Sentinel-3C or Sentinel-3D, an alternative approach based on the use near contemporaneous and collocated data will be required. ESA UNCLASSIFIED – For ESA Official Use Only Page 99/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 If a tandem calibration flight is not possible due to orbit incompatibility, the use of quasi simultaneous and contemporaneous crossover points of Sentinel-3C and/or Sentinel3D within 30 minutes and ±10 km provides an alternative, less performant, approach. The objective is to detect and quantify regional and seasonal differences between satellites and fully characterise such differences in support of the Copernicus Climate Change Service (3CS). Since the crossover technique is subject to greater uncertainty compared to the tandem calibration flight, a 12-month period is initially defined. In practice, it may be necessary to use a longer time period. However, since this has no direct impact on any operational aspect this has no impact on data availability or special operations for the mission: it is simple matching data. MRD-0360 For the handover from S3 to S3NG-T, if the Sentinel-3C/D orbit plane is used for S3NG-T satellites, S3NG-T shall ensure a tandem calibration flight, composed of a drift phase towards a tandem configuration, followed by a tandem operation phase and completed by a second drift phase towards a nominal operational orbit position, overlapping with Sentinel-3C and/or Sentinel-3D for a duration of at least 9 months. Note 1: Since S3NG-T satellites will be of a new design, it is imperative that the characteristics of the new system are introduced into the Sentinel-3 time series in a manner that does not introduce instability into the long time series. Sentinel-3 implemented a tandem calibration flight (see Donlon et al., 2016). Note 2: Ideally a 12-month tandem calibration phase would allow a full analysis of seasonal uncertainties at a global level as requested by GCOS (2016). This is the basis for the 12-month tandem flight flown by Jason-3 and Copernicus Sentinel-6. Note 3: A 9-month tandem calibration phase is considered sufficient (Ablain et al, 2019) to understand the technical differences between old and new technologies. MRD-0370 For the handover from S3 to S3NG-T, the following offset uncertainties at global scale should be ensured for the essential ocean parameters: • SSH offset uncertainty < 0.5 mm • SWH offset uncertainty < 1 cm • U10 windspeed offset uncertainty < 0.25 m/s Note 1: Since S3NG-T satellites will be of a new design, it is imperative that the characteristics of the new system are introduced into the Sentinel-3 time series in a manner that does not introduce instability into the long time series. Note 2: The numbers provided in this requirement for the SSH, SWH and wind speed offset uncertainty in the intermission cross-calibration represent 10% of the daily global variability of these three parameters at a global scale. ESA UNCLASSIFIED – For ESA Official Use Only Page 100/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 For the second satellite in the series, and for additional satellites, a tandem calibration flight at the start the new satellite lifetime naturally provides data to assess the old onorbit satellite and effectively constitutes a second tandem for the old satellite in a single operation. Sentinel-3 has implemented a highly successful tandem calibration phase of 5 months during Phase E1. Sentinel-6 used a 12-month Tandem phase with Jason-3 building on the previous heritage of the Jason missions. A 6-month tandem calibration phase is considered a minimum duration based on the assumption that uncertainties can be identified over a full seasonal cycle by using coverage of both north and south hemisphere measurements. The completion of tandem calibration flight is likely to include satellite commissioning activities related to the update of the new Satellite Characterisation and Calibration Data Base (SSCDB). MRD-0380 If a new satellite is added to the S3NG-T mission resulting in more than one satellite occupying the same orbit at the same time, a tandem calibration flight shall be flown as soon as practically possible, composed of a drift phase towards a tandem configuration, followed by a tandem operation phase of ≥ 100 days and completed by a second drift phase towards a nominal operational orbit position. Note 1: The user needs of the Copernicus Climate Change Service is for a stable time series of measurements that comply with the Climate Monitoring Principles (GCMP, GCOS, 2003). Note 2: The intent of the REQ is to improve bias and stability knowledge for the multisatellite data record when a new satellite is introduced to the constellation. Note 3: About 10 cycles (100 days) was the time length recommended to detect regional SSH biases with an uncertainty lower than ± 4 mm. Evolution of the standard deviation of the mean SSHA differences averaged per box of 1° x 3° between Jason-3 and S6-MF, according to the period length during the verification phase. Note 4: The tandem calibration phase is flown entirely during the Phase E1 commissioning phase (i.e. tandem flight to be completed independently of In Orbit commissioning). Note 5: A tandem calibration flight excludes the additional time required for the satellite to drift towards the tandem position and towards a nominal orbit position after a tandem phase. MRD-0385 For S3NG-T satellites flying at the same time, the following offset uncertainties should be ensured for the essential ocean parameters: • SSH offset uncertainty < 0.5 mm; (goal 0.25 mm) • SWH offset uncertainty < 1 cm; (goal 0.5 cm) • U10 windspeed offset uncertainty < 0.25 m/s; (goal 0.15 m/s). ESA UNCLASSIFIED – For ESA Official Use Only Page 101/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Note 1: When a new S3NG-T satellite is added to the mission, either to operate at the same time or to take over from another, the new system shall be introduced into the Sentinel-3NG-T time series in a manner that does not introduce instability into the combined products or the long time series. Note 2: The numbers provided in this requirement for the SSH, SWH and wind speed offset uncertainty in the intermission cross-calibration represent 10% of the daily global variability of these three parameters at a global scale. MRD-0390 The along-track separation of S3NG-T satellites, if configured in a tandem calibration phase, shall be nominally ≤30 seconds in time apart. Note 1: The shortest possible separation mitigates the uncertainty due to ocean geophysical space and time variability that complicates inter-comparison and inter-satellite bias; and uncertainty due to atmospheric space and time variability. Note 2: Sentinel-3A and Sentinel-3B flew a tandem calibration phase separated by 30s in time. The Jason-3 and Sentinel-6 Tandem flight will also separate spacecraft by 30 seconds in time. The ESA FLEX mission and Sentinel-3D will fly together separated by 6-15 seconds apart. Note 3: An along-track separation distance of up to 60s in time was used by the Sentinel-3A/B tandem calibration flight. Note 4: Separate requirements define the tolerance for along-track drift separation. Note 5: The achieved along-track separation between satellites depends on the launch injection that it is only known after launch. MRD-0400 Requirement deleted. The Sentinel-3A and B ground tracks were maintained during the tandem phase to much better than ±1 km allowing extremely useful comparisons between satellites to with 150m over ice sheets (e.g. Clerc et al, 2020) allowing detailed comparison of waveforms between Sentinel-3A and B over Antarctica (McMillan et al, 2021). An extremely high level of consistency, with close similarity of waveforms, both in terms of their magnitude and the distribution of backscattered power (i.e., waveform amplitude and shape). This consistency applies both to the waveform leading edge (corresponding to the surface return from the point of closest approach) and the trailing edge (corresponding to any subsurface and off-nadir contribution). Small shift of <100m did not appear to complicate the analysis demonstrating that the performance of both instruments is near-identical. Such a high degree of correlation in co-located waveforms acquired by both instruments, even over complex coastal terrain demonstrates that both satellites can be used interchangeably to monitor ongoing ice sheet evolution. More broadly, it also establishes the value of operating a tandem phase immediately after satellite launch and demonstrates that such operations should be performed when the Sentinel-3C and Sentinel-3D units enter service in the future (McMillan et al, 2021). Following this work, the S3NG-T ground track should be maintained to <1.0 km and ideally ±0.25 km. ESA UNCLASSIFIED – For ESA Official Use Only Page 102/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0410 The relative cross-track separation of satellites, if configured in a tandem calibration phase, shall be less than or equal to ±0.5 km (goal: ±0.25 km). Note 1: During tandem, the relative across track separation distance must be managed so that differences associated with each satellite measurement geometry are minimal. MRD-0420 Requirement deleted. MRD-0430 Requirement deleted. MRD-0440 If configured in a tandem calibration phase and excluding satellite commissioning activities, data shall be acquired in nominal acquisition mode from the satellite to be commissioned throughout the Tandem flight phase. Note 1: The principle is to assemble a dataset for analysis and establish differences between payload instruments that can be used to inter-calibrate measurements. This is best achieved by maintaining normal operations. Note 2: The data collected will be used to transfer calibration and characterisation data collected by operational satellite(s) and using that information to understand any differences with respect to new satellites. This may accelerate the uptake and application of mission data. 5.5.3 Nominal Operations Phase Requirements During the nominal operations phase, measurements will be continuously acquired from all payload components at full resolution except during calibration activities. MRD-0450 During the nominal operations phase, measurements shall be continuously acquired from all payload components, except during calibration activities. Note 1: Deleted. MRD-0460 Requirement Deleted ESA UNCLASSIFIED – For ESA Official Use Only Page 103/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 5.6 Orbit Requirements Sentinel-3 occupies a frozen sun-synchronous orbit (SSO) at 814.5 km altitude inclined to 98.65° with a 27-day repeat cycle (14+7/27 revolutions per day) and a local solar time at the descending node of 10:00 (similar to ENVISAT). This configuration satisfied a baseline requirement to provide coverage between 81.5°N and 81.5°S. This orbit has a sub-cycle at ~4 days that allows reasonable sampling of ocean mesoscale features within 10 days. The Copernicus Sentinel-3 sun-synchronous orbit (SSO) is optimized for operational ocean applications. It was chosen as a compromise between the needs of optical missions: • an ocean colour spectrometer requires a LTDN at high solar elevation, • a sea surface temperature thermal infrared radiometer dedicated to Sea Surface temperature requires a LTDN at 06:00 to minimise solar warming of the surface ocean during the day, • the topography mission required regular sampling of mesoscale (30-50 km over ~10 days) without consideration of measurement acquisition with respect to LTDN. The final orbit choice was a frozen sun-synchronous orbit at 814.5 km altitude inclined to 98.65° with a 27-day repeat cycle (14+7/27 revolutions per day) and a local solar time at the descending node of 10:00 (similar to ENVISAT). This configuration satisfied a baseline requirement to provide coverage between 81.5°N and 82.5°S. This orbit has a sub-cycle at ~4 days that allows reasonable sampling of ocean mesoscale features within 10 days. Two satellites operate in constellation on the same orbital plane separated by 140° in relative phase. However, this orbit choice is not fully optimised for ocean topography measurements based on a twin satellite constellation and could be improved. This is the fundamental reason for separating the Sentinel-3 mission into separate topography satellites and optical satellites. CMEMS recommends that high inclination orbits are needed to provide requite sampling and coverage and a sun synchronous orbit (SSO) is acceptable. MRD-0530 Deleted. MRD-0540 If more than one satellite in-orbit simultaneously occupy the same orbital plane, the relative phase between satellites shall be configurable. Note 1: This requirement allows any phase position in the orbit to accommodate a variable configuration satellite positioning. Note 2: This is heritage from Sentinel-3 and necessary to allow tandem calibration flight configuration. ESA UNCLASSIFIED – For ESA Official Use Only Page 104/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The rationale for maintaining a stringent satellite ground track to within ±1.0 km (ideally within ±0.25 km) to assist in consistency and performance of geophysical retrievals over ice sheets has been discussed in Sec. 5.5.1. This stringency also applies to routine operations since cross-track slopes in regions with marine geoid errors impact on the accuracy of ocean retrievals. In addition, cross track slopes may complicate retrieval of river water surface elevation estimates as discussed in Cohen et al. (2018) who demonstrate that the rivers in Version 1.0 of the 6 arc-minute Global River-Slope (GloRS) geospatial dataset have a median slope of ˜0.6m/km for 50% of the rivers. In reality, the number of larger and more mature rivers is located in areas of slowly varying terrain. In contrast, a large number of smaller width streams and tributaries dominate the headwaters of river basins typically in mountainous and hilly terrain (where slopes can reach as much as >2.6 m/km). Assuming the satellite track is maintained at ±1 km (3-sigma or 0.33km @1sigma) and noting that between each cycle the satellite will cross the river at a slightly different location within this tolerance, river slopes must be considered in terms of the water surface elevation estimation. MRD-0550 The satellite ground track shall be maintained to be within ±1.0 km (3sigma) (enhanced goal: ±0.25 km (3-sigma)) of the reference ground track defined at the sub-satellite point at nadir. Note 1: Deleted. Note 2: This requirement is linked to the performance of S3NG-T measurements over ice sheets and inland water. Like other altimetric satellites, S3NG-T will fly at 7 km/s and cover a region of 420 km in one minute, which will effectively provide a synoptic ‘snapshot’ of SSH variations. Some of the largest signals we observe with all altimeters are ocean tides. Although highfrequency open ocean barotropic tides are now well estimated from models and altimetry (Stammer et al., 2014), there are still large errors in the coastal regions and at high latitudes > 66°, regions that are not well covered by the Jason/Sentinel-6 altimeter series. The development of new and better tide models using hydrodynamic modelling is essential for ocean topography – particularly at higher latitudes (Quartly et al., 2018). As noted by Park et al., (1987), the choice of orbit inclination defines the coverage up to a certain latitude limit and is constrained by the desired density of coverage at the subsatellite track and by the time phasing of the repeat of the satellite's ground track as a function of inclination, which determines the aliased frequencies of the measurements of the oceanic tides. The choice of repetition period is constrained by trade-offs between temporal and spatial coverage and by the aliasing of tidal constituents. See Appendix VIII for altimeter orbit details related to tide measurement. ESA UNCLASSIFIED – For ESA Official Use Only Page 105/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0560 Should alternative orbits be chosen for S3NG-T compared to the operational sun-synchronous orbit of Sentinel-3A/B, they shall minimise tidal aliasing periods M2, N2, O1, K2, P1, and Q1 ocean tidal constituents based on orbit choice and the use of ocean tide models. Note 1: This requirement provides guidance to minimise the impact of ocean tides e.g. Parke et al., (1987), Lindsley (2011). Note 2: Deleted. Note 3: The aliasing of tides is a fundamental issue for altimeter sampling particularly in the aliasing band <180 days and the semi-annual/annual band Note 4: There should be no tidal aliasing at very long periods that requires an aliasing frequency >2 cycles per year. Note 5: An ability to resolve tidal waves of close frequency and separate these signals from each other within a reasonable time of observation is important (i.e. < mission lifetime). Ideally it should be possible to separate the main tidal constituents: S2 is the principal solar semidiurnal (infinitely aliased for sunsynchronous orbits such as Sentinel-3) K1 is the main diurnal tide (luni-solar declination diurnal aliased to 365.25 days for Sentinel-3), K2 is the luni-solar declination semidiurnal, M2 is the principal lunar semidiurnal tide (aliased to 157.5 days for Sentinel-3), O1 is lunar declinational diurnal (aliased to 277 days for Sentinel-2), N2 is the larger lunar ecliptic semidiurnal. Note 6: Deleted. Note 7: There should be minimal aliasing at annual and semi-annual frequencies. Note 8: Deleted. Note 9: It may be necessary to consider other tidal constituents if the S3NG-T orbit is different from that of Sentinel-3. MRD-0570 Should alternative orbits be chosen for S3NG-T compared to the operational sun-synchronous orbit of Sentinel-3A/B, the S3NG-T mission orbit choice shall, as much as practically possible, position aliasing frequencies of the main tidal constituents M2, N2, O1, K2, P1, and Q1 as far away from one another. Note 1: Deleted. Note 2: Deleted. Note-3: Deleted. ESA UNCLASSIFIED – For ESA Official Use Only Page 112/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0640 All on-board calibration data and supporting engineering data required to recalibrate the altimeter measurements shall be available to users. Note 1: Access to engineering data such as thermistor values, instrument state etc is essential to monitor the performance of the on-board calibration systems and to establish uncertainty budgets and uncertainty propagation and to recalibrate the instrument on-ground if required as part of reanalysis. Note 2: Supporting engineering data includes any on-board information considered necessary to reconstruct instrument calibration depending on the specific to the implementation of the on-board calibration system. Note 3: It is anticipated that this information will be available to users via Level 1a data products. MRD-0650 The S3NG-T baseline continuity mission shall be capable of continuous operation in fully-focused synthetic aperture radar. Note 1: Based on the preliminary in-flight performance of Copernicus Sentinel-6 Michael Freilich Poseildon-4 altimeter (Donlon et al 2021), an open burst interleaved functionality is preferred for nadir altimetry. Note 2: Sentinel-3 SRAL operates in SAR mode over all surfaces at Ku-band (i.e. no LRM capability is used). A fully focussed SAR mode capability, (Egido and Smith, 2017), is therefore considered the baseline for the S3NG-T mission. Note 3: This implies the radar is operated continuously during nominal scientific measurement operations. Note 4: Swath altimetry solutions may operate in different modes. Note 5: Deleted. Note 6: Deleted. MRD-0660 The S3NG-T baseline continuity mission shall be capable of an alongtrack resolution better than 300 m over all target surfaces after data processing. Note 1: This is baseline continuity for Sentinel-3. MRD-0670 The S3NG-T baseline continuity mission nadir altimeter shall be capable of generating pulse-width limited data in parallel with SAR data. Note 1: This requirement builds on the positive experience of Poseidon-4 interleaved mode operations aboard Sentinel-6. Note 2: For nadir SAR enabled solutions using an open-burst continuous radar chronogram solution, both LRM and SAR data can be generated on ground. ESA UNCLASSIFIED – For ESA Official Use Only Page 113/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0680 The S3NG-T baseline continuity mission altimeter shall be capable of measuring a range of surface elevation with respect to the ellipsoid to meet mission requirements. Note 1: This allows the mission to observe the highest elevations defined in the Hydrology mission. MRD-0690 The S3NG-T baseline continuity mission altimeter shall maintain tracking over ice surfaces, glaciers and ice sheet margins with slopes less than 1.25° (TBC). Note 1: McMillan et al. (2021) show that both Sentinel-3A and Sentinel-3B are capable of maintaining coherent signals over ice sheet margins. See also Wingham (1995). Note 2: Deleted, superfluous. Note 3: High resolution on-board DEM’s could be used to maintain altimeter echoes within the range window over ice margin regions. Note 4: This requirement will be assessed based on the final configuration of the technical solution. MRD-0700 The S3NG-T altimeter should maintain tracking over icebergs with a freeboard below 50m (enhanced goal: below 60 m (TBC)). Note 1: For small Icebergs the analysis of CryoSat-2 SARIN data shows that 90% of the icebergs detected have a freeboard less than 60m (there are some outliers but they are rare); Antarctic icebergs tend to be quite tabular and have a freeboard slightly lower than those form Greenland. Note 2: Tournadre et al. (2015) analyse large icebergs characteristics from altimeter waveforms analysis and provide a pdf of 5366 iceberg freeboard estimates as follows. ESA UNCLASSIFIED – For ESA Official Use Only Page 114/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0705 The S3NG-T altimeter shall maintain tracking over water reservoirs having a surface water elevation dynamic range up to 50m (enhanced goal: up to ≤60 m (TBC)) Note 1: Inland water reservoirs have extreme variations in surface water elevation depending of reservoir capacity, that must be mostly captured by Sentinel-3A SRAL for observed reservoirs (e.g. Zhang et al. 2020). MRD-0710 The S3NG-T baseline continuity mission altimeter range resolution shall be better than 0.5 m (enhanced goal: better than 0.3 m). Note 1: Range resolution impacts, primarily, the ability to properly sample specular echoes, such us from hydrology targets and over sea ice leads. MRD-0720 Orbit manoeuvres for ground track maintenance shall be scheduled to take place over areas that minimise the impact of data loss defined by the Marine and Coastal Mask and the Inland Water Mask. Note 1: Manoeuvres may be necessary in areas that could result in data loss. Independent on-ground Fiducial Reference Measurements (FRM) are used during flight to monitor and assess in-orbit calibration. A ground-based transponder is foreseen as part of the mission as part of the calibration and validation activities for the duration of the mission. Ground-based transponders allow verification of s0, range, datation, interferometry geometry among other parameters. Transponders are mandatory for Cal/Val of altimeter missions, cross-calibration of missions. Active transponders require accurate positioning (with FRM standard uncertainty) information with using GNSS and other positioning systems (e.g. DORIS beacon). A radiometer is needed to derive the wet tropospheric delay during transponder calibrations in addition to the GNSS-derived delays. As part of the Permanent Facility for Altimeter Calibration (PFAC) located in Crete, Greece (Mertikas et al., 2018, 2019), a Ku-band transponder has been established for pulse-width limited (and thus lower SNR) altimeter missions as an external calibration source (Mertikas et al., 2011; 2010) supported by with in-situ reference systems (DORIS ground station, GNSS reference points, radiometer for wet tropospheric correction, etc.). It is notable that a single Sentinel-3A and Sentinel-3B transponder pass over the PFAC during the Sentinel-3 tandem phase established a bias difference of just 1.5cm between the two altimeters. The PFAC is maintained operationally and is being upgraded with a second transponder to support Sentinel6MF located on Gavdos Island, Western Crete as part of the ESA Sentinel-6 calibration and validation activities. At present no Ka-band ground-based transponder exists and will need to be developed as part of the S3NG-Mission. The S3NG-T altimeter must carry appropriate functionality for use with ground-based transponders. ESA UNCLASSIFIED – For ESA Official Use Only Page 115/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0730 The altimeter shall be capable of acquiring measurements from a ground-based transponder for calibration purposes. Note 1: At this time only one European well-maintained Ku-band range transponder facility is at the disposal of Copernicus which is maintained at the PFAC in Crete. This transponder is used by Jason-2, Jasnn-3, Sentinel-6, Sentinel-3 and CryoSat-2. The location of the PFAC site is at the crossover points of Sentinel-3 and Sentinel-6 orbits. For information, the coordinates of the currently operational ESA PFAC transponder units are: Note 2: Recent developments using Sentinel-6 using on-ground corner reflectors may provide an alternative solution for in flight calibration. MRD-0740 It shall be possible to configure the altimeter by telecommand to acquire measurements from a ground-based transponder. Note 1: This functionality applies only to the space segment and allows on-board settings to be fully controlled for different transponder acquisitions (e.g. gain settings) that have been used on previous missions and different. MRD-0750 The S3NG-T baseline continuity mission shall provide range measurements with a noise floor below 0.8 cm at 1 Hz and Hs=2m (enhanced goal: ≤ 0.5 cm at ≥1 Hz and Hs=2 m) after ground processing. Note 1: See Table 2.4-1 for in flight performance of Sentinel-3. Note 2: Requirement is set for Sentinel-3 continuity, goal is equivalent to Sentinel-6 NTC performance. Note 3: The upper limit depends on Significant Wave Height (Hs) that defines the noise that will increase with increasing sea state. Note 4: Deleted. Note 5: Deleted. Note 6: Deleted. ESA UNCLASSIFIED – For ESA Official Use Only Page 116/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The Cross-Calibrated Multi-Platform wind vector analysis (CCMP, Ricciardulli and NCAR, 2017) is a near-global, high spatial (0.25° gridded product) and temporal (6 hourly) resolution gridded dataset of surface wind vectors from 1987-present between 78° North and South of the equator. A combination of inter-calibrated satellite data from numerous microwave radiometers and scatterometers together with in-situ data from moored buoys form the basis of the analysis. A best-fit solution to all of the available observations is derived using the ECMWF ERA-Interim reanalysis winds as a first guess. The temporal record is relatively stable, and are well suited to monitor daily to interannual variability. The analysis is known to perform poorly for precipitating conditions and under high wind (>15 m/s) conditions, and is therefore not recommended for studies of global wind trends. Figure 6.1.1 shows the Probability Density Function (PDF) of the CCMP data set between 1988-2020 at a grid size of 0.25° latitude x longitude between 78° North and South of the equator. Figure 6.1-1. Probability Density Function (PDF) of global ocean wind speeds for 1988-2020 derived from the Cross-Calibrated Multi-Platform wind vector analysis (CCMP, Ricciardulli and NCAR, 2017) 0.25 ° gridded product) and temporal (6 hourly) resolution of surface wind vectors from 1987-present between 78 ° North and South of the equator. (Credit: C. Gentemann.) MRD-0760 The nominal wind speed value over the ocean when computing the radar altimeter link budget shall be larger than 8.1 m s-1. Note 1: Figure 6.1-1 provides a PDF of the mean annual wind speed over the ocean surface. Note 2: This wind speed value corresponds to an average nominal normalised backscatter coefficient (sigma-0) value over the ocean of 13 dB for Ku-band and 11-dB for Ka-band; see Hossan et al., 2021. Note 3: System margins are typically added to these sigma-0 figures to achieve a minimum SNR of 11 dB as in the case of Sentinel-3 and Sentinel-6. Note 4: Removed, note not relevant for this requirement. ESA UNCLASSIFIED – For ESA Official Use Only Page 117/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0770 The nominal significant wave height (SWH) value to be assumed for any S3NG-T baseline continuity instrument calculations that depend on modelling waveforms shall be 2 metres. Note 1: Altimeter range measurement uncertainty increases with increasing sea state. Note 2: A mean sea state of Hs=2 m is defined based on wind speed of 8 ms-1 over the ocean at 10 m height (U10). In terms of the atmospheric attenuation experienced by the radar, Figure 6.1-2(a) shows shows the percentiles of atmospheric attenuation at Ku-band for valid altimeter observations only from Sentinel-6 data. This mostly represents the dry, wet and liquid contributions, since large rain impacts the range and thus leads to invalid Sea Surface Height measurements. At Ku-band, the percentiles of the atmospheric attenuation computed from ERA5 is consistent with the percentile of the attenuation computed from the radiometer. The difference for large attenuation is probably due to the weak representation of rain in the model, especially for strong small convection cells responsible for large attenuation (both in amplitude and location). Figure 6.1-2(b) shows the percentiles of atmospheric attenuation respectively at Kaband for valid altimeter observations only, from AltiKa data. As expected, the atmospheric attenuation for valid measurements is larger at Ka-band than at Ku-band with a median at about 1 dB. At Ka-band, a bias of about 0.2 dB is observed between the atmospheric attenuation retrieved from MWR observations and computed from ECMWF or ERA5 (those latter being very similar). There are some known issues on the retrieval of the wet tropospheric correction on AltiKa and since the same approach is used for the atmospheric attenuation, the latter is most probably also impacted. (a) (b) Figure 6.1: Atmospheric attenuation for valid measurements at Ku-band (a) and at Ka-band (b); courtesy, Bruno Picard. MRD-0780 The two-way atmospheric loss shall be assumed to be 0.5 dB in Ku-band and 1 dB in Ka-band when computing the radar altimeter link budget. Note 1: Values correspond to observed median values from Sentinel-6 and AltiKa in nominal atmospheric conditions without precipitation. ESA UNCLASSIFIED – For ESA Official Use Only Page 118/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0790 The following nadir-pointing waveform model shapes shall be used for S3NG-T baseline continuity mission radiometric performance calculations: Ocean echoes: the return echo waveform may be assumed to follow the Brown Model over ocean and some non-ocean surfaces; Ice echoes 1: with a trailing edge slope of -0.125 dB/ns; Ice echoes 2: with a trailing edge slope of -0.5 dB/ns; Ice echoes 3: with a trailing edge slope yielding a total echo power equal to the total power of an ocean echo with a normalised backscatter coefficient (s°) of 42 dB (TBC) in Ku-band and 40 dB (TBC) in Ka-band. Specular Echo: is defined for calm water/river, lake targets and sea ice leads, and is a purely specular return with the same shape and characteristics as the radar instrument impulse response and s° equivalent to that of the first Fresnel reflection. Note 1: Sentinel-3 SRAL specification. Note 2: A different Sigma0 model is required for swath altimeter solutions. Note 3: Nadir pointing Sigma0 models for Inland Waters are TBD Note 4: Vandemark et al. (2004) model the relationship between low-incidence Kaband Sigma0 measurements to wind speed (via its relation to the ocean mean surface capillary wave slope variance), modified for low wind speeds to allow specular reflection Note 5: Different echo shapes may be required for hydrology targets, ice targets and transponder Note 6: Specular returns coming from the Fresnel zone should not account for additional margin for speckle noise. MRD-0800 S3NG-T baseline continuity mission altimeter radiometric performances shall be met for different surfaces with echo shape characteristics and over the normalised backscatter coefficient (s°) range as specified in Table 6.2. Note 1: Note Deleted. Note 2: These figures include the scattering properties of ice and the effect of slope. No further correction for surface slope is required in using the figures. Note 3: Further adjustment for the scattering properties of ice and the effect of slope may be required. Note 4: A comparison of backscatter values observed with AltiKa over sea ice and leads is provided in Figure 6.1.2 Note 5: Vandemark et al. (2004) model the relationship between low-incidence Kaband Sigma0 measurements to wind speed (via its relation to the ocean mean surface capillary wave slope variance), modified for low wind speeds to allow specular reflection. Note 6: Consideration of all figures over river and lake targets must be considered. Note 7: A different Sigma0 model is required for swath altimeter solutions. ESA UNCLASSIFIED – For ESA Official Use Only Page 119/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Table 6.1: Normalized Radar Cross Section (NRCS, sigma-0) values for different surfacesn and echo types. Surface Type s ° Ku-band (all TBC) s ° Ka-band (all TBC) s ° C-band (all TBC) Ocean (including coastal) Ocean echoes: 8 dB < s° < 27 dB; Ice echoes 1: 2 dB < s° < 42 dB. Ice echoes 2: 2 dB < s° < 42 dB. Ocean echoes: 6 dB < s° < 25 dB; Ocean echoes: 12 dB < s° < 30 dB; Sea ice and Icebergs Ocean echo: 8 dB < s° < 27 dB; Ice echoes 1: 2 dB < s° < 42 dB; Ice echoes 2: 2 dB < s° < 42 dB; Ice echoes 3: 42 dB < s° < 57 dB. Specular Echoes: 42 dB < s° < 55 dB (TBC) Ice echoes 1, 2 and 3: 0 dB < s° < 55 dB; TBC For inland ice-sheets and ice-caps Ocean (Brown echo): 6 dB < s° < 25 dB; Ice echoes 2: -8 dB < s° < 40 dB; Ice echoes 3: -8 dB < s° < 40 dB; Ice echoes 2 and 3,: 0 dB < s° < 40 dB; TBC Ice sheet margins and glaciers Ocean echoes: 8 dB < s° < 27 dB; Ice echoes 1: –8 dB < s° < 42 dB; Ice echoes 2: –8 dB < s° < 42 dB Ice echoes 1,2,3: -10 dB < s° < 40 dB; TBC Inland waters (including frozen lakes) Ocean echoes: 8 dB < s° < 27 dB; Ice echoes 1: -8 dB < s° < 42 dB; Ice echoes 2: -8 dB < s° < 42 dB; Specular Echo: 42 dB < s° < 57 dB (TBC) Specular Echo (sinc2 response): 6 dB < s° < 55 dB; TBC ESA UNCLASSIFIED – For ESA Official Use Only Page 120/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Figure 6.1-3(a) shows data from the AltiKa altimeter revealing a dynamic range of 5 dB to +20 dB over global ocean surfaces. Figure 6.1-3(b) shows data from the AltiKa altimeter revealing a dynamic range of 0 dB to +50 dB including for ice floes and lead areas . Specular echoes over river targets may need to be confirmed. Figure 6.1-3(a). Dynamic range of AltiKa Kaband s 0 compared to Jason-2 Ku=band s 0 over the global ocean (A. Guillot). Figure 6.1-3(b). Distribution of AltiKa Kaband s 0 over ice floes and leads (A. Guillot). MRD-0810 The S3NG-T baseline continuity mission dynamic range of sigma-0 shall be 0 to 55 dB at Ku band. Note 1: Deleted. Note 2: Specular echoes from river and lake targets may require assessment depending on the payload configuration. MRD-0820 The S3NG-T baseline continuity mission total standard uncertainty of the sigma-0 measurement shall be better than 1dB (enhanced goal: 0.3dB), after appropriate calibration. Note 1: Sentinel-3 SRAL specification. MRD-0830 The S3NG-T baseline continuity mission stability in the measurement of sigma-0 shall be better than 0.3 dB per orbit at Level 1, assuming interruption of science measurements for internal calibration not more frequently than once per orbit. Note 1: The long-term drift error of Sentinel-3 SRAL Sigma0 internal calibration is specified as less than 0.3dB during the mission life-time. Note 2: Requirement is set equivalent to Sentinel-6 performance since S3NG-T will serve Copernicus climate service applications where stability is fundamental to the production of a climate record. ESA UNCLASSIFIED – For ESA Official Use Only Page 121/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-0840 The S3NG-T baseline continuity mission shall provide sigma-0 over the ocean surface with a long-term drift less than 0.5 dB (enhanced goal: 0.3 dB) after cross-calibration with other altimeter missions across the mission lifetime. Note 1: Requirement is set for Sentinel-3 continuity, goal is equivalent to Sentinel-6 NTC performance. Note 2: If multiple satellites are used in the S3NG-T this requirement bring homogeneity across the satellite fleet. MRD-0850 The stability of S3NG-T baseline continuity mission NTC sigma-0 measurements shall be better than 0.3 dB during any year period. Note 1: Requirement is set for Sentinel-3 continuity, goal is equivalent to Sentinel-6 NTC performance. Sentinel-6 Poseidon-4 implements an on-board Range Migration Correction (RMC) algorithm that provides an efficient approach to reduce the data volume by a factor two by truncating the processed waveforms. The approach uses approximations leading to Doppler stack misalignment: a fixed RMC matrix is applied for mean altitude; mean sea surface slopes are not accounted for and the accuracy of radial velocity estimates is relatively poor compared to that available on-ground. However, on-board RMC processing can be reversed on-ground without loss of useful information so that Doppler stack misalignment can be corrected (requiring I and Q signals) and accurate SAR processing is then applied on-ground. If I and Q signals are available on-ground, all processing options are possible including: optimal sea ice processing, fully-focused SAR, SAR multi-looking, RMC stacking, pulse-pair amongst others. Simulations suggest that 64 ranges bins and 16 Doppler bands are required to achieve good performance (64 and 128 range bin noise levels are identical for range, Hs and σ0). If on-board RMC is not reversible or fully configurable in flight it implies L2 processing must account for on-board errors due to assumptions and limitations of modelling - otherwise, large Hs and SSH errors are expected. This makes the L2 processing more complicated, risky and less processing options are possible. SAR Interferometry over inland waters is complex due to the specific elevation change induced by inland topography. The NASA/CNES SWOT mission uses substantial onboard processing to reduce data rates by computing interferograms over all ocean and land surfaces that are downlinked. However, over hydrology targets a suitable on-board processing approach has not been defined and large volumes of data are downlinked from which useful measurements can be extracted on-ground. It is anticipated that onboard processing approaches could be developed for hydrology targets once SWOT inflight data are available in the 2024/25 timeframe. MRD-0860 Requirement Deleted. ESA UNCLASSIFIED – For ESA Official Use Only Page 128/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1010 The S3NG-T baseline continuity mission shall embark a multi-channel microwave radiometer (MRAD) having channels centred at 23.8 GHz and 31.4GHz, (Enhanced continuity: one or more of 18.7 GHz, 89 GHz, 165.5 GHz and 172 GHz). MRD-1020 Level 2 S3NG-T mission MRAD measurements for all channels shall be co-registered and near contemporaneous with the altimeter measurements after ground processing. Note 1: This requirement allows the possibility to acquire data from different beams at different times as the satellite flies forwards. MRD-1030 Requirement deleted. MRD-1040 Requirement deleted. MRD-1050 Requirement deleted. MRD-1060 Requirement deleted. In the coastal zone and over rapidly changing atmospheric moisture structures including cloud structures, precipitation and land-sea breeze (amongst other phenomena) lead to more complex atmospheric temperature, pressure and moisture structures that are significantly different from open ocean conditions. Their influence may persist for several hundred km offshore. Furthermore, due to the input of freshwater from rivers and estuaries, bio-geo-chemical compounds and pollutants are more ubiquitous in this region. Natural compounds from biological decay form long-chain organic surfactants on the ocean surface in calm conditions that, together with pollutants on the ocean surface, inhibiting the growth of capillary waves (to which the radar altimeter responds) and modifying the surface emissivity (to which the microwave radiometer responds). Furthermore, these physical, bio-geo-chemical and atmospheric aspects and characteristics present a very complex sea surface topography structure and lead to specific measurement challenges compared to the open ocean. At the ocean-sea ice interface, other complex phenomena arise as sea ice in the marginal ice sone is highly variable and the atmospheric structures can be very different over the ice and ocean (e.g. katabatic winds from cold dry sea ice interiors or ice sheet interiors pour out across the ocean for several hundred km). Over terrestrial hydrology targets, depending on the water body size and persistence, similar atmospheric changes may exist complicating the altimeter measurements of Water Surface Elevation. Practically, the impact of a high performant beam efficiency can limit side-lobe contamination at L1 can be estimated and mitigated by analysing the contribution of power from the full beam (including side and other lobes) compared to the -3dB ESA UNCLASSIFIED – For ESA Official Use Only Page 129/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 footprint ellipse using accurate antenna gain patterns and knowledge of the brightness temperature characteristics of surrounding MRAD measurements. This requires that accurate antenna gain pattern knowledge is available for each channel and feed (if multiple feeds are employed) and a L1 data processing approach is used to adjust the - 3dB measurement and compensate for side-lobe contamination. Figure 6.2-2. Schematic example of how Brightness Temperature (Tb) changes as it approaches a boundary to illustrate the meaning of MRD-1040. Maintaining performance of MRAD at coastal and sea ice transitions is important. This is linked to the size of MRAD footprints and the quality of side-lobe compensation (a function of payload design and antenna gain pattern knowledge). When assessing the performance of MRAD at coastal and sea ice transitions and, as part of the MRAD total standard uncertainty assessment, a synthetic scene provides a powerful means to control the assumptions used (both geometrically and radiometrically). The following requirement is specified for coastal and sea ice transitions and the approach to be used to address each requirement is summarised in Figure 6.2-2. MRD-1070 The impact of radiometric discontinuities (e.g. thermal ocean fronts, at a coastal boundary, sea ice edge or ice shelf edge etc) on S3NG-T MRAD brightness temperature measurements shall be mitigated by appropriate side-lobe compensation. Note 1: Deleted, superfluous. Note 2: This requirement is designed to ensure that side-lobe corrections at an appropriate level. Note 3: This requirement implies a well-formed antenna beam and well characterised antenna side lobes and grating lobe patterns to be used in Level 1 processing. Note 4: This requirement implies that a suitable algorithm is used to compensate for side-lobe and e.g. grating lobe contamination at Level 1. Note 5: This requirement will be assessed theoretically by analysis. Note 6: It is expected that the “worst case” will be in the coastal zones where there are large radiometric discontinuities (temperature, emissivity, varied terrain and elevation etc). ESA UNCLASSIFIED – For ESA Official Use Only Page 130/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1080 The calibration of S3NG-T MRAD Level 1 brightness temperature in all channels shall be maintained during science measurement acquisition using an on-board calibration system. Note 1: Options to provide reference values include hot and cold load sources (including active cold loads and noise diodes). Note 2: The means that for all on-board calibration reference value temperatures should be reference to the International Temperature scale 1990 (ITS 1990) – see Preston-Thomas (1990) Note 3: The reflector characteristics must be known and the reflector temperature monitored in flight. Note 4: Vicarious calibration targets are not traceable to SI. Therefore, the on-board calibration system must be thoroughly characterised before flight and a strategy developed to periodically check calibration. Note 5: While not strictly traceable to SI, a view of deep space, when used together with frequency-specific galactic background radiance maps, offers a practical solution. Note 6: Active cold Load technologies provide one approach to in-flight calibration. MRD-1090 The S3NG-T baseline continuity mission MRAD shall be capable of regularly viewing deep space as a cold calibration reference. Note 1: Deep space is considered a reference target scene for calibration purposes (e.g. Farrar et al. 2016). Note 2: Deleted, space segment implementation choice. Note 3: Deleted. Note 4: Deleted. MRD-1100 Calibration and supporting engineering data required to recalibrate S3NG-T MRAD science measurements shall be available to users. Note 1: Access to engineering data is essential to monitor the performance of the S3NG-T MRAD calibration systems and to re-calibrate the instrument onground if required as part of reanalysis. Note 2: Supporting engineering data includes any on-board information considered necessary to reconstruct the calibration of S3NG-T MRAD and is specific to the implementation of the on-board calibration system e.g. thermistor values, scan position, feed temperature etc. Note 3: Access to pre-flight calibration information for uncertainty budget development is required by users. ESA UNCLASSIFIED – For ESA Official Use Only Page 131/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The consistency of brightness temperature retrievals from MRAD channels is important as L2 algorithms will use a combination of channels to derive measurements contained in geophysical products. In addition, depending on the implementation, MRAD may use multiple feeds each with their own characteristics in terms of calibration. The following requirements are designed to ensure that there is consistency both within a channel (regardless of the implementation used e.g. multiple feed chains) and between different bands in requirements addressing inter-channel and inter-band differences. MRD-1110 The full antenna gain pattern shall be known and made available to users for all MRAD feeds with sufficient information to resolve all sidelobes and grating lobes. Note 1: This information is fundamental for the proper production of Level 1 measurement data from Level 1a data because the energy from side lobes must be: (1) determined from the MRAD measurements and (2) used in Level 1 algorithms to adjust the calculated brightness temperature to compensate for these additional out of field radiance sources (3) the gain patters are required for use in the Level 1 uncertainty modelling. Note 2: When using L1 products, users may choose to apply different method depending on the specific application. Note 3: This information is typically derived from on-ground characterisation. Note 4: The antenna gain pattern may vary as a function of azimuth and must be well known with cuts sufficient to resolve all side-lobes. Note 5: The uncertainty of the antenna gain patterns will be used in the MRAD end to end uncertainty budget and performance. MRD-1120 Requirement deleted. MRD-1130 Requirement deleted. Geolocation is inextricably linked to Absolute Performance Error (APE) of the instrument pointing accuracy (attitude) and the uncertainty of this parameter expressed as an Absolute Knowledge Error (AKE, see ESSB-HB-E-003, 2011). These parameters are important inputs to the L1 and L2 scientific processing systems. Over the open ocean where no reference ground targets are available, geo-location relies on knowledge gained from instrument and platform pointing. Pointing errors may be systematic affecting all beams or may be attributable to individual beams (Purdy et al., 2006). Achieving good geolocation accuracy depends on excellent knowledge of a number of in-flight parameters including beam mis-pointing, roll, pitch, yaw uncertainty and pre-launch characterisation knowledge – all depending on the implementation of MRAD. This information must also be supplemented using a variety of statistical analyses used to validate geolocation uncertainty based on the use of in-flight data, ground control points and coastline/coastline gradient analyses (e.g. Purdy et al. 2006) and or other methods. ESA UNCLASSIFIED – For ESA Official Use Only Page 132/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 It is important to select a reference location for a L1 measurement for geolocation uncertainty validation which in the case of CIMR will be beam dependent. The antenna boresight provides the position of the peak of the -3dB antenna gain and is thus the location on the Earth surface that is most representative of the signal received. MRD-1140 The geo-location uncertainty of S3NG-T MRAD Level 1 measurement shall be better than 10% (3 sigma, zero-mean) of the footprint spatial resolution on ground, evaluated for each beam, at the antenna boresight position on ground, corresponding to mid-point for a given measurement integration interval. Note 1: Geolocation applies to all frequencies and all polarisations. Note 2: This requirement is to be met at L1. Note 3: This requirement should also be considered with the colocation with the altimeter data. MRD-1150 Requirement deleted. MRD-1160 Requirement deleted. MRD-1170 Requirement deleted. 6.2.1 Radio Frequency Interference (RFI) Mitigation Requirements. RFI is an increasing problem for satellite microwave radiometers due to the introduction of 5G telecommunication networks and is a major risk to S3NG-T MRAD data quality if not addressed properly using RFI detection and mitigation strategies implemented onboard (e.g. Kristensen et al. 2019) and on-ground (e.g. Uranga et al. 2018). Due to the potentially high data rates required to mitigate RFI using time-frequency domain analysis, on-board processing is likely to be preferred. In this approach, it is important that when RFI is detected information used to detect and mitigate the RFI contamination is also sent to ground so that this can be included in reprocessing activities (potential impact on NEΔT computation) and help to identify and close unauthorised RFI signals from ground (or space) infrastructure according to ITU regulations.. MRD-1180 MRAD shall be able to detect and mitigate Radio Frequency Interference (RFI) for frequency channels below 40 GHz. Note 1: RFI is a source of significant uncertainty in L1 products and must be clearly identified in the measurement data if they are to be scientifically useful by the operational user community. ESA UNCLASSIFIED – For ESA Official Use Only Page 133/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Note 2: To support the correct verification and evolution of on-board processor RFI detection and mitigation algorithms in a changing RFI environment (e.g. 5G networks) on-ground RFI management activities and re-processing is required. Note 3: RFI detection and mitigation could be performed on-board or as part of the ground processing depending on the available data rates and MRAD channel selection. Note 4: RFI must be flagged in the L1 and L2 products in the LF channels. Note 5. See figure 6.2.1-1 MRD-1190 Requirement deleted. MRD-1200 If Radio Frequency Interference (RFI) of a measurement is detected and mitigated, relevant data generated by the RFI processor for the affected measurement shall be available to users. Note 1: This requirement allows the performance of RFI filtering on-board to be assessed by the scientific and operational user community. Note 2: “relevant data” would include statistical indicators such as thresholds and kurtosis, etc. Typically, a spectrogram of RFI would be down linked to ground for further processing and RFI management (ITU reporting and RFI source management). Note 3: A diagnostic mode could be included (to be used as an example in commissioning) to downlink the data before and after RFI mitigation. This can be useful for RFI algorithm/parameters tuning. ESA UNCLASSIFIED – For ESA Official Use Only Page 134/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 6.3 Geodesy Requirements The heritage Sentinel-3 mission includes a suite of instruments for POD which is extremely important for a climate quality mission. A Doppler Orbitography and Radiopositioning Integrated from Space (DORIS) sub-system is used within the NRT processing. The instrument also provides an Ultra-Stable Oscillator (USO) that provides the clock for the altimeter retrievals and positional information allowing improved altimeter surface tracking and improving coastal and inland water retrievals. For Sentinel-6 Michael Freilich, Sentinel-3C and Sentinel-3D a Global Navigation Satellite System (GNSS-POD) includes the ability to track both GPS and Galileo constellations. The GNSS-POD provides the internal reference clock pulse to all the equipment on board including the payload. The instrument provides tracking data that are used in the POD processing. In terms of GNSS, a multi-frequency, multi-constellation approach is anticipated. Based on offline processing of Galileo dual-constellation pseudo-range and carrier phase measurements as well as broadcast ephemerides in a sequential filter with a reduced dynamic force model, navigation solutions for Sentinel-6 are extremely good with a representative position error of 10 cm (3D RMS) are achieved (Montenbruck et al, 2021; 2022, Darugna et al. 2022). For Sentinel-6, using satellite laser ranging (SLR) from selected high-performance stations, < 1 cm RMS consistency between SLR normal points and Galileo enabled GNSS-based orbits is obtained, which further improves to 6mm RMS when adjusting site-specific corrections to station positions and ranging biases (Montenbruck et al, 2021). While Galileo measurements exhibit 30–50% smaller RMS errors than those of GPS, the POD benefits most from the availability of an increased number of satellites in the combined dual-frequency solution (Montenbruck et al, 2021). In addition, Galileo GNSS-based onboard orbit determination can now reach a similar performance as the DORIS navigation system. It lends itself as a viable alternative for future remote sensing missions (Montenbruck et al, 2022). For the radial orbit component, a bias of less than 1mm is found from the SLR analysis relative to the mean height of 13 high-performance SLR stations (Montenbruck et al, 2022). In Sentinel-6 the Galileo enabled radial orbit component shows a bias of less than 1mm from the SLR analysis relative to the mean height of 13 high-performance SLR stations (Montenbruck et al, 2021). Sentinel-6 real time onboard velocity estimation accuracy (with respect to the CNES-DORIS reference) is already less than 0.1 mm/s using Galileo only and the dynamic model without HAS (Darugna et al, 2022). Galileo GNSSbased onboard orbit determination can now reach a similar performance as the DORIS navigation system (Montenbruck et al, 2022). Use of the Galileo E6 code High Accuracy Service (HAS) Service is presumed (https://www.gsc-europa.eu/sites/default/files/sites/all/files/E6BC_SIS_Technical_Note.pdf). Depending on the quality of its products, the upcoming Galileo HAS service might further improve such performance. It will provide orbit, clock, code and phase bias corrections, refining the GNSS orbits and clocks, which have been demonstrated to highly affect the P2OD accuracy and precision. The Galileo HAS will be based on the free transmission of Precise Point Positioning (PPP) corrections through the Galileo E6 ESA UNCLASSIFIED – For ESA Official Use Only Page 135/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 signal data component (E6-B) by the Galileo satellites providing improved performance compared to Sentinel-6 results of Montebruck et al (2021, 2022) and Darugna et al. (2022). By the time S3NG-T launches, the Galileo-2 system is expected to be operational with improved POD performance is anticipated. A Laser Retro-reflector Array (LRA), is used with global satellite laser ranging stations to provide independent tracking measurements for the POD processing or POD validation. Star-Trackers are used within the on-board Attitude and Orbit Control System (AOCS) providing attitude information (quaternions) are sensitive to platform ‘mis-pointing’ from the nominal local normal yaw steered. MRD-1210 In addition to any on-board Attitude Orbit Control System (AOCS) solution, individual unprocessed attitude sensor data (e.g. from each star-tracker) shall be available to users. Note 1: For example, quaternion data from star trackers with respect to the inertial reference frame is required by Users for geodesy for orbit modelling applications. Note 2: Information on the positioning of sensors relative to CoM is required by Users Note 3: Position, Velocity and Time information is required by users. MRD-1215 Should multiple satellite be used to address S3NG-T mission requirements, differences between the measurements derived from individual satellites shall be known and monitored on a daily basis. Note 1: Knowledge of differences across the constellation of satellites that comprise the S3NG-T mission is essential to avoid introducing artefacts into higher order applications (e.g. data assimilation) and products (e.g. Level 3 multimission maps). Note 2: This requirement implies a need for a robust inter-calibration methodology (e.g. based on the use of Sentinel 6 as a reference, minimization of crossover differences, use of transponders, corner cube reflectors, etc.) ESA UNCLASSIFIED – For ESA Official Use Only Page 136/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 7 DATA PRODUCT REQUIREMENTS The Sentinel-3 Surface Topography Mission provides Level 1 and Level 2 Near Real Time (NRT), Short-Time Critical (STC), and Non-Time Critical (NTC) data products. While these are currently delivered in discrete segments, the exact temporal or spatial extent of each product may vary depending on the processing level, geographic domain, and application needs. Future implementations may support user-driven customization of product granularity. 7.1 Product Delivery Timeliness and Availability Requirements In alignment with the operational standards of the Copernicus ground segment for the first-generation Sentinel series, stringent requirements are established for the timeliness and availability of Near Real-Time 3-hour (NRT3H) ocean product deliveries. However, other applications, such as those focused on sea ice and inland water, may require different near real-time latencies. To address the diverse needs of these applications, the following data product latency categories are specified. Table 7.1-1: S3NG-Topo mission latency requirements. The Timeliness specifies time elapsed from sensor acquisition to the user point of pickup. Application Domains are provided here for information only. Latency Timeliness and Availability Application Domain Near Real Time 3-hour (NRT3H) 97% of data within 3-hours Open Ocean, Coastal Areas. Near Real Time 6-hour (NRT6H) 97% of data within 6-hours Inland Waters (goal), Sea Ice Freeboard. Near Real Time 24-hour (NRT24H) 97% of data within 24-hours Inland Waters (threshold), Sea Ice Thickness Short Time Critical (STC) 97% of data within 36-hours Open Ocean, Coastal Areas. Non Time Critical (NTC) 97% of data within 30-days All Domains. MRD-1220 The S3NG-T mission shall provide users with Level 1 and Level 2 products within the timeliness and availability outlined in Table 7.1-1, tailored to the requirements of each specific application, as specified in subsequent requirements. Note 1: Timeliness is defined as the time taken from measurement acquisition to delivery of a product at the user point of pickup. Note 2: See definition of availability. ESA UNCLASSIFIED – For ESA Official Use Only Page 137/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 7.2 Product Format Requirements Heritage approaches in the altimetry community and within Copernicus follow NetCDF like format (https://www.unidata.ucar.edu/software/netcdf/) for Level 1 and higher level data products (e.g. Scharroo et al, 2016) . However, given the rapid evolution of cloudnative technologies, distributed computing frameworks, and data access standards, alternative data formats offering improved scalability, interoperability, and discoverability may become more suitable by the time S3NG-T reaches operations and should not be excluded. MRD-1230 S3NG-T Level 1 and higher-level data shall be formatted in an easily accessible format. Note 1: At the time of writing, Network common Data format (NetCDF) is the most commonly used data product format for altimetry, see https://www.unidata.ucar.edu/software/netcdf/, which fulfil this requirement. Note 2: Due to the fast evolution of cloud-native technologies, distributed computing frameworks, and data access standards, other data formats will become available and could be more suitable for the mission. Note 3: At the time of writing, the most commonly used data products follow Climate and Forecast (CF) Convention, see http://cfconventions.org/, fulfil this requirement. Note 4: At the time of writing, the current Attribute Convention for Data Discovery (ACDD) metadata convention (see http://cfconventions.org/ and http://wiki.esipfed.org/index.php/Category:Attribute_Conventions_Dataset_Di scovery ) or emerging frameworks such as the Spatio Temporal Asset Catalog (STAC) (STAC: SpatioTemporal Asset Catalogs), fulfil this requirement. 7.3 Level 0 Data Product Requirements Level 0 data products contain time stamped native instrument source packets data downlinked from the spacecraft. These are typically not distributed to users. 7.4 Level 1 Data Product Requirements 7.4.1 Level 1a Data Products L1a is the foundation product that carries all information from S3NG-T with all calibration and geolocation data available but not applied (i.e. the data remain in counts and instrument geometry and very close to a L0 product). MRD-1240 Level 1a data products shall be produced and made available to users. Note 1: This essential requirement provides flexibility for the ground processors and end user community because L1a products maintain data in native geometry. Note 2: L1a products need not necessarily be produced at NRT latency. ESA UNCLASSIFIED – For ESA Official Use Only Page 144/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Note 1: It is recognised that different data processing and data formats may be required for the different application domains. However, this requirement is aimed at achieving, as much as possible, Level 2 geophysical variable consistency at product boundaries between different operating modes and application domains (e.g. at the coastal ocean and open ocean boundary or SSH between open ocean and sea ice leads). 7.5.1 Ocean Product Requirements MRD-1450 The S3NG-T Level 2 nadir altimetry products shall contain measurements averaged at 20 Hz (enhanced goal: 80 Hz) and 1 Hz time intervals. Note 1: 1 Hz measurement is a standard used by satellite altimetry and provides continuity with Sentinel-3 Note 2: The S3NG-T MAG requested improved measurement sampling frequency compared to Sentinel-3 which provides Level 1a posting of 80 Hz and a Level 1b posting of 20 Hz. MRD-1460 The following reference data shall be included in S3NG-T ocean data product files: • Ocean depth (bathymetry) is given as negative values; land elevation as positive values; • Height of the geoid above the reference ellipsoid (WGS84) computed from the model with a correction to refer the value to the mean tide system i.e. includes the permanent tide (zero frequency); • Mean sea surface height above reference ellipsoid (WGS84); • Distance to the coastline (derived from the Marine Land Mask); • Others (TBD). Note 1: Reference data is used to compute geophysical quantities Note 2: The list of reference depends on the final altimeter design. ESA UNCLASSIFIED – For ESA Official Use Only Page 145/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 7.5.1.1 Sea Surface Height Requirements Table 7.5.1.1-1 specifies the total standard uncertainty (TSU) for the 1-Hz Level 2 sea surface height measurements and provides an apportionment to its main contributors. These figures are based on Sentinel-3A and Sentinel-3B in-flight performance (See Table 2.4-1 for in flight performance of Sentinel-3) and are adjusted based on anticipated S3NG-T developments. Table 7.5.1.1-1. Total Standard Uncertainty (TSU) budget for S3NG-T SSH estimation. All data specified for a 1 Hz integration time at Hs=2 m. (all values in cm, 1-sigma zero mean) Parameter (cm) NRT3H STC NTC REQ GOAL REQ GOAL REQ GOAL Range noise ≤0.8 ≤0.5 ≤0.8 ≤0.5 ≤0.8 ≤0.5 Ionospheric path delay ≤0.5 ≤0.3 ≤0.4 ≤0.3 ≤0.3 <0.3 Sea State Bias ≤2.0 ≤1.5 ≤2.0 ≤1.5 ≤1.5 ≤1.0 Dry tropospheric path Delay ≤0.7 ≤0.5 ≤0.7 ≤0.5 ≤0.5 ≤0.5 Wet tropospheric path delay ≤1.2 ≤1.0 ≤1.2 ≤0.7 ≤0.7 ≤0.7 TSU altimeter range measurement noise ≤2.61 ≤1.96 ≤2.59 ≤1.82 ≤1.93 ≤1.44 Radial orbit (RSS) ≤3.0 ≤3.0 ≤1.5 ≤1.0 ≤1.0 ≤0.8 TSU sea surface height (SSH) measurements ≤3.98 ≤2.47 ≤3.00 ≤2.08 ≤2.17 ≤1.65 MRD-1465 The total standard uncertainty (TSU) of the 1-second along-track averaged sea surface height (SSH) measurements over the ocean surface shall be less than the values specified in Table 7.5.1.1-1 for the prescribed data latencies. Note 1: This requirement may be validated by intercomparison with other altimeter missions or independent in situ measurements. Note 2: The altimeter range noise depends on Hs. For NTC products estimates (e.g. Sentinel-6 suggests uncertainties of 1.2 cm at 1 m Hs, 1.5 cm at 2 m Hs, 2.4 cm at 5 m Hs, and 3.2 cm at 8 m Hs. Note 3: The total RSS for sea surface height shall be met by any satellite instrument concept regardless of retrieval technique. Note 4: Deleted. Not relevant. Note 5: For a swath altimetry concept, the 1-Hz along-track averaged measurement should be translated to a pixel size equivalent to the nadir altimeter footprint, that is, ~6.6 km along-track by ~7.5. km across-track Note 6: For a swath altimetry concept, systematic errors, not apportioned in Table 7.5.1.1-1, should also be included in the TSU of the sea surface height measurements. ESA UNCLASSIFIED – For ESA Official Use Only Page 146/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1470 The contribution of the ionospheric path delay to the total standard uncertainty budget of the 1-second averaged altimeter range measurements over the ocean surface shall be less than the values specified in Table 7.5.1.1-1. Note 1: A dual frequency nadir looking altimeter should be able to meet this requirement based on Sentinel-3, Sentinel-6 and AltiKa inflight performance. Note 2: Ka-band is much less affected by the ionosphere than Ku-band. The attenuation due to the ionosphere is proportional to the inverse of the frequency squared, so the attenuation in Ka-band is about seven times lower than in Ku-band. (Steunou et al., 2015). AltiKa attains an ionospheric correction of 0.3cm (Verron et al 2015) that is the same as Sentinel-6 Kuband NTC performance Note 3: Requirement is set for Sentinel-3 continuity, goal is equivalent to Sentinel-6 NTC performance. Note 4: To be assessed using different techniques including comparison to GIM and other models (e.g. Zhao et al., 2021). A Sea State Bias correction is designed to compensate for the impact of sea state effects in the altimeter range estimate within the altimeter footprint. It includes three components: electromagnetic (EM) bias and skewness bias, (Gaspar et al., 1994). For a nadir altimeter like Sentinel-3, re-tracker bias includes the impacts of the waveform retracking algorithm used. The SSB correction varies from a few centimetres to a few decimetres depending on different sea states, and thus playing an important role in reducing SLA noise (Labroue et al., 2006). SSB corrections use U10 and Hs derived from altimeter Sigma0 measurements. Pires et al. (2016) and Tran et al. (2006) highlight the use of wave period to improve the SSB correction. In the coastal regions, SSB is more challenging due to the variable characteristics of Hs. Since understanding of the mechanisms causing the SSB is limited, there is scant theoretical modelling of the SSB correction (Peng et al., 2020). More advanced techniques may be required for swath altimeter solutions. Note that the use of a SAR Ka-band nadir pointing altimeter would potentially enable better sensitivity to surface roughness providing a means to analyse and reduce the impact of sea state on SSH measurements (e.g. smaller footprint compared to Ku-band instruments, vertical wave velocity, resonance with smaller waves to ˜8 m) depending on the instrument configuration. This is important since sea state is a major uncertainty term in altimetry measurements from space. New techniques of SSB must be developed to reduce this uncertainty ideally based on measurements. For example, use of fully-focused SAR altimeter data, measurement pulse correlations (e.g. Egido and Smith 2019) and off-nadir or nadir-pointing microwave radiometer measurements having sensitivity to surface ocean roughness (e.g. Tran et al., 2002; Tran and Chapron, 2006). The directional wave spectrum (e.g. from CFOSAT) may help improve knowledge of SSB and lead to new approaches. ESA UNCLASSIFIED – For ESA Official Use Only Page 147/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1480 The contribution of the sea state bias to the total standard uncertainty budget of the 1-second averaged altimeter range measurements over the ocean surface shall be less than the values specified in Table 7.5.1.1-1. Note 1: Progress toward more advanced Sea State Bias correction is required to improve radar altimeter measurements regardless of the technical concept. Note 2: This requirement may be validated by intercomparison with other altimeter missions, independent in situ sea state data and wave model outputs. Note 3: wave spectral information be available from S3NG-T (MRD-1650), these estimates may be used in the determination of the SSB correction. Atmospheric losses on the radar altimeter signal increase rapidly with frequency and at Ka-band they are larger than at Ku-band. A drawback of the use of Ka-band is that the attenuation of the signal due to liquid water (rain and clouds), that can be significant notably in the tropical regions. This is a constraining factor since limitation of the altimeter link budget imposes an attenuation of the signal. At Ka-band, Tournadre et al. (2009) suggest a 3dB atmospheric loss for the AltiKa mission. The CRISTAL mission sues a value of 1 dB at Ka-band. Tournadre et al. (2009) note that for light rain and/or cloud liquid water content even for small rain cells and/or clouds, can significantly distort the echo waveform and degrade the accuracy of the geophysical parameters inferred by waveform analysis. The Sentinel-3 dual frequency altimeter (Kuand C-band) uses differential attenuation to detect and flag precipitation that would not be possible for a single frequency Ka-band altimeter. Clouds and rain are characterized by sharp coherent along-track fluctuations of the off-nadir angle and a Matching Pursuit (MP) flagging algorithm has been developed and implemented for AltiKa based on the analysis of these fluctuations. Comparison to simulated operational dual-frequency flag shows that the MP flag performs better in detecting range errors and waveforms distortion, while its performances are inferior in detecting samples attenuated by rain. However, in-orbit AltiKa data show that the MP algorithm exceeds expectation and rain had little influence on data availability and quality (Verron et al 2020). MRD-1490 The contribution of the dry tropospheric path delay to the total standard uncertainty budget of the 1-second averaged altimeter range measurements over the ocean surface shall be less than the values specified in Table 7.5.1.1-1. Note 1: Deleted.. Note 2: Dry tropospheric path delay corrections are typically derived from numerical weather prediction models (e.g. Fernandez et al, 2021). Note 3: Requirement is set for Sentinel-3 continuity, goal is equivalent to Sentinel-6 NTC performance. Note 4: This requirement could be met using validated atmospheric model outputs. ESA UNCLASSIFIED – For ESA Official Use Only Page 148/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Note 5: This requirement assumes use of improved ECMWF model in line with the ECMWF 2021-2030 Strategy (https://www.ecmwf.int/sites/default/files/elibrary/2021/ecmwf-strategy-2021-2030en.pdf) MRD-1500 The contribution of the wet tropospheric path delay to the total standard uncertainty budget of the 1-second averaged altimeter range measurements over the ocean surface shall be less than the values specified in Table 7.5.1.1-1. Note 1: See Table 2.4-1 for in flight performance of Sentinel-3. Note 2: Wet tropospheric path delay measurements should be derived from microwave radiometer instruments that are part of the S3NG-T payload. Note 3: Requirement is set for Sentinel-3 continuity, goal is equivalent to Sentinel-6 NTC performance. Note 4: For inland waters, GNSS may be the best source for wet path delay retrieval (Fernandes et al., 2014; Vieira et al., 2018). In addition, GNSS plays a major role in filling the gap left by MWR in coastal regions. Note 5: This requirement could be validated using satellite sounder measurements, radiosonde profiles and atmospheric model outputs. The Sentinel-3 orbit is provided by the Copernicus POD Service (e.g. Fernandez et al, 2015) that continues to evolve (e.g. Peter et al, 2020). MRD-1510 At any point in the orbit, the contribution of the radial (ellipsoid normal) orbit position and velocity to the total standard uncertainty budget of the 1-second averaged altimeter range measurements over the ocean surface shall be less than the values specified in Table 7.5.1.1-1. Note 1: It is assumed that the Copernicus POD Service will provide orbit information. Note 2: Relativistic and ellipsoidal clock corrections should be accounted for. Note 3: Results of the Sentinel-3A and Sentinel-3B latest Copernicus POD Service (Regular Service Review) can be found at https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-3altimetry/pod/documentation MRD-1520 Requirement merged with MRD-1465. MRD-1530 Requirement deleted. ESA UNCLASSIFIED – For ESA Official Use Only Page 149/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1540 The surface elevation measurement drift of the S3NG-T mission altimeter system shall be below 0.5 mm/yr (TBC) over a 5 year period (TBC). Note1: This requirement refers to the residual drifts after calibration of known instrumental effects. Note 2: This requirement includes contributions of the S3NG-T altimeter, MWR, and POD suite. Contributions beyond the S3NG-T mission scope, such as ocean internal variability, ITRF geo-centre motion, gravity field trends, etc., are excluded. Note 3: It is anticipated that Sentinel-6NG will provide the reference mission for global and regional mean sea level. Note 4: This requirement may be verified with the use of transponders and/or corner reflectors. MRD-1550 For scales smaller than 1000 km S3NG-T SSH measurements shall have a total standard uncertainty below 1.7 cm (TBC) after inflight calibration for typical sea state values of 2 m. Note 1: This equates to a time scale of < ~3 minutes and addresses random noise and high-frequency correlated uncertainties. Note 2: Separation between longand short-scale uncertainties for end-to-end performance can be useful depending on the final technical concept. MRD-1560 For scales larger than 1000 km and smaller than 40000 km (TBC) S3NG-T SSH measurements shall have a total standard uncertainty below 1.4 cm (TBC) after inflight calibration for typical sea state values of 2 m. Note 1: This equates to a time scale of > ~3 minutes and addresses low frequency systematic uncertainties. Note 2: Separation between longand short-scale uncertainties for end-to-end performance can be useful depending on the final technical concept. Sea level anomaly is an essential variable for oceanic operational system as it gives outstanding information both on the small scales dynamics and climate change. MRD-1570 The S3NG-T mission shall provide Level 2 sea surface height anomaly measurements. Note 1: Sea surface height anomaly = altitude of satellite - corrected ocean altimeter range - geophysical corrections - mean sea surface ESA UNCLASSIFIED – For ESA Official Use Only Page 150/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1580 The S3NG-T mission shall provide Level 2 absolute dynamic topography measurements after ground processing. Note 1: The Absolute Dynamic Topography (ADT = SSH – geoid) cannot be directly measured by altimetry but can be derived from subtracting the Geoid to the altimetric Sea Surface Height (SSH) or, alternatively, from adding the MDT to the altimetric SLA: Note 2: This requirement implies application of a very high-resolution geoid for the S3NG-T mission 7.5.1.2 Sea State Requirements The sea surface wave field exhibits a very complex and highly dynamic moving surface (both vertically and horizontally) that impacts the quality of radar ranging with Hs varying from a few cm to considerably more than 20 m (e.g. Hanafin et al, 2021). , see Figure 7.5.1.2-1). MRD-1590 The S3NG-T mission shall provide Level 2 significant wave height, Hs, measurements for all sea surface height estimates provided by the altimeter payload. Note 1: Hs is a baseline requirement for continuity of Sentinel-3 measurements. Note 2: See Greilier et al. (2016). The term significant wave height is historical as this value appeared to be well correlated with visual estimates of wave height from experienced observers. It can be shown to correspond to the average 1/3rd highest waves (H1/3). Note 3: Swell is known to impact altimeter SSH measurements through aliasing (e.g. Moreau et al, 2018; Rieu et al, 2021) when orthogonal to the satellite ground track. Directional wind sea waves and swell estimates would help to monitor and mitigate these issues. Note 4: Spectral and directional spectral sea state parameters are the most important measurements to support future evolution of CMEMS wave forecasting models and CMEMS ocean atmosphere coupled models and C3S coupled systems since the exchange of heat, gas, and momentum occurs via the waves. Furthermore, at high resolution, it becomes extremely challenging to separate SSH for sea state since the sea state defines the scattering surface to which a radar instrument responds. This is already seen in moderate resolution nadir pointing altimeters (e.g. Boy et al., 2016, Moreau et al., 2018, amongst others)Note 5: This requirement ensures that sufficient information is available to interpret SSH measurements and apply sea state bias corrections. SSB corrections use U10 and Hs derived from altimeter Sigma0 measurements. Note 6: Pires et al. (2016) and Tran et al. (2006) highlight the use of wave period to improve the SSB correction. Note 7: In the coastal regions, SSB is more challenging due to the variable characteristics of Hs (Peng et al. 2020) ESA UNCLASSIFIED – For ESA Official Use Only Page 151/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1600 The S3NG-T mission shall provide Level 2 significant wave height, Hs, up to 20 m (enhanced goal: 25 m (TBC)). Note 1: This dynamic range includes Hs for extreme sea states. Note 2: The Sentinel-3 SRAL specification is 0-20m Note 3: This requirement may need to be validated for different dynamic range due to the limited number of extreme sea state reference measurements (extreme sea states are local phenomena and wave buoys, typically used to validate altimeter Hs do not function well in these conditions). Because of this, Sentinel-6 validated Hs over a range of only 0.5-8m. Note 4: Since Hs is used as part of the sea state bias estimation, all range measurements form the altimeter must be accompanied by an estimate of Hs. Note 5: The standard deviation/variance of the significant wave height measurement should be included in the Hs products to assist in SSB determination to capture gradient changes. MRD-1610 Merged with MRD-1590. MRD-1620 The S3NG-T mission shall provide Level 2 significant wave height, Hs, with a total standard uncertainty for 1-Hz measurements (or equivalent pixel size) below 15 cm plus 5% Hs in the range of 0.5 to 8 m (enhanced goal 0.5 - 15 meters). Note 1: This is baseline continuity from Sentinel-3. Note 2: Requirement is set for Sentinel-3 continuity, enhanced goal is equivalent to the Sentinel-6 requirement. Note 3: This requirement may be validated by intercomparison with other altimeter missions or independent in situ measurements. Note 4: Extreme sea states as seen in radar altimeters today (e.g. Hanafin et al., 2021) are local phenomenon and reference in situ wave buoys, typically used to validate altimeter Hs, do not function well in these conditions. As measurements are likely to be scarce for extreme sea states, validation of Hs measurements outside in the range of 0.5 to 8 m is considered sufficient for each satellite commissioning activities. MRD-1630 Merged with MRD-1620. MRD-1640 Merged with MRD-1620. User needs for maritime Surveillance and from European Maritime Safety Agency (EMSA) request new/additional products that are needed for search and rescue operations, sea surface pollution (including marine plastic debris) and vessel navigation/routing purposes. Only through wave induced flows can such activities lead to successful outcomes since the Stokes drift is fundamental to the ocean surface ESA UNCLASSIFIED – For ESA Official Use Only Page 152/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 dynamics and is completely different to the geostrophic flows inferred from SSH. Because of this, these products are also fundamental to the future evolution of Copernicus Wave Forecasting and coupled ocean-atmosphere model capability in the 2030 timeframe. Finally, many European directives and policies, particularly in the coastal zone, require at least Hs and ideally wave directional spectra for their successful implementation and monitoring of policy impact. MRD-1650 As a Secondary Mission Objective, the S3NG-T mission shall be capable of producing Level 2 Wave Directional Spectrum, E(k,j), estimates. Variation of wave energy in directions should have a directional resolution of ≤ 15° (enhanced goal: 10°) for wavelengths longer than 100 m (enhanced goal: 50 m). The uncertainty should be ≤ 10% for energy and wavelength quantities for Hs in the range of 0.5 to 8 m (enhanced goal 0.5 - 15 meters). Note 1: A 2D directional spectrum can be provided from nadir altimetry as explained by Altiparmaki et al. (2022). Should the directional wave spectrum not be feasible, a 1D wave spectrum should be provided. Note 2: This is a fundamental data product for many Copernicus applications in support of many European directives and policies. Note 3: Spectral and directional spectral sea state parameters are the most important measurements to support future evolution of CMEMS wave forecasting models and CMEMS ocean atmosphere coupled models and C3S coupled systems since the exchange of heat, gas, and momentum occurs via the waves. Furthermore, at high resolution, it becomes extremely challenging to separate SSH from sea state since the sea state defines the scattering surface to which a radar instrument responds. This is already seen in moderate resolution nadir pointing altimeters (e.g. Boy et al., 2016, Moreau et al, 2018 amongst others). CMEMS already make use of CFOSAT/SWIM directional wave spectrum measurements. Note 4: This requirement is significant for the improvement of ocean/waves coupled systems managed by CMEMS-MFCs. Accounting of directional wave energy leads to better parametrization of wind-waves growth in the wave model for storm events. Note 5: This requirement is relevant for better estimate of wave forcing into CMEMS ocean models (e.g. Wang et al, 2021) including wave-induced stress, Stokes drift and wave breaking induced turbulence for upper ocean layer mixing, marine debris (e.g. plastic, oil radioactive) tracking, and search and rescue. Note 6: As a Secondary Mission Objective, the S3NG-T Level 2 swell number for partition i, Hs(i), of the unambiguous directional wave spectrum E(k, j ) should have a combined uncertainty of £ 30 cm or £ 10 (TBC)% whichever is greater. Note 7: As a Secondary Mission Objective, the S3NG-T Level 2 peak period for partition i, Tp(i), of the unambiguous directional wave spectrum E(k, j ) should have a combined uncertainty of £ 1 s if Hs(i) ³ 30 cm. Note 8: As a Secondary Mission Objective, the S3NG-T Level 2 mean direction for partition i, j m(i), of the unambiguous directional wave spectrum E(k, j ) should have a combined uncertainty of £ 10 ° if Hs(i) ³ 30 cm. ESA UNCLASSIFIED – For ESA Official Use Only Page 153/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Note 9: As a Secondary Mission Objective, the S3NG-T Level 2 directional spread for partition i, s si, of the unambiguous directional wave spectrum E(k, j ) should have a combined uncertainty of £ 15 ° if Hs(i) ³ 30 (TBC)cm. Note 10: See Caudal et al., (2014). Note 11: The priority is to support wave forecasting models and coupled oceanatmosphere models used by Copernicus applications. MRD-1660 As a Secondary Mission Objective, the S3NG-T Level 2 Wave Directional Spectrum, E(k,j), data products shall be made available to users at NRT3H, STC and NTC data latencies. Note 1: This is a fundamental data product for many Copernicus applications in support of many European directives and policies. MRD-1670 As a Secondary Mission Objective, the S3NG-T Wave Directional Spectrum products shall consist of (TBC): 1. Unambiguous directional wave spectrum E(k,j); 2. αu(k, ji) wavenumber function; 3. ασ(k,ji) wave direction function; 4. Cross spectrum C’uσ (k,j); 5. Hs, Significant Wave Height; 6. Hsi, swell number for partition i; 7. Tpi, peak period for partition i; 8. jm,i, mean direction of partition i; 9. Uncertainty estimates for each measurement; 10. Other relevant information (TBD). Note 1: The exact content of wave directional spectrum products depends on the final implementation of S3NG-T. Note 2: A wave product is available from Sentinel-1 data sets but has a long-wave cut-off at ~150m and thus does not address wind seas that are necessary for wave modelling and coupled ocean-atmosphere models in the 2030 time frame. Note 3: The priority is to support wave forecasting models and coupled oceanatmosphere models used by Copernicus applications. Note 4: Ideally, uncertainty should be less than 5-10% for Hs, wavelength and direction, in 2030 we should expect less than 7%. ESA UNCLASSIFIED – For ESA Official Use Only Page 256/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The analysis of present/future user requirements drive CMEMS service evolution that, in turn, leads to revised specification for the upstream satellite observations. User requirements (e.g. observing the ocean currents at 1 km resolution with a 10% accuracy) do not translate directly into satellite observation requirements. They must go through the added value chain of the service. Use of satellites for data collection and location and for navigation and security issues (AIS) are not considered here. Those are very important uses of satellite technology but CMEMS is not (or barely) a prescriber for these applications. CMEMS requirements for future satellite observations are thus based on an analysis of the most important satellite observations required to improve/constrain future CMEMS products and services. Continuity of the present Copernicus satellite observing system should be first guaranteed as this is mandatory for maintaining the CMEMS service. [S3NG-T-UN-133: S3NG-T should guarantee the continuity of Sentinel-3 products] This holds, in particular, for the Sentinel 6 altimeter reference mission and the two satellite constellation of Sentinel 3 (altimetry, Sea Surface Temperature and Ocean Colour) and Sentinel 1 (SAR). This satellite observing system must, moreover, be complemented by an adequate and sustained in-situ observing system. Note also that the very fine scale observations provided by the two satellite constellation of Sentinel 2 provide highly valuable potential information for the CMEMS and its coastal applications. Improvement of CMEMS offer is, however, required to better serve existing users and to anticipate future needs. The main areas of improvements (see also previous section) are: 1. Improved space/time resolution to better monitor and forecast the ocean at fine scale and to improve the monitoring of the coastal zone. [S3NG-T-UN-134: S3NG-T should improve the space and time resolution of topography measurements compared to baseline continuity of Sentinel-3 products to address the coastal zone] 2. Better monitoring of biogeochemical state of ocean. 3. Better monitoring of the rapidly changing polar regions. [S3NG-T-UN-135: S3NG-T should provide topography measurements to better monitor the rapidly changing polar regions compared to baseline continuity of Sentinel-3 products] This will require major evolution of satellite observing capabilities. When moving to higher resolution, it will be fundamental to constrain CMEMS models with new observations. [S3NG-T-UN-136: S3NG-T should provide new topography measurements to constrain CMEMS models] The most important satellite-based observation is sea surface height (SSH) from altimetry. The SSH is an integral of the ocean interior properties and is a strong constraint for inferring the 4D ocean circulation through data assimilation. [S3NG-TUN-137: S3NG-T should provide enhanced performance of sea surface height measurements (the most important satellite observation for CMEMS) compared to Sentinel-3] Multiple nadir altimeters (at least 4 altimeters) are required (this was required 10 year ago) to adequately represent ocean eddies and associated currents in models. [S3NG-T-UN-138: S3NG-T should include multiple nadir altimeters flying in constellation to adequately represent ocean eddies and associated geostrophic currents in models] Much higher space/time resolution (e.g. 50 km / 5 days) will be needed in the post 2025 time period. [S3NG-T-UN-139: S3NG-T should provide topography products with a ESA UNCLASSIFIED – For ESA Official Use Only Page 257/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 horizontal gridded resolution of ≤50 km and a temporal resolution of ≤5 days] This can be achieved through a combination of swath altimetry (to be demonstrated with the SWOT mission to be launched in 2021) with nadir SAR altimetry. Satellite requirements for the GMES/Copernicus Marine Service have been detailed in the GMES Marine Core Service (MCS) implementation group report (Ryder, 2007) based on the recommendations of the GMES MCS Space Working Group (Le Traon et al., 2006). The nomenclature has been appended with [Copernicus] to add clarity since the requirements are only marginally met by Copernicus today and remain valid. General recommendations: 1. Continuity of observation is crucial. This is particularly critical around 2010 when data gaps could occur for several of the most critical observations. Decisions for developing the first of the GMES [Copernicus] satellites must then be taken most urgently. 2. It is more critical to establish satellite series for sustainable service availability than to try optimizing the specifications and designing for any one satellite and its instruments, if the latter leads to expensive, non-renewable satellites. Establishing satellite series should lead to significantly lower production costs. [S3NG-T-UN140: S3NG-T should provide a series of satellites as a sustainable service] [S3NGT-UN-141: S3NG-T should avoid expensive, non-renewable satellites but focus on a satellite series that should lead to significantly lower production costs] 3. GMES [Copernicus] should allow for research and technological developments. In particular, the possibility of embarking new instruments with the potential to meet GMES [Copernicus] needs should be considered. Wide Swath altimetry and geostationary ocean colour are the two most important new technology developments that will benefit the GMES MCS [Copernicus CMEMS] in the long run. Specific recommendations: 1. The Jason series (high accuracy altimeter system for climate applications and as a reference for other missions) is an essential and critical component of the GMES [Copernicus] satellite programme for MCS [CMEMS]. [S3NG-T-UN-142: S3NG-T should use the Sentinel-6 reference altimeter to homogenise topography measurements ] Planning of Jason-3 must be a priority for GMES [CMEMS]. 2. The MCS [CMEMS] requires a high resolution altimeter system with at least three altimeters in addition to the Jason series. Sentinel-3 should include a constellation of two satellites, flying simultaneously, providing adequate coverage and operational robustness. [S3NG-T-UN-143: A minimum of two satellites should be available in the S3NG-T]. Instrumentation costs for S3 should be reduced as much as possible to allow for a two satellite system. 3. Compared to the present design of S3 instrumentation, the priority for Sea Surface Temperature is for high accuracy dual view measurements. The large swath requirement has a much lower priority, in particular (but not only), if S3 is a two satellite system. As far as Ocean Colour is concerned, a sensor having a similar spectral resolution to MERIS is essential to meet the important shelf and coastal ocean water quality measurement requirements. The use of a SeaWiFS type of ESA UNCLASSIFIED – For ESA Official Use Only Page 258/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 instrument (reduced number of channels) would serve only the minimum operational requirements for the open ocean. 4. SAR data (Sentinel 1) are required for oil spill detection and sea ice monitoring. This is clearly a European core service that should be considered as part of the MCS [CMEMS]. The requirement is for at least one and preferably two SAR missions in addition to the other non-European missions (e.g. Radarsat). 5. Access to other European and non-European (e.g. NPOESS, RADARSAT) satellite data in real time is fundamental for the MCS [CMEMS]. 6. Ground segment requirements will be addressed in a specific report/note. The main recommendation is likely to be that the GMES ground segment should develop strong interfaces with EUMETSAT Ocean&Sea Ice SAF and with the MCS satellite Thematic Assembly Centres (TACs). CMEMS Recommendations For Polar And Sea Ice Monitoring: • Continuation and improvement of the sea ice thickness time series from CryoSat-2. [S3NG-T-UN-144: S3NG-T should provide measurements of sea ice thickness] This is required both for climate and operational sea ice monitoring activities (including assimilation in sea ice models). • Continuation of the altimetry sampling over the ocean in Polar Regions to constrain ocean models through data assimilation (e.g. for improved ocean currents). [S3NGT-UN-145: S3NG-T should provide continuity of Sentinel-3 topography measurements over polar regions] • Reliable retrieval of sea level in the leads to reach the retrieval accuracy required to monitor Climate Change. [S3NG-T-UN-146: S3NG-T should provide measurements of sea level in the sea ice leads to reach the retrieval accuracy required to monitor Climate Change.] • Continuation of SMOS like observations of thin sea ice below 0.5 m. • Sustainable operation of medium-resolution (5-10 km) multi-frequency and - polarization passive microwave observations of SST, sea ice lead fraction and sea ice concentration, area and extent. [S3NG-T-UN-147: S3NG-T should provide measurements of sea ice lead fraction] [S3NG-T-UN-148: S3NG-T should provide measurements of sea ice concentration] [S3NG-T-UN-149: S3NG-T should provide measurements of sea ice parameters using a radar altimeter and a microwave radiometer (required for tropospheric wet path delay correction of the altimeter measurements) within the altimeter footprint (at a resolution of 5-10 km)] • Automated production of ice chart-like products from a combination of SAR data and other data (e.g. bi-static SAR, passive microwave, multi-frequency SAR). • Reliable retrieval of ocean colour in the marginal ice zone. III.3 C3S requirements for climate series UN-NOTE articulates the following User Needs for the Copernicus climate change services in terms of climate records. ESA UNCLASSIFIED – For ESA Official Use Only Page 259/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 The requirements for C3S given by ECMWF have been consolidated based on the requirements presented during the first ad-hoc expert group meeting by CNRS on climate science needs, and by Met Office about waves: • Scientific requirements for S6-NG: Benoit Meyssignac (CNRS/LEGOS), after discussions with P.Bonnefond, J.Willis and C.Watson with inputs from R.de Conto, A.Ribes, A.Slangen, K.Richter, M.Ablain, P.Prandi, R.Jugier, A.Guerou, A.Blazquez, S.Labroue, T.Guinle, N.Picot, J.Benveniste, E.Obligis, Briefing for ESA Copernicus Next Generation Topography Constellation (ad-hoc) Expert Group Meeting, 17-18 Feb. 2020. • User requirements for satellite observations of wave spectra and currents – an operational modelling perspective: Andy Saulter (Met Office), Surge and Wave Modelling Team, Briefing for ESA Copernicus Next Generation Topography Constellation (ad-hoc) Expert Group Meeting, 17-18 Feb. 2020. C3S team endorses the requirements presented for climate science are relevant also for C3S. C3S however emphasizes that this may not reflect needs for operational ocean data assimilation, where the focus would need to be different: regional sea-level and uncertainty for sea-level instead of trend. For such requirements, C3S will follow requirements established by CMEMS for coastal and data assimilation in general. Additional requirements would be: • the need for an up-to-date land-ocean mask (especially in coastal regions where sand islands can move or build by humans) • A good bathymetry data (especially crucial for shallow waters or near underwater ocean bed rise). Requirements for wave products as presented by Met-Office reflect C3S needs. Expected performances for C3S should be: • Return frequency order 2 days or less • Spectral sensor resolves wavelengths from less than 40m (~5 seconds wave period in deep water) to up to 600m (~20 secs);; • Along track sampling at least as per present generation instruments; • Swath capability, samples up to +/-100km across track – improved spatial sampling per pass; • Certified significant wave height range: 0.1 – 15m; • Collocation with wind/surface stress and surface current measurements; • Coastal sampling capability of at least within 2km of coastline and 10m water depth. NB: If a wave ECV is part of GCOS, C3S does not plan to develop such ECV the next MFF timeframe. ESA UNCLASSIFIED – For ESA Official Use Only III.4 European Commission Copernicus Level 2 Product and Performance Needs for S3NG-T in the 2030-2050 timeframe The UN-COPURD articulates S3NG-T product performance user needs in Table III.4.1. Table III.4.1 S3NG-T product performance user needs derived from EC User Needs documentation. UR-ID Requirement Description Source Copernicus User Coverage Spatial Resolution (km) Temporal Resolution (Days) Delivery timeliness Uncertainty Stability S3NG-T-UN-P01 Sea Surface Height (Global Ocean) UN-COPURD CMEMS, C3S, CAMS, CGLMS, EMSA Global ocean ≤50 gridded ≤ 15 (g: ≤ 5) NTC ≤4 cm (g:≤2 cm) ≤(TBC) S3NG-T-UN-P02 Sea Surface Height (Global Ocean) CMEMS (2017) CMEMS, C3S, CAMS, CGLMS, EMSA Global ocean ≤50 gridded ≤ 5 NTC ≤4 cm (g:≤2 cm) ≤(TBC) S3NG-T-UN-P03 Sea Surface Height (SSH) - 50 km, 15 days (C3S REQ UR-C3S-152) UN-COPURD C3S Global ocean ≤50 grid ≤10 ≤10 ≤4 cm (g:≤1 cm) (global mean) over a grid mesh of 50-100 km < 0.3 mm/yr (global mean) S3NG-T-UN-P04 Sea Surface Height (Coastal Ocean) UN-COPURD CMEMS, C3S, CAMS, CGLMS, EMSA Coastal ocean ≤50 grid ≤10 NTC ≤4 cm (g:≤2 cm) ≤(TBC) S3NG-T-UN-P05 Sea Surface Height (Coastal Ocean) UN-COPURD CMEMS, C3S, CAMS, CGLMS, EMSA Regional Coastal ocean ≤10 (g:≤5) ≤1 (g:≤0.5) NTC ≤3 cm (g:≤2 cm) ≤(TBC) S3NG-T-UN-P06 Global sea level (SSH) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤50 grid ≤10 ≤60 ≤0.2 cm at annual tome scales < 0.1 mm/yr <0.05mm/yr2 ESA UNCLASSIFIED – For ESA Official Use Only Page 261/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 over more than a decade S3NG-T-UN-P07 Global sea level (SSH, GCOS-154, C3S REQ: URC3S-45) C3S(2020) CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤50 ≤10 ≤10 ≤0.4 cm (g:≤0.2 cm ) (global mean); 1cm over a grid mesh of 50 - 100 km < 0.3 mm/yr (global mean) S3NG-T-UN-P08 Regional sea level (SSH) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤100 (g:≤50) ≤10 ≤60 ≤1cm over a grid mesh of 50 - 100 km < 0.5 mm/yr over more than a decade S3NG-T-UN-P09 Regional sea level (SSH GCOS-154) C3S(2020) CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤100 (g:≤50) ≤10 ≤60 ≤1cm over a grid mesh of 50 - 100 km < 0.5 mm/yr over more than a decade S3NG-T-UN-P10 Regional sea level (SSH GCOS-154C3S REQ URC3S-46) C3S(2020) CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤25 ≤7 ≤7 ≤1cm over a grid mesh of 50 - 100 km < 1 mm/yr (for a grid mesh of 50 - 100km) S3NG-T-UN-P11 Significant Wave Height (Hs) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤1 ≤1 (TBC) ≤5 cm ≤(TBC) S3NG-T-UN-P12 Significant Wave Height (Hs) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global Ocean ≤10 ≤1 (TBC) ≤10 cm ≤(TBC) S3NG-T-UN-P13 Significant Wave Height (Hs) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Regional Ocean ≤5 (g:≤1) ≤3 hour (g:≤1 hour) NRT3H ≤5 cm ≤(TBC) S3NG-T-UN-P14 Significant Wave Height (Hs) UN-NOTE CMEMS, C3S, CAMS, Global Ocean ≤5 (g:≤1) ≤3 hour (g:≤1 hour) NRT3H ≤5 cm ≤(TBC) ESA UNCLASSIFIED – For ESA Official Use Only Page 262/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 CGLMS, EMSA S3NG-T-UN-P15 Significant Wave Height (Hs, GCOS-154) (C3S REQ. URC3S-44) C3S(2020) CMEMS, C3S, CAMS, CGLMS, Global and Coastal Ocean ≤25 ≤3 hour NRT3H ≤10 cm ≤5c m [year?] S3NG-T-UN-P16 Directional Wave Energy Spectrum UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Costal Regional ≤1 km (g: ≤50 m Coastal regions ≤1 hour (g:≤ 0.5 hour) NRT1H ≤0.1 m2/Hz ≤(TBC) S3NG-T-UN-P17 Wave direction - 0.5 - 1 h UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Costal Regional ≤5 ≤1 hour (g:≤ 0.5 hour) NRT1H ≤0.05 s ≤(TBC) S3NG-T-UN-P18 Wave direction - 24 h UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Costal Regional ≤1 ≤1 NRT3H ≤0.05 s ≤(TBC) S3NG-T-UN-P19 Wave period (Tp, Tm01, Tm02) - 0.5 - 1 h UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Costal Regional ≤5 ≤1 hour (g: 0.5 hour) NRT3H ≤0.05 s ≤(TBC) S3NG-T-UN-P20 Wave period (Tp, Tm01, Tm02) - 24 h UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Costal Regional ≤ 1 ≤1 NRT3H ≤0.05 s ≤(TBC) S3NG-T-UN-P21 Wind speed over the ocean at 10m height (U10) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Coastal and Global ≤5 ≤1 hour NRT1H ≤ 2 m/s ≤(TBC) S3NG-T-UN-P22 Wind direction over the ocean at 10m height (U10) UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global ≤5 ≤1 hour NRT1H ≤5° ≤(TBC) S3NG-T-UN-P23 Surface Water Elevation over Rivers UN-COPURD CMEMS, C3S, CAMS, CGLMS, EMSA Global Rivers ≥100m width (g:≥50 m) ≤1 NRT3H ≤10 cm (g:≤5 cm) ≤(TBC) ESA UNCLASSIFIED – For ESA Official Use Only Page 263/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-P24 Surface Water Elevation over Rivers UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global Rivers ≥100m width ≤1 NRT3H ≤10 cm ≤(TBC) S3NG-T-UN-P25 Surface Water Elevation over Lakes and reservoirs UN-COPURD CMEMS, C3S, CAMS, CGLMS, EMSA Global Lakes ≥ 1 km² ≤1 NRT3H ≤10 cm (g:≤5 cm) ≤(TBC) S3NG-T-UN-P26 Surface Water Elevation over Lakes UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Continental coverage ≥1 ≤1 NRT3H ≤3 cm ≤(TBC) S3NG-T-UN-P27 Surface Water Elevation over Lakes UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Europe ≥0.5 ≤0.5 NRT3H ≤50 cm ≤(TBC) S3NG-T-UN-P28 Surface Water Elevation over Lakes UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global 100 km grid ≤1 month NTC ≤10 cm ≤(TBC) S3NG-T-UN-P29 Lake Level (GCOS-154 requirement, C3S REQ. UR-C3S-64) UN-COPUN C3S Lake level of all lakes in the Global Terrestrial Network for Lakes (GTN-L) N/A ≤1 month ≤1 month ≤50 cm ≤10 cm S3NG-T-UN-P30 Water extent UN-NOTE CMEMS, C3S, CAMS, CGLMS, Continental ≤0.2 ≤1 NRT3H 10 % (relative) 5% (for 70 largest lakes) ≤(TBC) S3NG-T-UN-P31 Water extent UN-NOTE CMEMS, C3S, CAMS, CGLMS, Continental ≤0.1 ≤30 NTC 10 % (relative) 5% (for 70 largest lakes) ≤(TBC) S3NG-T-UN-P32 Sea Ice Thickness UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Ocean coverage ≤25 ≤30 NTC ≤0.1 cm ≤(TBC) ESA UNCLASSIFIED – For ESA Official Use Only Page 264/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-P33 Sea Ice Thickness UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Regional coverage ≤0.1 ≤1 NTC ≤ 1 cm ≤(TBC) S3NG-T-UN-P34 Sea ice type UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Ocean coverage ≤1 km (g: 100 m) for shipping and search and rescue <10 m (g: ≤3m) in highly infested ice regions ≤1 NRT3H ≤(TBC) % false rate in classification ≤(TBC) S3NG-T-UN-P35 Snow cover on sea ice UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Ocean coverage ≤0.5 (g:≤ 0.1 on complex terrain) ≤1 NRT3H ≤5 cm ≤(TBC) S3NG-T-UN-P36 Ice Sheets and Shelves drift UN-NOTE C3S, CAMS, CGLMS, Continental ≤ 0.1 (g:≤0.02)m ≤30 NTC ≤0.1 m ≤(TBC) S3NG-T-UN-P37 Ice Sheets and Shelves elevation data UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.1 ≤30 NTC ≤0.1 m/yr ≤(TBC) S3NG-T-UN-P38 Ice Sheets elevation data UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.1 ≤30 NTC ≤10 cm ≤(TBC) S3NG-T-UN-P39 Ice Sheets surface elevation change (GCOS-154 requirement, C3S REQ URC3S-53) UN-COPUN C3S, Continental ≤0.1 ≤30 ≤30 days 0.1 m/yr 0.1 m/yr S3NG-T-UN-P40 Glacier area extent UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.03 (g:≤ 0.015 m) 1 year (at the end of the ablation season) NTC < 5% ≤(TBC) ESA UNCLASSIFIED – For ESA Official Use Only Page 265/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-P41 Glacier elevation data - High-Res UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.3 (g: ≤0.2) 1 year NTC ≤10 cm ≤(TBC) S3NG-T-UN-P42 Glacier elevation data - Medium-Res UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.1 (g:≤0.03) 10 years NTC ≤1 m ≤(TBC) S3NG-T-UN-P43 Lake ice cover UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.3 ≤1 NRT3H ≤0.1 ≤(TBC) S3NG-T-UN-P44 Lake ice thickness UN-NOTE C3S, CAMS, CGLMS, Continental ≤0.1 1 month NTC ≤2 cm (g:≤1 cm) ≤(TBC) S3NG-T-UN-P45 Permafrost Active Layer Thickness UN-NOTE C3S, CAMS, CGLMS, Continental Sites for bioclimate zones 24 h - 1 week NTC/ STC ≤2 cm ≤(TBC) S3NG-T-UN-P46 Sea Ice Area UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Regional ≤1 km (g:≤0.1 for shipping and search and rescue, ≤0.03 in highly infested ice regions) ≤1 (g: ≤ 3 h in highly infested ice regions) NRT3H/ STC ≤5% ≤(TBC) S3NG-T-UN-P47 Sea Ice Concentration - 1 week UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Ocean ≤15 (g:≤10) ≤1 week NTC ≤5% ≤(TBC) S3NG-T-UN-P48 Sea Ice Concentration - 24 h UN-NOTE CMEMS, C3S, CAMS, CGLMS, EMSA Global; Regional ≤1 km (g:≤0.1 for shipping and search and rescue, ≤0.03 in highly ≤1 (g: ≤ 3 h in highly infested ice regions) NRT3H/ STC ≤5% ≤(TBC) ESA UNCLASSIFIED – For ESA Official Use Only Page 272/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 1690, MRD-1700, MRD-1710, S3NG-T-UN-031 S3NG-T should recognize definitions of transitional waters (according to the nomenclature of Water Framework Directive), coastal waters (according to the nomenclature of Water Framework Directive) and to marine waters SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040 S3NG-T-UN-032 S3NG-T should ensure consistency between land and ocean products SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-033 S3NG-T should provide measurements to observe coastal submesoscale dynamics SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-034 S3NG-T should provide hydrological inputs (e.g. discharge estimates) to the coastal ocean from rivers, estuaries and ice melt SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1760, MRD1765 S3NG-T-UN-035 S3NG-T should provide estimates of bathymetry in the coastal ocean SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-036 S3NG-T should provide topography measurements to support climate adaptation applications in the coastal zone SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-037 S3NG-T should provide topography measurements in support of biogeochemical ocean forecasts (e.g. sea state, winds, ocean currents) SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD- ESA UNCLASSIFIED – For ESA Official Use Only Page 273/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 1690, MRD-1700, MRD-1710, S3NG-T-UN-038 S3NG-T should provide ocean products at global scale and regional scale for EU policy implementation, support to Regional Fisheries Management Organisations (RFMO), at coastal scale to support aquaculture SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-039 S3NG-T should support policy implementation by European Fisheries Control Agency (EFCA) and EU Member States sustainable fisheries management and reporting SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-040 S3NG-T ocean products (e.g. geostrophic currents, waves, winds) should be available to support industrial and coastal fisheries SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-041 S3NG-T should provide topography measurements in support of fisheries applications SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-042 S3NG-T should provide topography measurements in support of seasonal and annual fish stock assessment SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-043 S3NG-T should provide topography measurements in support of long-term climate impacts on fisheries (e.g. sea level rise, ocean currents) SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, ESA UNCLASSIFIED – For ESA Official Use Only Page 274/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-044 S3NG-T should provide topography measurements at different time scales :near real time, daily to monthly and climate scales SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-045 S3NG-T should provide topography measurements to support model based products for fisheries applications SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-046 S3NG-T should provide topography measurements (e.g. wind, wave SSH, ocean currents) in support of monitoring suspended sediments SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-047 S3NG-T should provide river discharge measurements aquaculture activities in the coastal ocean activities SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-048 S3NG-T should provide topography measurements close to shore area SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-030 S3NG-T-UN-049: S3NG-T should provide topography measurements in support of fisheries and aquaculture in the pan-European context including the exclusive economic zones of European Member states SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD- ESA UNCLASSIFIED – For ESA Official Use Only Page 275/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 1690, MRD-1700, MRD-1710, S3NG-T-UN-050 S3NG-T should provide measurements in support of aquaculture and fisheries spatial planning SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-051 S3NG-T should provide topography measurements of waves, winds and ocean surface currents to support applications in energy, maritime transport, marine environmental monitoring. SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-052 S3NG-T should provide consistent information from the open ocean to the coastal ocean SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1440, MRD1530 S3NG-T-UN-053 S3NG-T should provide measurements in support of submesoscale dynamics SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-054 S3NG-T should provide measurements in support of vertical mixing and diffusivity in the ocean SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-055 S3NG-T should provide topography information on the interactions between inland waters, estuaries and costal ocean SWD (2019) Ares(2020)1823662 - 30/03/2020 ESA UNCLASSIFIED – For ESA Official Use Only Page 276/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-056 S3NG-T should provide information on sea ice and continental ice SWD (2019) Ares(2020)1823662 - 30/03/2020 to MRD-1970 S3NG-T-UN-057 S3NG-T should provide new products (e.g. wave, wind and current) in real time in support of oil spill pollution SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1650, MRD1670, MRD-1680, MRD-1690, MRD1700, MRD-1710, MRD-1720, MRD1730 S3NG-T-UN-058 S3NG-T should provide information on sea-ice over the Arctic SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1770 to MRD-1970 S3NG-T-UN-059 S3NG-T should provide topography measurements of lake surface ice SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-790 S3NG-T-UN-060 S3NG-T should provide measurements of snow cover and snow parameters SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1900, MRD1910, MRD-1920, MRD-1920, MRD1930, MRD-1940, S3NG-T-UN-061 S3NG-T should provide measurements to support ECV for sea ice and glaciers SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-062 S3NG-T should provide measurements to support ECV for glaciers SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-063 S3NG-T should provide topography products in support of Arctic maritime safety SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-064 S3NG-T should provide topography measurements of seasonal snow cover SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1900, MRD1910, MRD-1920, ESA UNCLASSIFIED – For ESA Official Use Only Page 277/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 MRD-1920, MRD1930, MRD-1940, S3NG-T-UN-065 S3NG-T should provide topography measurements of sea ice thickness SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1800 S3NG-T-UN-066 S3NG-T should provide topography measurements of sea ice concentration SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1950 S3NG-T-UN-067 S3NG-T should provide topography measurements of sea ice [thickness] distribution SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1850 S3NG-T-UN-068 S3NG-T should provide topography measurements of icebergs SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1860, MRD1870 S3NG-T-UN-069 S3NG-T should provide topography measurements of ice sheet elevation SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1960, MRD1970 S3NG-T-UN-070 S3NG-T should provide topography measurements of glacier elevation SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1960, MRD1970 S3NG-T-UN-071 S3NG-T should provide topography measurements to support ice mass balance estimates SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1960, MRD1970 S3NG-T-UN-072 S3NG-T should provide topography measurements of ice sheet mass change estimates SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1960, MRD1970 S3NG-T-UN-073 S3NG-T should provide topography measurements of sea level rise SWD (2019) Ares(2020)1823662 - 30/03/2020 ESA UNCLASSIFIED – For ESA Official Use Only Page 278/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-074 S3NG-T should provide topography measurements of sea level in polar regions SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1530, MRD1840 S3NG-T-UN-075 S3NG-T should provide topography measurements of sea level anomalies in polar regions SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1840 S3NG-T-UN-076 S3NG-T should provide topography measurements of ocean currents in polar regions SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1840 – currents derived from SSH. S3NG-T-UN-077 S3NG-T should provide topography measurements of significant wave height in polar regions SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1620, MRD1630, MRD-1640 S3NG-T-UN-078 S3NG-T should provide topography estimates of river discharge in polar regions SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1765 S3NG-T-UN-079 S3NG-T should provide topography measurements of sea ice parameters at a resolution of < 5 km SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1790, MRD1800, MRD-1810, MRD-1820 , MRD-1850 S3NG-T-UN-080 S3NG-T should provide topography measurements of sea ice parameters at a sub-daily revisit SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-590, MRD1770 S3NG-T-UN-081 S3NG-T should provide topography measurements of glacier elevation SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1790 S3NG-T-UN-082 S3NG-T should provide topography measurements of sea ice in Antarctica SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1770 ESA UNCLASSIFIED – For ESA Official Use Only Page 279/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-083 S3NG-T should provide topography measurements for lakes larger than 50 ha SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040 S3NG-T-UN-084 S3NG-T should provide topography measurements for major river networks SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040 S3NG-T-UN-085 S3NG-T should provide measurements of snow over land surfaces in support of hydrology applications SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1900, MRD1910, MRD-1920, MRD-1920, MRD1930, MRD-1940, S3NG-T-UN-086 S3NG-T should provide topography measurements of lake ice surfaces SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD800 S3NG-T-UN-087 S3NG-T should provide topography measurements in support of GCOS water ECV products consistent with existing time series SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1760 S3NG-T-UN-088 S3NG-T should provide water surface elevation of river catchments SWD (2019) Ares(2020)1823662 - 30/03/2020 S3NG-T-UN-089 S3NG-T should support monitoring hydrological dynamics (river flow and lake storage) of river catchments SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1760, MRD1765 S3NG-T-UN-090 S3NG-T should provide topography products in support of costal bathing waters and the Water Framework Directive SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-030, MRD1530 S3NG-T-UN-091 S3NG-T should provide inland water information in support of international disaster risk reduction such as drought SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, MRD-1765 ESA UNCLASSIFIED – For ESA Official Use Only Page 280/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-092 S3NG-T should provide inland water topography products in support of management of rivers and channels, floods, discharge, hydro-power, and waterways SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, MRD-1765 S3NG-T-UN-093 S3NG-T should provide topography measurements of water surface elevation for lakes scales smaller than 50 ha SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040 S3NG-T-UN-094 S3NG-T should provide topography measurements of water surface elevation over reservoirs SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040 S3NG-T-UN-095 S3NG-T should provide topography measurements of water surface elevation over small rivers SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040 S3NG-T-UN-096 S3NG-T should provide topography measurements of water surface elevation with sufficient fidelity to monitor the target (river, lake, reservoir) dynamics SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, S3NG-T-UN-097 S3NG-T should provide topography measurements of water surface elevation to support hydrological modelling for climate, emergency and land applications SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, S3NG-T-UN-098 S3NG-T should provide topography measurements of water surface elevation to support hydrological modelling for agriculture support and urban/local planning SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, S3NG-T-UN-099 S3NG-T should provide topography measurements of water surface elevation to support hydrological modelling for agriculture at seasonal and yearly periods SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, S3NG-T-UN-100 S3NG-T should provide topography measurements of water surface elevation for 1 ha minimum mapping unit SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-040, MRD1760, S3NG-T-UN-101 S3NG-T should provide topography measurements in support of water resources in the cryosphere as climate records SWD (2019) Ares(2020)1823662 - 30/03/2020 ESA UNCLASSIFIED – For ESA Official Use Only Page 281/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-102 S3NG-T should provide topography measurements of water surface elevation for rivers >10m in width UN-NOTE MRD-040, MRD1760, S3NG-T-UN-103 S3NG-T should provide topography measurements of water surface elevation with daily revisit UN-NOTE MRD-040, MRD590, MRD-1760, S3NG-T-UN-104 S3NG-T should provide topography measurements of water surface elevation with hourly revisit for small rivers UN-NOTE MRD-590 S3NG-T-UN-105 S3NG-T should provide topography measurements of water surface elevation with weekly revisit for drought monitoring UN-NOTE MRD-040, MRD590, MRD-1760, S3NG-T-UN-106 S3NG-T should provide topography measurements of water surface elevation with global coverage UN-NOTE MRD-040 S3NG-T-UN-107 S3NG-T should provide topography measurements for climate monitoring, ocean, polar and inland, at national to regional scale SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-360, MRD370, MRD-380, MRD-830, MRD850, MRD-1090, MRD-1540 S3NG-T-UN-108 S3NG-T should provide topography measurements in support of seasonal forecasting SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-109 S3NG-T should provide topography measurements to support regional to local climate adaptation strategies SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD1690, MRD-1700, MRD-1710, S3NG-T-UN-110 S3NG-T should provide topography measurements yearly to support sustainable energy and climate action plan (SECAP) reporting. SWD (2019) Ares(2020)1823662 - 30/03/2020 MRD-1520, MRD11530, MRD1620, MRD-1630, MRD-1640, MRD- ESA UNCLASSIFIED – For ESA Official Use Only Page 288/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 S3NG-T-UN-149 S3NG-T should provide measurements of sea ice parameters using a radar altimeter and a microwave radiometer (required for tropospheric wet path delay correction of the altimeter measurements) within the altimeter footprint (at a resolution of 5-10 km) CMEMS (2017) S3NG-T-UN-150 S3NG-T should address the product needs described in Table 14.2.1 UN-COPURD, UNNOTE, C3S(2020), CMEMS(2017) All Requirements ESA UNCLASSIFIED – For ESA Official Use Only APPENDIX V TECHNICAL SUMMARY OF THE SENTINEL-3 TOPOGRAPHY MISSION The Copernicus Sentinel-3 mission, part of the first generation of Copernicus satellites, is designed to ensure the long-term collection and operational delivery of high-quality measurements to Copernicus ocean, land, and atmospheric services. Mission Requirements are set out in the Sentinel-3 Mission Requirements Traceability Document (MRTD, Donlon, 2011). A full description of the Sentinel-3 Mission is provided in Donlon et al. (2016) Sentinel-3 is designed to monitor the global environment through measurements that will also be used to constrain and drive global-local numerical prediction models in support of Global Monitoring for Environment and Security (GMES, now called Copernicus) user needs. Figure V-1 The Copernicus Sentinel-3 Mission identifying key features of the mission payload. The aim of the Sentinel-3 (Donlon, 2011) mission is: To provide continuity of ENVISAT type measurement capability in Europe to determine sea, ice and land surface topography, temperature, ocean and land surface radiance/reflectance, and atmospheric measurements with high accuracy, timely delivery and in a sustained operational manner for GMES users. Sentinel-3 is based on the heritage of demonstrated European measurement techniques, platform design, instrument design, and data processing systems that are required features to ensure a robust mission design in an operational system such as Copernicus (as opposed to scientific explorer missions that are designed to pioneer new measurement techniques and technologies to explore new parameters of interest ESA UNCLASSIFIED – For ESA Official Use Only Page 290/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 in pursuit of scientific knowledge). There are two primary mission components hosted on the same satellite platform: • A Sentinel-3 topography mission providing altimetry measurements, • A Sentinel-3 optical mission providing visible and infrared measurements simultaneously and contemporaneous with the topography mission (that is not discussed further in this MRD). Sentinel-3 altimetry contributes to global climate Observing System (GCOS, 2016) climate monitoring activities via direct contributions to monitoring ocean mesoscale dynamics, ocean waves, ice mass balance, sea ice variability, river and lake stage heights and sea level ECV. The mission addressed measurement stability as a design feature for each Sentinel-3 instrument to ensure that long-term stability is maintained and accurately known throughout their entire mission lifetime through careful calibration and validation operations. While the Sentinel-3 optical mission is not discussed further in this MRD it must be noted that it brings an enormous benefit because optical sensors together with an altimeter, viewing the same area of the ocean, at the same time, allows unprecedented synergy. In particular, the interpretation and positioning of SRAL along-track altimeter data within the context of the mesoscale circulation field of the ocean. This has been fully exploited in Copernicus applications of quasi-geostrophy (e.g. Rio et al, 2016), bringing improved estimates of geostrophic ocean currents and ocean dynamics. In addition, the collocated and contemporaneous multi-sensor measurements are also beneficial for multi-variate ocean data assimilation models. Sentinel-3 Primary Objective-4 is dedicated to altimetry and forms the basis for the S3NG-T baseline continuity mission: Sentinel-3 shall provide, in a NRT operational and timely manner, a generalised suite of high-level primary geophysical products with a consistent quality, a very high level of availability (>95%), high accuracy and reliability and in a sustained operational manner for GMES users. Products shall include as priority: • Global coverage Sea Surface Topography (SSH) for ocean and coastal areas, • Enhanced resolution SSH products in the Coastal Zones and sea ice regions, • Global coverage Ocean Surface Wind Speed measurements, • Global coverage Significant Wave Height measurement, • Ice products (e.g., ice surface topography, extent, concentration). Sentinel-3 Secondary Objective-6 is including altimetry measurements over river and lake targets: Sentinel-3 shall provide in an operational and timely manner, a generalised suite of high-level secondary geophysical products with a consistent quality, a very high level of availability (>95%), high accuracy and reliability and in a sustained operational manner for GMES users. Products shall include as priority: • Inland water (lakes and rivers) surface height data(now elevated to a Primary Mission Objective for S3NG-T). ESA UNCLASSIFIED – For ESA Official Use Only Page 291/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 To achieve the mission aim and objectives, the Sentinel-3 satellite carries the following topography payload instruments: • A dual-frequency SAR radar altimeter (SRAL), derived from elements of ENVISAT RA-2, CryoSat SAR Interferometric radar altimeter (SIRAL) and Jason-2/Poseidon-3 heritage called the SAR Radar ALtimeter (SRAL) instrument. • A Microwave Radiometer (MWR) instrument, which supports the SRAL to achieve the overall altimeter mission performance by providing the wet tropospheric correction derived from ENVISAT MWR heritage. • A Precise Orbit Determination (POD) package including a Global Navigation Satellite Systems (GNSS) instrument, a Doppler Orbit determination and Radio-positioning Integrated on Satellite (DORIS) instrument (e.g. Auriol et al., 2010) and a Laser retro-reflector (LRR). Each Sentinel-3 satellite is a low Earth-orbit moderate size satellite compatible with small launchers including Vega and Rokot. The satellite layout is driven by the need to provide a large face viewing cold-space for thermal control and, a modular design for payload accommodation and simplified management of all on-board interfaces. In order to satisfy the demanding sampling, coverage and revisit requirements, the Sentinel-3 mission is designed as a constellation of two identical satellites on-orbit at the same time. The complete mission includes a series of four satellites (Sentinel-3A, B, C and D) each having 7-year lifetime and consumables for 12 years, to operate over a 20-year period. During full operations two identical satellites will be maintained in the same orbit plane with a phase delay of 140°. Sentinel-3A was launched on 16 February 2016 from Plesetsk, Russia using a Rockot launcher. The SRAL and MWR instruments were switched-on 1 March and 29 February 2016 respectively. On 12 April 2016, after six weeks of acquisition in LRM mode, Sentinel-3A SRAL was switched to SAR mode and has operated in SAR mode continuously and over all surfaces (the first altimetry mission to use this mode at global scale). After 5 months of commissioning, routine operations started in July 2016. Sentinel-3B was launched on the 25 April 2018 from Plesetsk, Russia using a Rockot launcher. On 8 May 2018 the SRAL instrument was switched-on. On 6 of June 2018 Sentinel-3B reached a tandem configuration 30 seconds ahead Sentinel-3A orbit. During 4 months of tandem flight, different configurations were defined to assess the Sentinel-3B instrument performances and to improve our understanding of SAR vs LRM measurement modes. In particular, Sentinel-3B acquisition was switched between Closed Loop (CL) and Open Loop (OL) mode. On 16th October 2018 Sentinel-3B reach its final orbit position which was at a phase angle of 140° apart from the Sentinel-3A ground track. Sentinel-3C and Sentinel-3D are foreseen for launch in the 2023-2026 time frame as replacements for the A and B units providing baseline continuity until the 2035+ timeframe. ESA UNCLASSIFIED – For ESA Official Use Only Page 292/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 Figure V-2 Sea Surface Height Anomaly (SSHA) at 1 Hz sampling coverage of Sentinel-3A and Sentinel-3B ground tracks for a 2-week period in January 2021. Three levels of timeliness are required for Sentinel-3 topography products within Copernicus depending on the specific application: • Near-Real-Time 3-hour (NRT3H) products, delivered to users in less than 3 hours after acquisition of data by the sensor, • Short time critical (STC) products, delivered to users in less than 48 hours after the acquisition that include improved orbit ephemeris data and, • Non-time critical (NTC) products delivered not later than 1 month after acquisition that include final orbit ephemeris data. Originally, the Sentinel-3 Mission was developed with Sentinel-3A and Sentinel-3B placed at 180° phase relative to each other in the same orbit plane. The impact of this planning resulted in a non-uniform spacing of Sentinel-3A and Sentinel-3B ground tracks. After 4 days, ground tracks would not be uniformly interleaved but close together (~57km) resulting in highly correlated measurements. In other locations large (~600 km) areas remained un-observed. Considering the characteristic time and space scales of mesoscale ocean circulation (5-10 days, 50-500 km) this was far from optimal. To improve ocean topography sampling with minimal impact on the optical mission the relative orbit phase was modified to a 140° relative satellite phasing as explained in Donlon (2015b). In this configuration, the impact of a second satellite altimeter is significantly improved for CMEMS since ocean mesoscale features are now better sampled at the 4-day sub-cycle of the Sentinel-3 orbit. This configuration remains the baseline for a two-satellite constellation as shown in Figure V-2. ESA UNCLASSIFIED – For ESA Official Use Only Page 293/327 S3NG-T - Mission Requirements Document Issue Date: 15/10/2025, Ref: ESA-EOPSM-S3NG-MRD-3821, Version:1.0 V.1 Sentinel-3 SAR Radar Altimeter (SRAL) The Sentinel-3 Radar Altimeter (SRAL) instrument (Le Roy et al, 2009) is a fully redundant dual-frequency (Ku and C-band) nadir-looking radar altimeter that employs Synthetic Aperture Radar (SAR) closed-burst altimetry technologies (e.g., Raney, 1995) inherited from the CryoSat (Wingham, 1999) mission. SRAL acquires topography data over all types of surfaces covered by the Sentinel-3 mission (ocean, coastal areas, sea ice, ice sheets, ice margins, in-land waters). Key design elements of the SRAL instrument are provided in Table V.1-1 that includes the following sub-systems: • A dual-frequency (Ku/C band) near nadir pointing antenna of 1.2 m diameter with a focal length 0.43 m mounted on the Earth pointing face of the satellite, • Delay Doppler synthetic aperture radar (SAR) capability in Ku-band, conventional low-resolution (LRM) mode for C-band, • A Radio Frequency Unit (RFU) comprised of solid-state power amplifiers in Ku and C bands, diplexers used to route signals in the transmit or receive chains, a signal demodulation and “de-ramp” system, and gain controlled amplifiers to slave the echo level. • A Digital Processing Unit (DPU) that manages all communication interfaces between the satellite platform (telemetry and tele-commands), a chirp generator, full sequencing of the instrument, received signal sampling and all elements of the required for tracking. • Two identical DPU and RFU systems mounted inside the satellite platform provide cold redundancy. Table V.1-1. Technical characteristics of the Sentinel-3 SAR Radar Altimeter (SRAL) instrument. Parameter Ku band C band Frequency 13.575 GHz 5.41 GHz Bandwidth 350 MHz (320 used) 320 MHz (290 used) Antenna footprint 18.2 km 48.4 km Radius of 1st resolution cell 823 m 865 m Low Resolution Mode (LRM) Pulse repetition frequency (PRF) 1924 Hz 274.8 Hz LRM Tracking Modes Closed loop and Open loop Synthetic Aperture Radar (SAR) mode SAR (PRF) 17825 Hz SAR along track resolution 291 m (Orbit height 795 km) – 306 m (Orbit height 833 km) SAR across track resolution > 2km depending on Hs Doppler bandwidth 15055 Hz Tracking modes Closed loop and Open loop Antenna size 1.2 m diameter, focal length 0.43 m [Document text truncated for crawler view.]