Technical requirements for ACTRIS Mobile Platforms operating automatic sun/sky/lunar photometers

22 December 2025 1 / 12 Technical requirements for ACTRIS Mobile Platforms operating automatic sun/sky/lunar photometers Emitter CARS & ARES Version 01 Revision 01 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 2 / 12 Table of contents APPLICABILITY OF THE DOCUMENT ................................................................................................................. 3 ACRONYMS ...................................................................................................................................................... 3 REFERENCE DOCUMENTS ................................................................................................................................. 3 1 INTRODUCTION ....................................................................................................................................... 4 2 PLATFORM AND MOUNTING SPECIFICATIONS ......................................................................................... 4 2.1 INSTRUMENT AND ADAPTATION TO MOVING PLATFORM DESCRIPTION ................................................................... 4 2.2 MOBILE PLATFORM SPEED/OSCILLATION SPECIFICATIONS ................................................................................... 6 2.3 LOCATION ON PLATFORM: ........................................................................................................................... 7 2.4 VIBRATION ISOLATION: ................................................................................................................................ 7 2.5 MAINTENANCE: ......................................................................................................................................... 7 3 ENVIRONMENTAL PROTECTION .............................................................................................................. 8 3.1 SALT AND SPRAY MANAGEMENT: .................................................................................................................. 8 3.2 HOUSING AND WEATHERPROOFING: .............................................................................................................. 9 4 POWER SUPPLY AND DATA SYSTEMS .................................................................................................... 11 4.1 POWER REQUIREMENTS ............................................................................................................................ 11 4.2 DATA ACQUISITION AND TRANSMISSION ....................................................................................................... 11 4.3 TIME SYNCHRONIZATION ........................................................................................................................... 11 5 4MOTION AND NAVIGATION DATA INTEGRATION ................................................................................ 11 5.1 MOTION SENSORS AND NAVIGATION DATA: ................................................................................................... 11 6 CALIBRATION AND TRACEABILITY .......................................................................................................... 12 6.1 CALIBRATION FREQUENCY: ......................................................................................................................... 12 6.2 DUTY CYCLE MANAGEMENT: ....................................................................................................................... 12 7 OPERATIONAL SAFETY AND COMPLIANCE. ............................................................................................ 12 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 3 / 12 Applicability of the document These technical requirements apply to ACTRIS Mobile Platforms operating automatic sun/sky/lunar photometers. Acronyms • ACTRIS - Aerosol, Clouds and Trace gases Research InfraStructure • ACTRIS GA – ACTRIS General Assembly • ARES – Aerosol Remote Sensing unit of the ACTRIS Data Centre • ARS – Aerosol Remote Sensing • CARPORT – CARS workflow management portal • CARS - Centre for Aerosol Remote Sensing • CLU – Cloud Remote Sensing unit of the ACTRIS Data Centre • CCRES - Centre for Cloud Remote Sensing • DC – Data Centre • MP – Mobile Platform • NF - National Facility • NRT – Near-real time (less than 3 days from the measurements) • OP – Observational Platform • PI – Principal Investigator • RI Comm – Research Infrastructure Committee • RRT – Real-real time (less than 3 hours form the measurement) • SCC – Single Calculus Chain Reference documents 1. Documentation on technical concepts and requirements for ACTRIS Observational Platforms 2. ACTRIS NF Labelling Plan 3. Descriptions of the workflows between ACTRIS components 4. ACTRIS Data Management Plan 5. CARS implementation plan 6. ACTRIS vocabulary 7. Labelling of the ACTRIS National Facilities operating Aerosol Remote Sensing instruments 8. Measurement Guidelines for automatic sun/sky/lunar photometers 9. Standard Operation Procedure for automatic sun/sky/lunar photometers 10. Quality Assurance Procedure for automatic sun/sky/lunar photometers 11. ACTRIS CIMEL Photometer Guide for Users ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 4 / 12 1 Introduction Detailed instructions are provided for the deployment, operation, quality control, and calibration of CIMEL photometers within the AERONET-ACTRIS network, for both stationary and mobile/marine sites. Through rigorous facility-based calibration, weekly local and remote QC, and network-level verification, every CIMEL CE318T photometer remains a reliable and accurate instrument for climate research, atmospheric monitoring, and environmental policy applications. This Guide summarizes the main relevant information all users / instruments PI must be aware of. This document provides Technical requirements for ACTRIS Mobile Platforms operating automatic sun/sky/lunar photometer 2 Platform and mounting specifications 2.1 Instrument and adaptation to moving platform description The standard sun/sky/lunar photometer used for ACTRIS Mobile Platforms is the CIMEL CE318-T, described in Section 1. This instrument is the latest standard within the AERONET network and is capable of performing automatic measurements of direct Sun irradiance, sky radiances, and Moon irradiance. To ensure proper data acquisition under mobile conditions, the photometer must be complemented with a system that accurately determines the orientation and motion of the platform in real time. Specifically, this system should provide continuous attitude information, including heading, roll, and pitch, with an estimated uncertainty of < 0.2°. Such systems have evolved in recent years (e.g., SIMRAD-H60, Trimble ABX-Two, as described in Torres et al., 2025), with the latest recommendation being the use of the inertial GPS unit SBG-Ellipse D. This unit continuously transmits the platform’s attitude data to one of the photometer’s communication ports. Combined with geo-localization from the photometer’s internal GPS, the CIMEL electronic system can then determine the precise position of the Sun and Moon, ensuring accurate pointing. For maritime deployments, the use of a protective air-blowing system (airshield) around the instrument’s optical window is strongly recommended. This system prevents the accumulation of sea spray and helps ensure uninterrupted optical measurements in harsh marine environments. In addition, it is recommended to replace the standard CE318-T resistive wet sensor—which has been shown to be prone to corrosion—with an advanced weather unit. The latest version of this unit includes an optical rain sensor and an anemometer to monitor wind speed, enabling the system to automatically stop measurements during both rain and extreme wind conditions. This protects the instrument from strong winds and high waves. Unlike standard AERONET ground-based instruments, which suspend operation only in the presence of rain, this configuration halts measurements under both rain and high winds (detailed information in section 2). The weather unit connects to the photometer via the standard humidity sensor port. For land-based mobile applications, such as cars or vans, the use of the airshield and the advanced weather unit is optional. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 5 / 12 The schematic in Figure 1, adapted from Figure 1 in Torres et al. (2025), illustrates all the aforementioned components. The central part shows the triple version of the CIMEL CE318-T photometer, the latest standard instrument in the AERONET network, originally designed for groundbased installations. On the right, the inertial GPS unit is depicted, providing attitude data (heading, roll, pitch) and continuously transmitting this information to the photometer’s communication ports. On the left, the air pumping unit is displayed, supplying clean compressed air to the base of the collimator to prevent sea spray deposition. In addition, a weather system—including a non-corrosive optical rain sensor and an anemometer—is integrated to stop operation during rain or high winds. Figure 1: Schematic of the CIMEL CE318-T photometer adapted for ACTRIS Mobile Platforms. The inertial unit (right) is mandatory for all mobile applications, providing real-time attitude data. The airshield and advanced weather unit (left) are specific to maritime deployments, protecting the instrument under marine conditions, though they may also be used for other mobile platforms. Source: Pictures of Module 3 taken from the User Guide of the Trimble ABX-Two. The advantage of using the standard AERONET CE318-T photometer for the shipborne system lies in its full compatibility with AERONET calibration, which allows for the extension of quality control and quality assurance (QC/QA) procedures to all measurements. This compatibility significantly simplifies the data processing workflow. The term AERONET compatible implies that ACTRIS mobile photometers follow the same measurement protocols and schedules as ground-based instruments, use identical filters, and undergo the same data processing procedures. Additionally, the calibration of Sun/Moon direct measurements and sky radiances is performed in the same manner as for terrestrial photometers, ensuring consistency and accuracy across the network. Therefore, the measurements and Standard Operating Procedures (SoP) presented in Sections 1 and 2 for the photometer at National Facilities are the same for the mobile version. The only softwarerelated differences do not concern the measurement sequence itself, but rather the tracking and acquisition frequency. Specifically, mobile instruments perform continuous tracking during Sun/Moon ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 6 / 12 measurements instead of the static tracking used in fixed installations (as described in Torres et al., 2025, and implemented in the instrument’s EPROM). For mobile photometers, platform motion can sometimes affect the quality of the measurements, leading to a lower fraction of data passing the AERONET Level 1.5 cloud screening. To obtain a number of valid observations comparable to ground-based sites, it is therefore recommended to increase the frequency of direct Sun/Moon measurements to once every three minutes (approximately twice as often as at regular sites), while sky measurements follow the same protocols as standard instruments. This acquisition mode, known as O’NEILL mode, is programmed in all instruments of the network and, although uncommon, is also activated at some fixed sites. 2.2 Mobile Platform speed/oscillation specifications The successful operation of ACTRIS mobile photometers depends not only on the instrument design but also on the dynamic characteristics of the platform. The first installation of a CE318-T photometer on a ship, aboard the R.V. Marion Dufresne (one of the pioneer mobile platforms in the ACTRIS context), demonstrated that the system can operate reliably under slow and low-amplitude attitude (heading/pitch and roll) oscillations. A full-year analysis of attitude data from the R.V. Marion Dufresne shows that pitch and roll standard deviations over 7-minute intervals (corresponding to a typical almucantar scan) are generally in the range of 0.4–0.5°, with dominant oscillation frequencies around 0.1 Hz (oscillation periods ~10 s, corresponding to an effective angular speed of ~30°/s for typical oscillation ranges). Under such conditions, the continuous tracking system is able to maintain Sun/Moon lock throughout the measurement sequence, and the resulting AOD retrievals consistently pass the AERONET Level 1.5 quality criteria. Similar analyses from other installations on large research vessels, such as the R.V. Gaia Blu and the R.V. Sarmiento de Gamboa, confirms these results. In contrast, experience on smaller and faster research vessels (e.g., the 20-m NOAA R.V. Shearwater) has shown that when pitch and roll exceed 2–3°, even under relatively calm conditions, the tracking system struggles to maintain alignment, preventing stable AOD acquisition. These results demonstrate that the current system cannot be operated on any type of platform without restriction, and that there are amplitude and speed limits associated with platform motion. Based on shipborne data and laboratory tests using a motion simulation platform, preliminary limits have been identified. Reliable operation is achieved when platform oscillations remain below approximately 1° in heading, pitch, and roll. In addition, a dynamic limit appears around 50°/s of effective angular speed (assuming harmonic oscillations), above which tracking performance degrades. Work is ongoing, in collaboration with CIMEL Electronique, to further quantify these thresholds using simulation experiments, with the objective of improving robustness and extending performance margins under more challenging conditions. One approach under development involves implementing adaptive tracking algorithms based on Proportional-Integral-Derivative (PID) control, which would dynamically adjust tracking speed in response to pitch and roll fluctuations. At present, installations on platforms with larger and faster attitude motions can only be achieved by employing stabilizing mounts. One such system was tested aboard the R.V. Shearwater and demonstrated improved performance under highly dynamic conditions. However, these mounts are ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 7 / 12 significantly more expensive (four to five times the cost of the inertial unit) and their long-term reliability in harsh marine environments remains uncertain. 2.3 Location on Platform: For ACTRIS Mobile Platforms, the general guidelines for instrument installation follow the same principles as for fixed sites (described in section 2 related to SOP): the photometer should ideally be located in a position free from obstructions, shadows, and reflective surfaces. For land-based vehicles (e.g., cars, vans), the photometer is normally mounted on the roof, preferably in a central position to maximize sky visibility and ensure balanced support. For shipborne deployments, the system can be affixed to standard ship railings using stainless steel clamps with rubber padding, ensuring stability under marine conditions. The installation position is selected in coordination with the vessel’s crew to optimize sky visibility.. Although complete unobstructed views are generally not achievable (due to the presence of masts, cranes, and other ship structures) the goal is to minimize obstructions as much as possible. In addition to sky visibility, other factors must be considered: ● ease of access for installation and maintenance, ● avoiding proximity to exhausts or other contamination sources, ● mounting at a sufficient height to reduce exposure to sea spray and waves, ● minimizing electromagnetic interference from large antennas, which may affect the performance of the inertial GPS unit. 2.4 Vibration Isolation: Mount should incorporate damping or isolation mechanisms to reduce vibration-induced pointing instability. During installation, check that the mount is securely fixed. Engine vibrations must not resonate with the measuring head. Similarly, as the mount will be exposed to wind, do not hesitate to strengthen the support to which it will be fixed. 2.5 Maintenance: Maintenance requirements for ACTRIS mobile photometers are minimal and comparable to those of fixed sites, consisting mainly of occasional filter cleaning and visual inspections, as described in the SOP document. There are a few minor operations to be carried out on the boat to ensure it runs correctly.: Check the mount (robot): →Visually and daily Check the data: → On the data website. At least once a week Check applications on the PC: → Weekly Check the 4Q detector: → Clean it as it is not protected from sea spray. Monthly ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 8 / 12 Check the Airshield box: → Monthly and change the filters annually. Check the robot's operation: → If requested For shipborne deployments, the ideal scenario is to combine continuous data transmission (typically less than 5 kilobytes per hour for near-real-time processing) with remote diagnostics (e.g., via TeamViewer), which enables reliable unattended operation for extended periods at sea, often lasting several months. 3 Environmental Protection 3.1 Salt and Spray Management: To prevent sea spray contamination, an airshield system supplies clean, dry air to the base of the photometer’s collimator, generating a continuous protective overpressure that prevents salt deposition and particle intrusion. A flexible butyl-lined hose connects the pump to the photometer, minimizing pressure loss while allowing free movement of the tracking head. A filter installed at the air intake ensures that only clean air enters the system. The pump operates continuously to safeguard the optics, even when measurements are paused, but may be manually stopped when the tracking head is removed. (a) (b) Figure 2: Views of the Airshield box (a); and path to the collimator (b) The AirShield consists of a high-performance air blower that drives double-filtered compressed air to a special injector head that creates an air barrier in front of the CIMEL sun photometer front window. The system can be operated on 110-230V AC power and is ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 9 / 12 intended for outdoor use only. The AirShield soiling prevention system increases the longterm accuracy of sensor readings and reduces cleaning frequency requirements by providing a constant air barrier to prevent dust and dirt from settling. The airshield box is designed to accommodate a free flow of 860 liters per minute. It is connected to the tube fittings of the CIMEL sun photometer's collimator through a 3-meter flexible hose (25∅ - UV protected) and a 1-meter butyl pipe. Within the airshield box, the MICRONEL air blower provides the required airflow. The blower is powered by a power supply protected by a 4A fuse, connected to the European power grid (100/240VDC). It is capable of continuous operation for three years without the need for maintenance. A 24VDC FAN ensures continuous air refreshment for the blower to prevent any issues. The modified collimator is a standard CIMEL collimator designed for the CE318-T version. Two holes were created on the collimator tube near the optical lenses of the optical head. The airshield directs air through a hose and a butyl pipe to the collimator. Two 90° fittings are used to connect the butyl pipe. The butyl pipe is highly flexible and can withstand a wide temperature range, from -40°C to 120°C. Figure 3: View of the modified collimator 3.2 Housing and Weatherproofing: The CE318-T photometer used on ACTRIS Mobile Platforms is designed to operate under outdoor conditions, but additional measures are required in harsh marine environments. In particular, the use of a dedicated weather unit is recommended for shipborne applications. This unit called METEOBOX, connects to the photometer via the standard humidity sensor port and follows an enhanced safety