Quality Assurance Procedure for aerosol high-power lidars

ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 1 / 28 Quality Assurance Procedure for aerosol high-power lidars Emitter CARS & ARES Version 01 Revision 16 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 2 / 28 Table of contents APPLICABILITY OF THE DOCUMENT............................................................................................. 4 ACRONYMS ................................................................................................................................. 4 REFERENCE DOCUMENTS ........................................................................................................... 4 1 INTRODUCTION ................................................................................................................... 5 2 OVERVIEW OF QA TESTS AND SUBMISSION ......................................................................... 6 3 QA SUBMISSION PROCESS ................................................................................................... 6 3.1 Before Submission .............................................................................................................................. 6 3.2 During Submission .............................................................................................................................. 7 3.3 After Submission ................................................................................................................................. 7 4 TELECOVER TEST .................................................................................................................. 9 4.1 About the test ..................................................................................................................................... 9 4.2 Environmental conditions ................................................................................................................. 10 4.3 Test procedure .................................................................................................................................. 10 4.3.1 Biaxial systems .................................................................................................................................... 11 4.3.2 Coaxial systems ................................................................................................................................... 11 4.4 Schedule............................................................................................................................................ 12 4.5 Internal analysis ................................................................................................................................ 12 4.5.1 Example ............................................................................................................................................... 12 4.6 Filling the QA logbook ....................................................................................................................... 13 5 POLARIZATION CALIBRATION ............................................................................................. 14 5.1 About the test ................................................................................................................................... 14 5.2 Test procedure .................................................................................................................................. 15 5.3 Environmental conditions ................................................................................................................. 15 5.4 Schedule............................................................................................................................................ 15 5.5 Internal analysis ................................................................................................................................ 16 5.5.1 Example ............................................................................................................................................... 16 5.6 Filling the maintenance logbook ....................................................................................................... 16 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 3 / 28 6 RAYLEIGH FIT TEST ............................................................................................................. 17 6.1 About the test ................................................................................................................................... 17 6.2 Environmental conditions ................................................................................................................. 17 6.3 Test procedure .................................................................................................................................. 17 6.4 Schedule............................................................................................................................................ 17 6.5 Internal analysis ................................................................................................................................ 18 6.5.1 Example ............................................................................................................................................... 18 6.6 Filling the maintenance logbook ....................................................................................................... 18 7 ZERO BIN TEST ................................................................................................................... 19 7.1 About the test ................................................................................................................................... 19 7.2 Environmental conditions ................................................................................................................. 20 7.3 Test procedure .................................................................................................................................. 20 7.3.1 Elastic channels ................................................................................................................................... 20 7.3.2 Inelastic channels ................................................................................................................................ 21 7.4 Schedule............................................................................................................................................ 22 7.5 Internal analysis ................................................................................................................................ 22 7.6 Filling the maintenance logbook ....................................................................................................... 22 8 EXTENDED DARK SIGNAL MEASUREMENT ......................................................................... 23 8.1 About the test ................................................................................................................................... 23 8.2 Environmental conditions ................................................................................................................. 23 8.3 Test procedure .................................................................................................................................. 23 8.4 Schedule............................................................................................................................................ 24 8.5 Internal analysis ................................................................................................................................ 24 8.6 Filling the maintenance logbook ....................................................................................................... 24 9 QA TEST SUMMARY ........................................................................................................... 25 10 ADDITIONAL RESOURCES ............................................................................................... 26 ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 4 / 28 Applicability of the document These procedures apply to the ACTRIS aerosol remote sensing National Facilities and to the EARLINETACTRIS associated lidar stations. Acronyms ACTRIS - Aerosol, Clouds and Trace gases Research InfraStructure AHL – Aerosol High Power Lidars CARS - Centre for Aerosol Remote Sensing ND – Neutral Density (Filter) NF - National Facility PBS – Polarizing Beam Splitter PMTPhotoMultiplier QA – Quality Assurance Reference documents [1]. Labelling of the ACTRIS National Facilities operating Aerosol Remote Sensing instruments [2]. Guidelines for EARLINET-ACTRIS associated stations to access QA services [3]. Technical requirements for ACTRIS Aerosol Remote Sensing Observational Platforms [4]. Technical requirements for ACTRIS Mobile Platforms operating aerosol high-power lidars [5]. Standard Operation Procedure for aerosol high-power lidars [6]. ATLAS manual ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 5 / 28 1 Introduction The current document will act as a guideline and must be adapted to the particularities of each instrument and to the operator’s personal experience. In case the information does not apply to your lidar instrument, please contact the appropriate representative within CARS for further support: Nikos Siomos » on data handling and processing using ATLAS email: [email protected] Doina Nicolae » on Actris procedures and rules email: [email protected] Volker Freudenthaler » on optics related aspects and QA tests email: volker.freudent[email protected]i-muenchen.de Aldo Almodeo » on instrument operation and QA procedures email: [email protected] Livio Belegante » on instrument operation and QA procedures email: [email protected] All safety procedures that apply within your research institution must be applied and must not be in contradiction with the guidelines found in this document. Before using this document, please read carefully all user manuals from all components and modules that are part or are connected to each lidar instrument. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 6 / 28 2 Overview of QA tests and Submission 3 QA submission process Each station will be provided with a dedicated folder on a shared cloud platform, which will serve as the central location for submitting all QA test data and related documentation. If a lidar station does not have access to its shared folder or is unaware of the folder link, the Principal Investigator (PI) must contact carp[email protected]o to obtain the necessary information. In addition to the test data, a dedicated “System Info” folder must be created on the station’s shared folder for each lidar instrument. This folder should contain all available technical details about the instrument, including information on optical components, laser specifications, instrument schematics, and photos of the emission and receiving units, GHk calculus info. A dedicated folder named “logs” must be created. This folder should contain the instrument logbook corresponding to the instrument's mode of operation: • For continuous measurements, please use the following link to download the appropriate logbook. • For scheduled measurements, please use the following link to download the corresponding logbook. 3.1 Before Submission Prior to initial submission, the Principal Investigator (PI) must ensure that the operational configurations are properly defined in the SCC. The PI is required to establish two distinct operational configurations: • Daytime Configuration: Optimized for lidar measurements under daylight conditions. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 7 / 28 • Nighttime Configuration: Optimized for lidar measurements during nighttime operations. Each lidar operator must perform a series of quality assurance measurement tests for every operational lidar configuration that the PI intends to use during the test period. These configurations must undergo rigorous testing to ensure consistency and accuracy before submission. Only standard validated SCC lidar products should be included in the operational configurations. Experimental products must only be submitted within the SCC experimental lidar configuration. Note: Certain lidar setups or applications may require a single configuration that accommodates both daytime and nighttime operations. The QA submission follows a predefined schedule, with two calls for QA tests each year: the first call will be open from April to May, and the second from September to October. Each station will be assigned to either the first or the second call. The Principal Investigator (PI) should plan the QA measurements so that an initial set of data is collected and processed by the station within the assigned period. If the results are not satisfactory, the PI should still have sufficient time to optimize the instrument and perform a second round of measurements. If the second set meets the quality criteria, the PI can proceed to the next step. Note: The PI must perform regular internal QA tests (e.g., once per month or every two months) to monitor instrument performance. These tests may become essential if two consecutive QA reports indicate a deterioration of instrument performance. In such cases, the SCC operational configuration can only be validated for the period when the instrument operated reliably, and the monthly QA tests provide the necessary evidence for this assessment. Without these additional tests, it will not be possible to validate the operational configuration for the corresponding time period. 3.2 During Submission • Upload the test data to the designated station folder provided by CARS. • Store the data in Atlas format, ensuring compatibility with analysis tools. Refer to the Atlas manual at (https://github.com/nikolaos-siomos/ATLAS). • The submission package must include: o config_file.ini o settings_file.ini o Radio sounding data, stored in a predefined folder named "radiosondes". o The exported CSV file corresponding to the SCC operational configuration for which the tests are submitted. Note: If any of the QA tests or required files are missing, the QA analysis cannot be carried out. 3.3 After Submission Once the submission is complete, the following steps will be undertaken: • Please notify CARS about the availability of the new test sets by sending an email to car- [email protected]. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 8 / 28 • After analyzing the QA tests and verifying the operational configurations, a test report will be submitted for each configuration. • The test report will specify the validity period for each configuration. • If the lidar PI is satisfied with the report - determining that additional tests would not improve the parameters detailed in the report - the PI must submit a confirmation message (via email or later via CARPORT) to both CARS and ARES, requesting that the operational configuration be locked for the specified period suggested within the test report. Note: If the reported parameters for any lidar configuration remain unchanged, or if CARS deems that any changes to these parameters are minor, the validity period of the previously locked lidar configuration may be extended as specified in the test report. Otherwise, new lidar configurations will be established for the up-coming operational period. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 9 / 28 4 Telecover test 4.1 About the test Deviations of near range signals from different parts of the telescope and the comparison of such deviations of different lidar channels and with theoretical ray-tracing simulations can reveal the distance of full overlap and possible reasons for the deviations from the ideal case. In the near range region, we do not have a calibration method for a lidar system, where almost never clean air conditions can be assumed. But shortcomings of the optical and opto-mechanical design or misalignments have their largest effect in the near range. A test for this range is based on the fact that the backscattered photons collected by different parts of the telescope of a lidar system must give the same range dependency of the partial signals, and if not, the range dependency of the whole signal is uncertain. With ray tracing simulations we see that ray bundles collected by different telescope parts reach the signal detector in different paths through the optical receiver and hit the optical components under different incident angles (see Figure 1), with possibly different transmission. Possible causes for the differences are laser tilt, telescope misalignments, displacement of field and aperture stops (vignetting, defocus), optical coating effects of, e.g., beam-splitters and interference filters with spatial inhomogeneity or angle dependency of the transmission (see Figure 2), or spatial inhomogeneity of the detector sensitivity (Simeonov et al. 1999). The geometrical overlap function, which is mainly determined by the size and location of the telescope's field stop, is just the most obvious feature producing differences in different telecover signals (Freudenthaler, V., et al., 2018). Figure 1: Ray bundles through the receiver optics of a typical lidar setup from the top part of the telescope (left) and from the bottom part (right), from near range (green) and far range (blue) have different paths and incidence angles on the optical elements (Freudenthaler, V., et al., 2018). Figure 2: Optical elements in a typical lidar receiver optics which can influence the transmission of the ray bundles due to vignetting (red arrows) or angular transmission dependency (blue arrows) (Freudenthaler, V., et al., 2018) ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 16 / 28 5.5 Internal analysis • The internal analysis made by the AHL operator should be performed using the ATLAS software. • Information on how to process and analyze the QA tests using the ATLAS software is provided in the user manuals and the dedicated CARS training courses. 5.5.1 Example Figure 5: The gain ratio signals retrieved with the Atlas tool for 355xcpt channel. The corrected linear depolarization ratio (orange line – right plot) is similar to the measured linear depolarization ratio (blue line – right plot). Deviation from the molecular is 0.0005±7.7e-5. 5.6 Filling the maintenance logbook • All QA tests performed on the AHL must be recorded in a dedicated logbook. A template of this logbook is provided in the following links: scheduled or continuous measurements. • The logbook must be stored in the dedicated station folder provided by CARS. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 17 / 28 6 Rayleigh fit test 6.1 About the test The comparison of lidar signals in clean air ranges with the signals calculated from air density and temperature profiles from radiosondes is the only absolute calibration of lidar signals. To be able to calibrate lidar signals with the so-called Rayleigh (molecular) backscatter signals, the optoelectronic detection systems must have a high dynamic range. The Rayleigh-fit is a normalization of the range corrected lidar signal to the calculated attenuated molecular backscatter coefficient (βm attn, Rayleigh signal) in a range where we assume clean air without aerosols and where the calculated signal fits the lidar signal within the noise limits. 6.2 Environmental conditions • The Rayleigh fit test is an extended normal lidar measurement performed in clear atmospheric conditions - without cirrus clouds. • The test must be performed using the same lidar setup as for normal measuring conditions. 6.3 Test procedure The Rayleigh fit test signals are preprocessed similar to a normal measurement, and therefore it is mandatory to collect a Dark signal before each test for all analogue channels. In case of photon counting channels, the additional dark signal is not required. For analogue channels: • Cover the telescope so that no light reaches the detection unit (either using a mechanical cover or an automatic shutter). • Collect a statistically representative Dark measurement for your instrument (5-20 minutes as a general rule). • Remove the telescope cover (or open the shutter). For all channels: • Collect at least 1 hour measurement and up to 6 hours (10 seconds/profile is advised). NOTEs: 1. Radiosonde data or model data must be provided. 2. The radiosonde/model data must be less than 18 hours after/before the end/beginning of the Rayleigh measurement in ascii or in SCC netcdf radiosonde format. 3. If the radiosonde site is more than 50 km away from the AHL location, modeled data is preferred. 4. Even if the Rayleigh fit test is performed for a daytime configuration, the test can be performed during nighttime in stable atmospheric conditions. 6.4 Schedule • The Rayleigh test must be performed and submitted to CARS every 12 months or after each instrument upgrade for each operational configuration used within the validity period. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 18 / 28 6.5 Internal analysis • The internal analysis made by the AHL operator should be performed using the ATLAS software. • Information on how to process and analyze the QA tests using the ATLAS software is provided in the user manuals and the dedicated CARS training courses. 6.5.1 Example Figure 6: Left: Photon counting lidar signal (blue) averaged over 1 h and calculated Rayleigh signal (red) from local radiosonde data of the same night (normalization around 16km). Right: the relative deviation from the calculated Rayleigh signal. 6.6 Filling the maintenance logbook • All QA tests performed on the AHL must be recorded in a dedicated logbook. A template of this logbook is provided in the following links: scheduled or continuous measurements. • The logbook must be stored in the dedicated station folder provided by CARS. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 19 / 28 7 Zero bin test 7.1 About the test A trigger-delay between the actual laser pulse emission and the assumed zero range of the signal recording (zero bin) can cause large errors in the near range signal up to about 1 km range. Especially the inversion of the Raman signals can be distorted dramatically, because the signal slope in the near range changes very much when the zero-bin for the range correction is varied. Hence it is worth some effort to verify that the zero-bin is really where we assume it to be. In case pre-trigger samples are recorded, the zero-bin can easily be detected due to the signal peak from stray light diffusely reflected from the laboratory walls. As the distance to the laboratory walls is not well defined, a diffuse scattering target blocking the laser path (see Figure 7 top) can be used together with a small hole aperture above the telescope to decrease the signal height to within the detection range of the detectors. In case no pre-trigger samples are recorded, the zero-bin can be detected by means of a near range target with a known distance to the lidar. Alternatively the sufficiently attenuated outgoing laser pulse can be fed into an optical fiber with sufficient length s and the fiber output positioned at the aperture of the telescope (see Figure 7 bottom). White open-cell foam often used for instrument packing and a piece of cheap communication fiber (see Figure 8) served us well for this purpose. With this a signal pulse can be measured with a delay dt = s / v = s / c * n with respect to the outgoing laser pulse, with c = speed of light in vacuum, v = speed of light in the fiber with refractive index n at the wavelength of the receiver channel. (sections from Freudenthaler, V., et al., 2018) Figure 7: Lidar trigger delay test setup with diffuse reflector (top) close to the laser and (bottom) with a foam block as beam diffuser/attenuator and a fiber delay line to achieve a controlled trigger pulse delay. The fiber must be as short as possible in order to minimize the wavelength dependent (refractive index) delay error (Freudenthaler, V., et al., 2018). ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 20 / 28 Figure 8: Open-cell foam and communication fiber used for the zero-bin measurement (Freudenthaler, V., et al., 2017). 7.2 Environmental conditions The environmental conditions are not relevant for this test. 7.3 Test procedure 7.3.1 Elastic channels Using optical fiber: • Start the AHL using the same procedure as for a normal measurement. • Place the telescope cover so that no light enters the telescope (except the light exiting the fiber end). Figure 9: The telescope cover (including the fiber support) (Freudenthaler, V., et al., 2017) • Place the other end of the fiber (with the open-cell foam) in front of the laser emission. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 21 / 28 Figure 10: Fiber + open-cell foam in front of the laser emission (Freudenthaler, V., et al., 2017). • Record 100 samples – single shot profiles if possible. If this is not possible, use the lowest number of shots possible for the recorder. Using diffuse reflector • Start the AHL using the same procedure as for a normal measurement. • Place the telescope cover so that only a small part of the diffused light enters the telescope (see Figure 7, 9). • Place the diffuser in front of the laser emission. • Record 100 samples – single shot profiles if possible. If this is not possible, use the lowest number of shots possible for the recorder. NOTE: 1. Check that the maximum signal does not exceed the detection range (100 V) in case of analogue channels and does not exceed the saturation limit in case of photon counting channels. An optimal value should be around 25% of the detection range. 2. Remember to use protective glasses. Limit the access to the lidar location while performing the test. 7.3.2 Inelastic channels The trigger delay test must also be performed for inelastic channels. Since the trigger delay test is focused on the electronic modules, the elastic PMT/APD detectors could be connected to the inelastic electronic modules to perform the zero bin test for inelastic channels. • Switch the signal cables of the inelastic channels to the PMT/APD of the elastic channels. • See the test procedure used for the elastic channels: optical fiber and diffused reflector. • Switch back the signal cables similar to the initial layout. NOTE: Please see the electronics user manual or contact the electronics manufacturer to get information on how to unplug and plug the signal cable to each detector unit. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 22 / 28 7.4 Schedule • The zero bin test must be performed and submitted to CARS one time and after each instrument upgrade. The zero bin test will be valid for all lidar configurations used within the validity period. 7.5 Internal analysis • The ATLAS software could be used to visualize the recorded signals and determine the zero bin value. NOTE: The zero altitude is determined by the first bin of the signal peak (Figure 11). Figure 11: Zero bin assessment: first bin of the signal peak (red line) 7.6 Filling the maintenance logbook • All QA tests performed on the AHL must be recorded in a dedicated logbook. A template of this logbook is provided in the following links: scheduled or continuous measurements. • The logbook must be stored in the dedicated station folder provided by CARS. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 23 / 28 8 Extended Dark signal measurement 8.1 About the test If signal distortions are independent of the lidar signal itself but synchronous with the laser repetition, they can be determined with so-called dark-measurement. The measured dark-signals without atmospheric backscatter from the laser can be subtracted from the normal lidar signals just as the skylight background or the analogue DC-offset, but as a range dependent offset. The dark measurement is like a normal measurement with laser and Q-switch trigger etc., but with fully covered telescope or a blocking of the optical path inside the receiver with a shutter, so that no light from the atmosphere and from the backscattered laser pulse is collected by the detectors. In such signals we can see EM-interferences from the electro-magnetic laser pulses or other electronic interferences which are synchronous to the laser trigger, but also rests of analogue low frequency noise, which can never be completely removed by means of spatial or temporal averaging. As there are different sources of such disturbances with different effects on averaged lidar signals, we currently don't have a standardized procedure for the dark measurements and cannot use them for the evaluation of the lidar signal quality in a standardized way. However, if after sufficient temporal averaging of the dark measurement the signal distortions are stable, which means not changing by further temporal averaging, the dark signals can be subtracted from the atmospheric signals to improve their accuracy. Because it is not practical to make the dark measurements for a timespan comparable to the atmospheric measurements, the subtraction of the dark measurement with the same smoothing length as the atmospheric measurement would considerably increase the signal noise in the far range. On the other hand, with a high dark signal smoothing in the near range the high frequency interspersions could not be removed. We therefore recommend to not smooth the dark signal in the near range and to start smoothing only when it would increase the signal noise. Furthermore, we found that the near range interspersions can change quite fast. Hence it is necessary for each channel to test the temporal stability of the dark signal regularly before using it for signal correction. 8.2 Environmental conditions The environmental conditions are not relevant for this test. 8.3 Test procedure The test is generally used for the analogue channels. In case of photon counting channels, the dark measurements should be submitted only once (and after every instrument upgrade) to check if external sources of electronic noise are affecting the signal. • Cover the telescope so that no light reaches the detection unit (either using a mechanical cover or an automatic shutter). • Collect a Dark measurement set (>30 minutes as a general rule). It is suggested to use 10 seconds/profile in order to detect short term fluctuations. • Remove the telescope cover (or open the shutter). ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 24 / 28 8.4 Schedule • The Dark test must be performed and submitted to CARS one time and after each instrument upgrade for all channels used for the SCC operational configurations. NOTE: 1. Even though the Dark measurement is performed before each measurement, the extended >30 minutes Dark measurement should be considered as an additional test. 2. The extended dark test can be submitted together with the Rayleigh fit test. 3. Do not switch off the channels that are not involved in the test since it might change the state of the lidar instrument. • The Dark measurement must be performed using exactly the same instrument setup as for the normal measurements, without moving cables or other equipment. 8.5 Internal analysis • The internal analysis made by the AHL operator should be performed using the ATLAS software. • Information on how to process and analyze the QA tests using the ATLAS software is provided in the user manuals and the dedicated CARS training courses. Figure 12: Temporal variability of the dark profiles for an and pc channels. 8.6 Filling the maintenance logbook • All QA tests performed on the AHL must be recorded in a dedicated logbook. A template of this logbook is provided in the following links: scheduled or continuous measurements. • The logbook must be stored in the dedicated station folder provided by CARS. ACTRIS Centre for Aerosol Remote Sensing C . A . R . S . 22 December 2025 25 / 28 9 QA test summary QA test CARS submission schedule Test details Telecover + alignment pictures Every 12 months. After major upgrades. For each operational lidar configuration. Biaxial Setup Dark measurement for analogue detection (5-20 minutes) + Stable atmospheric conditions: - at least three consecutive N-E-S-W iterations (e.g. each iteration must include 10 seconds profiles, 40 seconds / sector, resulting a total averaged profile/sector of at least 120 seconds). Un-stable atmospheric conditions: - at least five consecutive N-E-S-W iterations (e.g. each iteration must include 10 seconds profiles, 40 seconds / sector, resulting a total averaged profile/sector of at least 200 seconds). Coaxial Setup Dark measurement for analogue detection (5-20 minutes) + Stable atmospheric conditions: - at least three consecutive N-E-S-W iterations followed by three in-out iterations (e.g. each iteration must include 10 seconds profiles, 40 seconds / sector, resulting a total averaged profile/sector of at least 120 seconds). Un-stable atmospheric conditions: - at least five consecutive N-E-S-W iterations followed by five in-out iterations (e.g. each iteration must include 10 seconds profiles, 40 seconds / sector, resulting a total averaged profile/sector of at least 200 seconds). Rayleigh fit Every 12 months. After major upgrades. For each operational lidar configuration. Dark measurement for analogue detection (5-20 minutes) + - 1-6 hours normal measurement + (recommended: 10 seconds/profile) Polarization Calibration Every 12 months. After major upgrades. For each operational lidar configuration. ±45° measurement (5 minutes each calibrator position, recommended: 10 seconds / profile) Extended Dark Once. After major upgrades (an or pc). For all channels in op. configurations. >30 minutes (10 sec/profile) Zero Bin Once. After major upgrades. For all channels in op. configurations. 100 samples – single shot profiles (or lowest number of shots possible for the recorder) NOTE: please adapt each QA test based on the specificity of each AHL.