Atmospheric Lidar (ATLID): Pre-Launch Testing and Calibration of the European Space Agency Instrument That Will Measure Aerosols and Thin Clouds in the Atmosphere

atmosphere

Article ATmospheric LIDar (ATLID): Pre-Launch Testing and Calibration of the European Space Agency Instrument That Will Measure Aerosols and Thin Clouds in the Atmosphere

João Pereira do Carmo 1, Geraud de Villele 2, Kotska Wallace 1,*, Alain Lefebvre 1, Kaustav Ghose 1, Thomas Kanitz 1, François Chassat 2, Bertrand Corselle 2, Thomas Belhadj 2 and Paolo Bravetti 2

1 European Space Agency–ESTEC, 2200 AG Noordwijk, The Netherlands; [email protected] (J.P.d.C.); [email protected] (A.L.); [email protected] (K.G.); [email protected] (T.K.) 2 Airbus Defense and Space, CEDEX 4, 31402 Toulouse, France; [email protected] (G.d.V.); [email protected] (F.C.); [email protected] (B.C.); [email protected] (T.B.); [email protected] (P.B.) * Correspondence: [email protected]

Abstract: ATLID (ATmospheric LIDar) is the atmospheric backscatter Light Detection and Ranging (LIDAR) instrument on board of the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) mission, the sixth Earth Explorer Mission of the European Space Agency (ESA) Living Planet Programme. ATLID’s purpose is to provide vertical profiles of optically thin cloud and aerosol layers, as well as the altitude of cloud boundaries, with a resolution of 100 m for altitudes of 0 to 20 km, and a resolution of 500 m for 20 km to 40 km. In order to achieve this objective ATLID emits short duration laser pulses Citation: do Carmo, J.P.; de Villele, in the ultraviolet, at a repetition rate of 51 Hz, while pointing in a near nadir direction along track of G.; Wallace, K.; Lefebvre, A.; Ghose, the satellite trajectory. The atmospheric backscatter signal is then collected by its 620 mm aperture K.; Kanitz, T.; Chassat, F.; Corselle, B.; telescope, filtered through the optics of the instrument focal plane assembly, in order to separate Belhadj, T.; Bravetti, P.; et al. and measure the atmospheric Mie and Rayleigh scattering signals. With the completion of the full ATmospheric LIDar (ATLID): instrument assembly in 2019, ATLID has been subjected to an ambient performance test campaign, Pre-Launch Testing and Calibration followed by a successful environmental qualification test campaign, including performance calibra- of the European Space Agency tion and characterization in thermal vacuum conditions. In this paper the design and operational Instrument That Will Measure Aerosols and Thin Clouds in the principle of ATLID is recalled and the major performance test results are presented, addressing Atmosphere. Atmosphere 2021, 12, 76. the main key receiver and emitter characteristics. Finally, the estimated instrument, in-orbit, flight https://doi.org/10.3390/atmos predictions are presented; these indicate compliance of the ALTID instrument performance against 12010076 its specification and that it will meet its mission science objectives for the EarthCARE mission, to be launched in 2023. Received: 30 November 2020 Accepted: 23 December 2020 Keywords: LIDAR; UV laser; high spectral resolution; aerosols Published: 6 January 2021

Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- 1. Introduction: The EarthCARE mission ms in published maps and institutio- EarthCARE [1–5] is a joint collaborative mission of ESA and Japan Aerospace Ex- nal afﬁliations. ploration Agency (JAXA) with the objective to improve our understanding of the cloud- aerosol-radiation interactions and Earth radiative balance, so that they can be modelled with better reliability in climate and numerical weather prediction models [6]. To provide Copyright: © 2021 by the authors. Li- atmospheric observations globally, the EarthCARE satellite (Figure1), will be placed in a censee MDPI, Basel, Switzerland. Sun-synchronous orbit with a repeat cycle of 25 days. A low average altitude to ground of This article is an open access article 408 km has been selected in order to enhance the performance of the two active instruments. distributed under the terms and con- The quasi-polar orbit will allow to cover all latitudes from equator to ±83◦. ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Atmosphere 2021, 12, 76. https://doi.org/10.3390/atmos12010076 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, 76 2 of 15 Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 15

FigureFigure 1. 1.Artist’s Artist’s view view of of the the EarthCARE EarthCARE satellite satellite in-orbit. in-orbit Reproduced. Reproduced with with permission permission of of European Euro- pean Space Agency. Space Agency.

TheThe EarthCARE EarthCARE payload payload consists consists of of four fou instrumentsr instruments that that will, will, in ain synergetic a synergetic manner, man- retrievener, retrieve vertical vertical profiles profiles of clouds of clouds and aerosols, and aerosols, and the and characteristics the characteristics of the of radiative the radiative and micro-physicaland micro-physical properties, properties, to determine to determine flux gradientsflux gradients within within the atmospherethe atmosphere as well as well as topas top of atmosphere of atmosphere radiance radiance and and flux. flux. Specifically, Specifically, EarthCARE EarthCARE scientific scientific objectives objectives are: are: • Observation ofof thethe verticalvertical profilesprofiles ofof naturalnatural andand anthropogenicanthropogenic aerosols aerosols on on a a global global scale,scale, theirtheir radiativeradiative propertiesproperties andand interactioninteraction withwith clouds;clouds; • Observation of the the vertical vertical distributions distributions ofof atmospheric atmospheric liquid liquid water water and and ice ice on on a a global scale,scale, theirtheir transporttransport byby cloudsclouds andand theirtheir radiativeradiative impact;impact; • Observation ofof cloudcloud distributiondistribution (‘cloud(‘cloud overlap’),overlap’), cloudcloud precipitation precipitation interactions interactions and thethe characteristicscharacteristics ofof verticalvertical motionsmotions withinwithin clouds;clouds; • RetrievalRetrieval ofof profilesprofiles ofof atmosphericatmospheric radiativeradiative heatingheating andand coolingcooling throughthrough thethe combi-combination ofof thethe retrievedretrieved aerosolaerosol andand cloudcloud properties.properties. EarthCARE payload comprises three instruments provided by ESA: a High Spectral EarthCARE payload comprises three instruments provided by ESA: a High Spectral Resolution UV ATmospheric LIDar (ATLID) [7–11], a Multi-spectral Imager (MSI) and a Resolution UV ATmospheric LIDar (ATLID) [7–11], a Multi-spectral Imager (MSI) and a Broad-Band Radiometer (BBR) [12]; and the Cloud Profiling Radar (CPR) [13] with Doppler Broad-Band Radiometer (BBR) [12]; and the Cloud Profiling Radar (CPR) [13] with Dop- capability, provided by JAXA. The two active payloads, a LIDAR and a radar, penetrate pler capability, provided by JAXA. The two active payloads, a LIDAR and a radar, pene- into clouds and collect data, at a microscopic level, on atmospheric constituents and their movement.trate into clouds 3D scene and constructioncollect data, willat a microscopic be used to derive level, radiativeon atmospheric transfer constituents products from and thetheir active movement. instrument 3D scene observation construction data and will models. be used Calculated to derive radiative radiances transfer and fluxes products can thenfrom be the compared active instrument against theobservation BBR measurements data and models. of radiated Calculated thermal radiances versus and reflected fluxes solarcan then fluxes be atcompared the top against of the atmosphere; the BBR measurements conversion of to radiated flux is performedthermal versus analytically reflected usingsolar fluxes Angular at the Dependence top of the Modelsatmosphere and; theconversion three BBR to flux views is performed [14]. The Multi-spectralanalytically us- Imagering Angular provides Dependence contextual Models information and the about three the BBR relatively views [14]. small The scenes Multi observed-spectral byImager the activeprovides instruments, contextual as information well as providing about the spectral relatively scene small content scenes to observed aid with by un-filtering the active theinstruments, BBR data. as The well combined as providing dataset spectral will be scene an evolution content to of aid the with existing un-filtering observations the BBR of thedata. A-Train The combined satellites dataset CloudSat, will Cloud–Aerosol be an evolution LIDARof the existing and Infrared observations Pathfinder of the Satellite A-Train Observationssatellites CloudSat, (CALIPSO), Cloud and–Aerosol Aqua. LIDAR On one and hand, Infrared the impacts Pathfinder of climate Satellite change Observations can be further(CALIPSO), monitored. and Aqua. On the On other one hand, the advancedimpacts of payloads climate change will provide can be aerosol-cloud further moni- andtored. radiation On the measurementsother hand, the to advanced further refine payloads their will modelling provide of aerosol direct- andcloud indirect and radiation effects onme theasurements radiative to budget further [15 refine]. Co-registration their modelling of of the direct multi-instrument and indirect effects payload on datathe radia- is a keytive aspect budget for [15 the]. Co mission.-registration The instrumentof the multi viewing-instrument geometry payload can data be is seen a key in Figureaspect 2for, whichthe mission illustrates. The the instrument satellite ground viewing track, geometry the CPR ca beamn be seen at normal in Figure nadir, 2, thewhich ATLID illustrates beam de-pointedthe satellite backward ground track, by 3◦ theto reduceCPR beam any specularat normal reflection, nadir, the the ATLID across beam track de swath-pointed of thebackward MSI with by its 3° offsetto reduce in the any anti-sun specular direction reflection to mitigate, the across sun-glint track swath and finally of the the MSI 3BBR with

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its offsetviews in in the nadir, anti forward-sun direction and backward to mitigate directions sun-glint required and finally to retrieve the 3 BBR the topviews of atmospherein nadir, forwardemitted and ﬂux. backward directions required to retrieve the top of atmosphere emitted flux.

FigureFigure 2. EarthCARE 2. EarthCARE payload payload viewing viewing geometry. geometry.

2. ATLID2. ATLID Measurement Measurement Principle Principle ATLIDATLID is a is high a high spectral spectral Resolution Resolution LIDAR LIDAR (HSRL) (HSRL),, operating operating in the in theultraviolet ultraviolet spec- spec- traltral domain domain (355 (355 nm) nm),, which which takes takes advantage advantage of the ofdistinct the distinct interaction interactionss of light ofwith light mol- with molecules and aerosols that lead to different spectra scattering effects. Whereas the Brow- ecules and aerosols that lead to different spectra scattering effects. Whereas the Brownian nian motion of molecules induces a wide broadening of the incident light spectrum, the motion of molecules induces a wide broadening of the incident light spectrum, the scat- scattering with an aerosol does not affect the spectral shape of the incident light. Conse- tering with an aerosol does not affect the spectral shape of the incident light. Conse- quently, a simple means of separating the backscattering contributions of aerosols and quently, a simple means of separating the backscattering contributions of aerosols and molecules consists of filtering the backscattered spectrum with a high spectral resolution molecules consists of filtering the backscattered spectrum with a high spectral resolution filter centred on the laser emitted wavelength. In this way the instrument is able to separate filter centred on the laser emitted wavelength. In this way the instrument is able to sepa- the relative contribution of aerosol (Mie) and molecular (Rayleigh) scattering, which allows rate the relative contribution of aerosol (Mie) and molecular (Rayleigh) scattering, which the retrieval of the aerosol optical depth. Co-polarised and cross-polarised components of allows the retrieval of the aerosol optical depth. Co-polarised and cross-polarised compo- the Mie scattering contribution are also separated and measured on dedicated channels. nents of the Mie scattering contribution are also separated and measured on dedicated While CALIPSO is missing the HSRL capability to determine directly the extinction-to- channels.backscatter While ratio, CALIPSO Aeolus is missing is not equipped the HSRL to capability determine to thedetermine particle directly depolarisation the extinc- ratio. tionTherefore,-to-backscatter ATLID ratio, will allowAeolus for is the not first equipped time aerosol to determine typing (e.g., the particle smoke and depolari dust)s basedation on ratio.measured Therefore, intensive ATLID particle will allow properties. for the Thefirst operating time aerosol wavelength typing (e.g., in the smoke UV spectral and dust) range basedwas on selected measured as the intensive molecular particle scattering properties. is high enough The operating to measure wavelength more accurately in the extinc-UV spectraltion profiles range was and selected aerosols/thin as the cloudsmolecular thickness scattering and is because high enough laser technology to measure (Nd:YAG more accuratelylaser with extinction frequency profiles tripling and conversion) aerosols/thin is available clouds thickness for operation and inbecause this spectral laser tech- region. nologyAdditionally, (Nd:YAG eye laser safety with versus frequency field of tripling view dimension conversion) were is available also taken for into op considerationeration in thisfor spec thetral selected region. operational Additionally wavelength., eye safety versus field of view dimension were also taken intoAs consideration displayed in Figure for the3, selected ATLID measures operational atmospheric wavelength. profiles, in a direction close to theAs nadir, displayed with ain vertical Figure resolution3, ATLID measures of about 100atmospheric m from ground profiles, to in an a altitudedirection of close 20 km, to theand nadir, of 500 with m from a vertic altitudeal resolution 20 km to of 40about km. 100 The m instrument from ground transmitter to an altitude emits of short 20 km, laser andpulses of 500 with m from a repetition altitude rate 20 km of 51 to Hz,40 km. corresponding The instrument to about transmitter 285 m spatial emits sampling short laser for a pulseslocal with accumulation a repetition of rate two of shots, 51 Hz along, corresponding the horizontal to about track 285 of the m spatial satellite. sampling Atmospheric, for a localbackscattered accumulation photons of two are shots, collected along by the ATLID horizontal receiver track using of the a 620 satellite mm diameter. Atmospheric telescope., backscattered photons are collected by ATLID receiver using a 620 mm diameter telescope.

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FigureFigureFigure 3. 3. 3.ATLIDATLID ATLID atmospheric atmospheric atmospheric measurement measurement measurement principle principle. principle. .

3.3. 3.ATLID ATLIDATLID Instrument Instrument Instrument D Design esignDesign AirbusAirbusAirbus Defence Defence Defence and and and Space Space Space in in in Toulouse Toulouse Toulouse was was was responsible responsible responsible for for for the the the design, design, design, assembly assembly assembly and and and test of ATLID. The instrument was designed as a self-standing instrument, as seen in testtest of of ATLID. ATLID. The The instrument instrument was was designed designed as as a selfa self-standing-standing instrument, instrument, as as seen seen in in Fig- Fig- Figure4 , thereby reducing the mechanical coupling between instrument/platform inter- ureure 4, 4the, therebyreby reducing reducing the the mechanical mechanical coupling coupling between between instrument instrument/platform/platform interfac interfaceses faces and allowing better ﬂexibility in the satellite integration sequence. andand allowing allowing better better flexibility flexibility in in the the satellite satellite integration integration sequence. sequence.

FigureFigureFigure 4. 4.4.ATLID ATLID on on trolley trolley trolley after after after completion completion completion of ofits of its integration. its integration. integration. Reproduced Reproduced Reproduced with with permission with permission permission of of of Airbus © . AirbusAirbus © ©..

AATLIDTLIDATLID instrument instrumentinstrument developmentdevelopment development is is isbased basedbased on onon a aprotoa proto-flightproto-flight-flight model modelmodel (PFM) (PFM)(PFM) approach: approach:approach: criticalcriticalcritical sub sub-systemssub-systems-systems such suchsuch as asas laser laser laser transmitter, transmitter, transmitter, beam beam beam expander, expander, expander, beam beam beam steering steering steering mechanism mechanism mechanism andandand detector detector detector front front-endfront-end-end have have have been beenbeen subject subjectsubject to toto specific specificspecific efforts effortsefforts in inin terms termsterms of ofof early earlyearly breadboards, breadboards,breadboards, electricalelectricalelectrical model modelsmodels ors or or qualification qualification qualification models models models to to to minimi minimise minimises riskse risks. risks. The. The The instrument instrument instrument is is isbased based based on on on a a a bi-static architecture consisting of two independent main sections, the emitter chain and the receiver chain. ATLID functional architecture can be seen in Figure5.

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Atmosphere 2021, 12, 76 bi-static architecture consisting of two independent main sections, the emitter chain5 and of 15 the receiver chain. ATLID functional architecture can be seen in Figure 5.

FigureFigure 5. 5. ATLIDATLID functional functional architecture. architecture.

OnOn the the emitter emitter chain chain,, the the Transmitter Transmitter Assembly (TxA), designed and manufactured byby LEONARDO LEONARDO ( (Pomezia,Pomezia, Italy Italy),), is is the the laser laser source source for for the the ATLID ATLID LIDAR. LIDAR. It It comprises comprises a a powerpower laser laser Head Head (PLH), (PLH), seeded seeded via via a a fibre fibre optic optic by by a a reference reference laser laser head (RLH), (RLH), and and its its associatedassociated transmitter transmitter laser laser electronics electronics (TLE). (TLE). There There are are two two fully fully redundant redundant transmitters transmitters (in(in cold cold redundancy), redundancy), each each including including both both laser laser heads heads (PLH (PLH and and RLH) RLH) and and electronics (TLE).(TLE). The The PLH PLH design design is is based based on on a a diode diode-pumped-pumped triple tripledd Nd:Yag Nd:Yag laser laser,, providing a highhigh energy energy pulse pulse at at 355 355 nm. nm. In In the the nominal nominal measurement measurement mode mode it it is is operated operated in in steady steady-- statestate mode mode with with 51 51 Hz Hz pulse pulse repetition repetition frequency frequency (PRF). (PRF). While While the the laser laser transmitter transmitter is is largelylargely inheriting inheriting from from the the Aladin Aladin instrument instrument developme developmentnt for for the the AEOLUS AEOLUS mission mission [ [1515]],, aa significant significant evolution evolution has has been been achieved achieved by by the the fact fact that that the the ATLID ATLID PLH PLH is is sealed and and pressuripressurisedsed in in order order to to improve its tolerance to laser-inducedlaser-induced contamination.contamination. Both Both TxA TxA flightflight models models,, nominal nominal and and redundant redundant units, units, demons demonstratedtrated compliance compliance with with the themain main requirements:requirements: pulse pulse ener energygy >35 >35 mJ, mJ,pulse pulse duration duration <35 <35ns and ns andlaser laser beam beam divergence divergence < 300 < µrad300 µ (<45rad (<45µradµ afterrad afterEmission Emission Beam Beam Expander Expander).). AtAt the the heart heart of of the the receiver receiver chain chain the the High High Spectral Spectral Resolution Resolution Etalon Etalon (HSRE), (HSRE), devel- devel- opedoped by RUAG SpaceSpace (Zurich,(Zurich, Switzerland), Switzerland) differentiates, differentiates and and filters filters the the Mie Mie and and Rayleigh Ray- leighcomponents components of the of backscatter the backscatter signal, signal, routing routing these these (including (including splitting splitting co-polarised co-polari fromsed fromcross cross polarised polari light)sed light) towards towards the relevantthe relevant detection detection channels. channels. The The key key performance performance re- requirementquirement of ofthe the HSRE HSRE is achieving is achieving high high UV transmittance UV transmittance on the on Mie the co-polarised Mie co-polari channelsed channelwith a full-width-half-maximumwith a full-width-half-maximum (FHWM) (FHWM) of 0.3 pm.of 0.3 The pm. unit The conceptunit concept is based is based on a onFabry-Perot a Fabry-Perot (FP) (FP) etalon, etalon used, used in combination in combination with with polarisation polarisation beam-splitters beam-splitters (PBS) (PB andS) quarter-wave plates. The Fabry-Perot etalon acts as a filter, transmitting only the narrow and quarter-wave plates. The Fabry-Perot etalon acts as a filter, transmitting only the nar- Mie signal and reflecting the wider Rayleigh signal. row Mie signal and reflecting the wider Rayleigh signal. The instrument implements a co-alignment control loop, making use of a Beam The instrument implements a co-alignment control loop, making use of a Beam Steer- Steering Mirror (BSM) at transmitter chain level and a Co-Alignment Sensor (CAS) at ing Mirror (BSM) at transmitter chain level and a Co-Alignment Sensor (CAS) at receiver receiver chain level, in order to track and ensure that the emitted laser beam is aligned with chain level, in order to track and ensure that the emitted laser beam is aligned with the the receiver field of view. This active alignment approach allows to correct for structural, thermo-elastic deformations that could be expected after launch or in orbit.

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receiver field of view. This active alignment approach allows to correct for structural, thermo-elastic deformations that could be expected after launch or in orbit.

4. ATLID Performance Test Overview 4.1. Performance Verification Approach The LIDAR performance verification consists of the analysis of recordings of the instrument receiver response to an atmospheric echo simulation and of the validation of instrument transmitter characteristics. The LIDAR instrument is tested in ambient for all performances that are not sensitive to air to vacuum effects: timing calibrations, detection alignment and functional operations, such as receiver to transmitter co alignment functions (Figure 6 right). Then final tests are performed in a thermal vacuum test chamber (Figure 6 left), where real flight conditions are reproduced; in this way performance is evaluated according to actual thermal control stability. Also, vacuum conditions can affect optical coating phase shifts, change their characteristics and impact instrument performance. A limited set of key performance indicators are also measured at various stages of the satellite campaign on ground (in ambient conditions), in order to check the performance health of the ATLID, for instance upon delivery, after integration onto the satellite platform, before and after tests at satellite level. A dedicated in vacuum check of performance is also made during the satellite level thermal vacuum campaign.

4.2. Receiver Measurement Setup The instrument’s high sensitivity to the simulated echo characteristics shall be dis- criminated from the possible bias or instabilities of the test setup. The simulated echo Atmosphere 2021, 12,source 76 is a tuneable laser coupled to an optical system that control its divergence, line of 6 of 15 sight, wavelength, polarisation and pulse timings. Any uncontrolled variation of these parameters will induce a performance variation that will degrade the test accuracy. The measurement4. ATLID setup Performance used for Testthe instrument Overview characterisation is shown in Figure 6. It transmits 4.1.a beam Performance of the simplified Verification atmospheric Approach echo:  Generated by aThe frequency LIDAR-tripled, performance Nd:YAG verification, narrow- consistsband Optical of the Ground analysis Support of recordings of the Equipmentinstrument (OGSE), UV receiver laser response(source-pack to an outside atmospheric the vacuum echo simulation chamber), and which of the is validation of fibre-coupled;instrument transmitter characteristics. The LIDAR instrument is tested in ambient for all  Through partperformances B of OGSE that FPA are (OGSE not sensitive multi-purpose to air to focal vacuum plane effects: assembly timing, outside calibrations, the detection vacuum chamber,alignment detailed and functional in Figure operations, 6), whichsuch allows as receiverfor laser to beam transmitter parameters co alignment ad- functions justment, (Figure including6 right). polari Thensation, final line tests-of are-sight performed and radiometric in a thermal level vacuum, and then test chamberfree (Figure6 path; left), where real flight conditions are reproduced; in this way performance is evaluated  Through OGSEaccording collimator to actual COL70 thermal (70 cm control diameter, stability. inside Also, the vacuum vacuum conditions chamber); canup affect optical to ATLID coatingtelescope phase input. shifts, change their characteristics and impact instrument performance.

Figure 6. ATLIDFigure test 6. ATLID conﬁguration test configuration in vacuum in chamber, vacuum withchamber, FPA (Focalwith FPA Plane (Focal Assemblies) Plane Assemblies) parts A&B parts and collimator A&B and collimator COL70. Reproduced with permission of Airbus © . COL70. Reproduced with permission of Airbus ©.

A limited set of key performance indicators are also measured at various stages of the satellite campaign on ground (in ambient conditions), in order to check the performance health of the ATLID, for instance upon delivery, after integration onto the satellite platform, before and after tests at satellite level. A dedicated in vacuum check of performance is also made during the satellite level thermal vacuum campaign.

4.3. Transmitter Measurement Setup The two ATLID emitters have been characterised in terms of: (1) Laser pulse energy; (2) Laser beam divergence. Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 15

4.3. Transmitter Measurement Setup Atmosphere 2021, 12, 76 The two ATLID emitters have been characterised in terms of: 7 of 15 (1) Laser pulse energy; (2) Laser beam divergence. The additionalThe measurement additional measurement setup used setup for the used characteri for thes characterisationation is shown in is shownFigure in Figure7 7 and indicatedand indicatedwith FPA withpart FPAA in partFigure A 6 in (left Figure). It6 can (left). collect It can the collect radiation the from radiation both from both nominal andnominal redundant and lasers redundant (yellow lasers circles) (yellow into circles) single path into singletoward path COL70 toward inside COL70 the inside the vacuum chamber.vacuum Laser chamber. A beam Laser (bottom A beam circle) (bottom is reflected circle) is on reflected front side on frontof P0, side while of P0,laser while laser B B beam (topbeam circle) (top is circle)going isthrough going throughprism P0 prism and P0is reflected and is reflected on back on side. back The side. wedge The wedge angle angle of thisof prism this prism is sufficiently is sufficiently large largeto avoid to avoid any parasitic any parasitic images images within within the FF the (Far FF (Far Field) Field) imageimage pattern pattern (<100 (<100µrad).µ Arad). camera A camera system system is placed is placedat the output at the outputof COL70 of COL70sys- system, tem, outsideoutside the vacuum the vacuum chamber chamber on FPA on part FPA B part, as per B, asFigure per Figure 6. 6.

Figure 7. FPA partFigure A, OGSE7. FPA periscope part A, OGSE setup periscope for TxA beam setup collection for TxA intobeam COL70. collection Reproduced into COL70 with. Reproduced permission of Airbus ©. with permission of Airbus © . 5. Test Results 5. Test results5.1. Transmitter Test Results 5.1. Transmitter5.1.1. Test Laser Results Pulse Energy 5.1.1. Laser PulseThe Energy objective was to estimate the energy knowledge via the internal laser photodiode monitoring. To this purpose, the absolute energy was measured before Thermal Vacuum The objective was to estimate the energy knowledge via the internal laser photodiode (TVAC) test, acquiring the entire laser beam spot. For TVAC test two additional photodi- monitoring. To this purpose, the absolute energy was measured before Thermal Vacuum odes were installed and calibrated on the OGSE, one on its main bench (FPA part B) and (TVAC) test, acquiring the entire laser beam spot. For TVAC test two additional photodi- one inside the vacuum chamber behind a partially reﬂective mirror (FPA part A). These odes were installed and calibrated on the OGSE, one on its main bench (FPA part B) and two sensors aimed at providing a measurement reference external to the laser, all along the one inside the vacuum chamber behind a partially reflective mirror (FPA part A). These laser optical path, during the whole TVAC test. two sensors aimed at providing a measurement reference external to the laser, all along Atmosphere 2021, 12, x FOR PEER REVIEW An example result of the telemetry calibration is shown in Figure8a. Figure8b shows3 of 15 the laser optical path, during the whole TVAC test. the overall, internal photodiode reading during the entire test, for both emitters: energies An example result of the telemetry calibration is shown in Figure 8a. Figure 8b shows between 35 and 40 mJ were reached. the overall, internal photodiode reading during the entire test, for both emitters: energies between 35 and 40 mJ were reached.

(a) (b)

Figure 8. Example of Energy measurementsmeasurements performedperformed during during TVAC TVAC via via the the internal internal laser laser photodiode: photodiode: (a )( Energya) Energy telemetry telem- etrycalibration calibration plotting plotting calibrated calibrated detector detector measurement measurement of emitter of emitter energy (y)energy against (y) internalagainst laserinternal photodiode laser photodiode (x); (b) Overview (x); (b) Overview of Energy measurements during the entire vacuum phase (mJ). of Energy measurements during the entire vacuum phase (mJ). 5.1.2. Laser Beam Divergence With this test a measurement of the far field pattern (FFP) of the emitted laser beams was performed, thanks to the imaging capability of the COL70 collimator OGSE. The measurements were performed in vacuum, collecting OGSE images on a shot-to-shot basis (instantaneous FFP). Post processing averaging was done to compute integration over an equivalent of 10 km horizontal span. Additional measurements were also done to verify the capability to thermally adjust the external beam expander (EBEX) defocus and to quantify the thermal sensitivity; in this second phase, emission defocus calibration (EDC) mode is commanded, inducing a slow scan of EBEX temperature. A special test procedure is used to optimise the number of steps (down to 4 or 5 steps) for this ground test, in order to reduce test duration compared to a finer stepping in flight. Figure 9 shows the far field measurement results during EDC; as expected, the divergence is in the 15–45 µrad range (with angular resolution lower than 2 µrad and accuracy within ±5 µrad). Far field pattern images are reported in the figure in correspondence to the different tests’ EBEX temperatures.

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(a) (b)

AtmosphereFigure2021 8., Example12, 76 of Energy measurements performed during TVAC via the internal laser photodiode: (a) Energy telem-8 of 15 etry calibration plotting calibrated detector measurement of emitter energy (y) against internal laser photodiode (x); (b) Overview of Energy measurements during the entire vacuum phase (mJ).

55.1.2..1.2. Laser Laser Beam Beam D Divergenceivergence WithWith this test a measurement of the far field field pattern (FFP) of the emitted laser beams was performedperformed,, thanks to the imaging capability of the COL70 collimator OGSE. The measurementsmeasurements were performedperformed inin vacuum,vacuum, collectingcollecting OGSE OGSE images images on on a a shot-to-shot shot-to-shot basis basis(instantaneous (instantaneous FFP). FFP). Post Post processing processing averaging averaging was was done done to computeto compute integration integration over over an anequivalent equivalent of 10of km10 km horizontal horizontal span. span. Additional Additional measurements measurements were were also donealso done to verify to ver- the ifycapability the capability to thermally to thermally adjustthe adjust external the external beam expander beam expander (EBEX) defocus(EBEX) anddefocus to quantify and to quantifythe thermal the sensitivity;thermal sensitivity in this; second in this phase,second emissionphase, emission defocus defocus calibration calibration (EDC) ( modeEDC) modeis commanded, is commanded, inducing inducing a slow a slow scan scan of EBEX of EBEX temperature. temperature. A specialA special test test procedure procedure is isused used to to optimise optimise the the number number of of steps steps (down (down to to 4 or4 or 5 steps)5 steps) for for this this ground ground test, test in, in order order to toreduce reduce test test duration duration compared compared to to a finer a finer stepping stepping in flight.in flight. FigureFigure 99 showsshows thethe farfar fieldfield measurementmeasurement resultsresults duringduring EDC;EDC; as expected, the diver- µ µ gencegence is is in in the the 15 15–45–45 µradrad range range (with (with angular angular resolution resolution lower th thanan 2 µradrad and accuracy ± µ within ±55 µrad).rad). Far Far field field pattern pattern images images are are reported reported in in the the figure figure in correspondence to the different tests’ EBEX temperatures. the different tests’ EBEX temperatures.

Figure 9. Far ﬁeld measurement results during EDC; the variation of beam divergence (right scale) is measured and averaged for various EBEX temperatures (thermistor readings on the (left scale)). The yellow diamonds indicate averaged divergence measurements at 86% encircled energy, with associated camera acquired far ﬁeld pattern images overlaid. The blue circles and green squares indicate averaged measurement, in x and y, of angular width at 1/e2 of maximum. The solid lines indicate thermistor acquired temperature measurements.

The FFP was also acquired during the simulation of orbital cycle effects, i.e., for a ±1 ◦C temperature oscillation on the transmitter thermal interface. Figure 10 shows the evolution of encircled energy divergence during about 6 cycles. The FF images corresponding to the minimum and maximum temperatures are reported inside the plot. Atmosphere 2021, 12, x FOR PEER REVIEW 4 of 15

Figure 9. Far field measurement results during EDC; the variation of beam divergence (right scale) is measured and averaged for various EBEX temperatures (thermistor readings on the (left scale)). The yellow diamonds indicate averaged divergence measurements at 86% encircled energy, with associated camera acquired far field pattern images overlaid. The blue circles and green squares indicate averaged measurement, in x and y, of angular width at 1/e2 of maximum. The solid lines indicate thermistor acquired temperature measurements.

The FFP was also acquired during the simulation of orbital cycle effects, i.e., for a ±1 Atmosphere 2021, 12, 76 °C temperature oscillation on the transmitter thermal interface. Figure 10 shows the9 ofevo- 15 lution of encircled energy divergence during about 6 cycles. The FF images corresponding to the minimum and maximum temperatures are reported inside the plot.

FigureFigure 10. 10.Far Far ﬁeldfield measurementmeasurement results during simulation of of orbital orbital cycles cycles (± (±1 1°C◦ Con on the the laser) laser);; thethe variation variation ofof beambeam divergencedivergence (left(left scale scale,, diameter diameter of of 86% 86% encircled encircled energy energy)) plotted plotted over over time time (overlaid, camera acquired far field images). (overlaid, camera acquired far ﬁeld images).

5.2.5.2. RadiometricRadiometric Tests Tests TheThe ATLID ATLID receiver receiver has has been been characterised characterised in in terms terms of: of: • PolarisationPolarisation • Field-Of-ViewField-Of-View • RadiometricRadiometric stabilitystability • DarkDark current current

5.2.1.5.2.1. ATLIDATLID Receiver receiver Polarisationpolarisation Characterisationcharacterisation TheThe objective objective here here was was to to characterise characteris thee the response response of theof the ATLID ATLID receiver receiver for each for each of the of threethe three channels, channels, for both for polarisationboth polaris axes,ation andaxes, over and at over least at one least free one spectral free spectral range of range HSRE. of ToHSRE. this purpose, To this purpose, the OGSE the (Figure OGSE 11 (Figure) emission 11) line-of-sightemission line is-of centred-sight inis centred instrument in instru- ﬁeld- of-view,ment field radiometric-of-view, radiometric level is adjusted levelto is beadjusted in the instrumentto be in the measurementinstrument measurement dynamic, and: dy- •namic,Scans and: of the OGSE laser wavelength are performed, for vertical and horizontal linear polarisation directions, and the channels’ relative responses are extracted; • RotationsScans of the of theOGSE OGSE laser beam wavelength linear polarisation are performed, orientation for vertical are performed, and horizontal for OGSE linear Atmosphere 2021, 12, x FOR PEER REVIEW source-packpolarisation laser directions, wavelength and the set channel at HSREs’ relative response responses peak, and are the extracted channels’; 5 relativeof 15

 responsesRotationsare of the extracted. OGSE beam linear polarisation orientation are performed, for OGSE source-pack laser wavelength set at HSRE response peak, and the channels’ relative responses are extracted.

(a) (b)

Figure 11. Example of polarisationFigure 11. Example measurements of polari performedsation measurements during TVAC; performed channels’ during relative TVAC responses; channels’ versus: relative (a) OGSE re- frequency scan; (b) OGSEsponse linears versus polarisation: (a) OGSE rotation. frequency scan; (b) OGSE linear polarisation rotation.

5.2.2. ATLID Field-Of-View Characterisation The objective here was to evaluate the reception field-of-view angular diameter, in order to assess the collecting solid angle of the instrument. The same test shall also permit to the reference receiver line-of-sight with respect to the co-alignment sensor. To this purpose, the OGSE emission line-of-sight is scanned around the centre of the instrument field- of-view, radiometric level is adjusted to be in the instrument measurement dynamic, and the channels’ responses are extracted and normalised. The angular width of the normalised response gives the angular diameter of the instrument reception field-of-view, as can be seen in Figure 12.

Figure 12. Example of instrument field-of-view characterisation measurements performed during TVAC; channels’ normalised responses vs OGSE Line-of-Sight scan.

5.2.3. ATLID Radiometric Stability Characterisation The objective was to characterise the relative instrument response variation for each of the three science channels, with varying environment. To this purpose, the OGSE emission line-of-sight is centred in the instrument field-of-view, OGSE laser wavelength is set at HSRE response peak and the thermal environment in the vacuum tank is managed in order to describe the equivalence of the foreseen orbital thermal cycle. The channels’ signals are extracted and their stabilities during thermal cycle are calculated. As the OGSE frequency drift during the measurement contributes to signal variation during the test, its

Atmosphere 2021, 12, x FOR PEER REVIEW 5 of 15

(a) (b) Atmosphere 2021, 12, 76 10 of 15 Figure 11. Example of polarisation measurements performed during TVAC; channels’ relative responses versus: (a) OGSE frequency scan; (b) OGSE linear polarisation rotation.

5.2.2.5.2.2. ATLID ATLID Field Field-of-View-Of-View C Characterisationharacterisation TheThe objective objective here here was was to to evaluate evaluate the the reception reception field field-of-view-of-view angular angular diameter diameter,, in in orderorder to to assess assess the the collecting collecting solid solid angle angle of of the the instrument. instrument. The The s sameame test test shall shall also also permit permit toto the the reference reference receiver receiver line line-of-sight-of-sight with with respect respect to the to co the-alignment co-alignment sensor. sensor. To this To pur- this pose,purpose, the OGSE the OGSE emission emission line-of line-of-sight-sight is scanned is scanned around around the centre thecentre of the instrument of the instrument field- offield-of-view,-view, radiometric radiometric level is level adjusted is adjusted to be in to the be ininstrument the instrument measurement measurement dynamic, dynamic, and theand channel the channels’s’ responses responses are extracted are extracted and normali and normalised.sed. The angular The width angular of widththe normal- of the isnormaliseded response response gives the gives angular the angulardiameter diameter of the instrument of the instrument reception reception field-offield-of-view,view, as can beas seen can bein Figure seen in 12 Figure. 12.

FigureFigure 12. 12. ExampleExample of of instrument instrument field ﬁeld-of-view-of-view characteri characterisationsation measurements measurements performed performed during during TVACTVAC;; channels channels’’ normali normalisedsed responses responses vs vs. OGSE OGSE Line Line-of-Sight-of-Sight scan scan..

5.2.3.5.2.3. ATLID ATLID R Radiometricadiometric S Stabilitytability C Characterisationharacterisation TheThe objective objective was was to to characteri characterisese the relativerelative instrumentinstrument response response variation variation for for each each of ofthe the three three science science channels, channels, with with varying varying environment. environment. To To this this purpose, purpose, the the OGSE OGSE emission emis- sionline-of-sight line-of-sight is centred is centred in thein the instrument instrument ﬁeld-of-view, field-of-view OGSE, OGSE laser laser wavelength wavelength is is set set at atHSRE HSRE response response peak peak and an thed the thermal thermal environment environment in the in vacuumthe vacuum tank tank is managed is managed in order in orderto describe to describe the equivalence the equivalence of the of foreseen the foreseen orbital orbital thermal thermal cycle. cycle. The channels’ The channels’ signals sig- are nalsextracted are extracted and their and stabilities their stabilities during thermal during cyclethermal are cycle calculated. are calculated. As the OGSE As the frequency OGSE frequencydrift during drift the d measurementuring the measurement contributes contributes to signal variation to signal during variation the test,during its contribution the test, its is calculated and accounted for in the measured radiometric stability assessment, in order to assess the impact of thermal cycle only.

5.2.4. ATLID Dark Current Characterisation

The objective was to record the dark signal map and the dark noise in operational conditions at a −30 ◦C setpoint for the detector CCD. To this purpose, the OGSE does not emit any signal and ATLID is in dark current calibration (DCC) mode. The output of this test allows filling the calibration database to be provided with the instrument and to be used by the EarthCARE ground processer. The dark noise all along the atmospheric profiles is below the defined 2.2 e- rms success criteria for all channels, validating the ATLID performance model budgeted value. Atmosphere 2021, 12, x FOR PEER REVIEW 6 of 15 Atmosphere 2021 , 12, x FOR PEER REVIEW 6 of 15

contribution is calculated and accounted for in the measured radiometric stability assess- contribution is calculated and accounted for in the measured radiometric stability assessment, in order to assess the impact of thermal cycle only. ment, in order to assess the impact of thermal cycle only. 5.2.4. ATLID Dark Current Characterisation 5.2.4. ATLID Dark Current Characterisation The objective was to record the dark signal map and the dark noise in operational The objective was to record the dark signal map and the dark noise in operational conditions at a −30°C setpoint for the detector CCD. To this purpose, the OGSE does not conditions at a −30°C setpoint for the detector CCD. To this purpose, the OGSE does not emit any signal and ATLID is in dark current calibration (DCC) mode. The output of this emit any signal and ATLID is in dark current calibration (DCC) mode. The output of this test allows filling the calibration database to be provided with the instrument and to be test allows filling the calibration database to be provided with the instrument and to be used by the EarthCARE ground processer. The dark noise all along the atmospheric pro- used by the EarthCARE ground processer. The dark noise all along the atmospheric pro- Atmosphere 2021, 12, 76 files is below the defined 2.2 e- rms success criteria for all channels, validating the11 ATLID of 15 files is below the defined 2.2 e- rms success criteria for all channels, validating the ATLID performance model budgeted value. performance model budgeted value.

6. ATLID 6.Performance ATLID ATLID PerformancePerformance Update and UpdateU pdateFlight andand Prediction FlightFlight Prediction POredictionverview Overview Overview 6.1. Performance6.1. P Performanceerformance Analysis AnalysisAnalysis The performanceThe performance prediction predictionis made using isis mademade the test usingusing results thethe test testobtained results results with obtained obtained an artificial, with with an an artiﬁcial, artificial, atmosphericatmospheric echo simulation echo simulation to reconstruct toto reconstructreconstruct the instrumentthe the instrument instrument performance; performance; performance; as shown above as as shown shown above above during theduring instrument the instrument testing, no testing,testing, direct atmospheric nono directdirect atmosphericatmospheric test is performed test test is is performed. .performed In particular In. In, particular, theparticular the, the test sourcetesttest spectrum source is spectrum a monochromatic isis aa monochromatic,monochromatic, monomode, monomode,monomode, longitudinal,longitudinal longitudinal laser line at laser laser 355 line nm,line at at 355 355 nm, nm, while in flightwhile the in received ﬂightflight the backscattered receivedreceived backscatteredbackscattered signal spectrumsignal signal will spectrum spectrum be broaden will will be beed broadened broadenby Rayleighed by by Rayleigh Rayleigh backscattering superimposed onto a variable Mie spectrum line (see Figure 13). backscatteringbackscattering superimposed superimposed onto a variable onto Miea variable spectrum Mie line spectrum (see Figure line (13see). Figure 13).

Figure 13. IFigurellustration 13. IIllustrationofllustration the backscattered ofof thethe backscatteredbackscattered spectrum. spectrum.spectrum.

Also, the echoAlso, will the be echo partially willwill bebe depolari partiallypartiallysed depolariseddepolari by the sedatmospheric bybythe the atmospheric atmospheric scene; for scene;example, scene for; for example, example, Sahara aerosolsSahara can aerosols depolarise can depolarisedepolarise up to 30% upofup the toto 30% 30%polarised of of the the light. polarised polarised light. light. The instrumentThe instrument performance performance model is updated modelmodel is isbased updatedupdated on the basedbased instrument on on the the instrument instrumenttest results test test results results shown above.shownshown In above.above.particular InIn particular, ,particular the receiver the, the receiverchannel receiver channelcross channel talk crosss crossare talks ve talkrified ares verified are and ve calibrated.rified and calibrated. and calibrated. This This concernsThisconcerns at concerns first at the first atpolarisation thefirst polarisation the polarisation cross crosstalk characteristic talkcross characteristic talk characteristic between between co between polar co polar and co andcross polar cross and polar cross polar channelpolarchannelss that channel thatis checked iss that checked is via checked viainput input beam via beam input of pure of beam pure Co Co ofpolarisation, pure polarisation, Co polarisation, or orpur puree Cross Cross or purpo- polarisation.e Cross polarisation.larisation. TheThe spectralspectral The crosscross spectral talktalk is iscross also also confirmed, talkconfirmed, is also byconfirmed, by verifying verifying by the the verifying instrument instrument the spectrum instrumentspectrum response spectrum of response ofresponsethe the HSRE. HSRE. of The the Th crosseHSRE. cross talk talkTh correctione correctioncross talk needs needcorrection ares are shown shown need ins inare Figure Figure shown 14 .14 in. Figure 14.

HSR Filter X-talk Radiometric Atmosphere Polarisation Polarisation HSR LF0ilter X-talk Radiometric Atmosphere Spectral X-talk coLr0rections conversion X-talk X-talk Spectral X-talk corrections cLo1nbversion L1b Co-po AMB Co-po AMB RAY signal Co-po AMB Co-po AMB RAY signal Co-po molecular Co-po AMB CoM-pool. mCtoelecular Mol. Cte Co-po AMB Co-po APB Co-po APB MIE signal Co-po APB Co-po APB MIE signal Co-po particular Part. Cte Co-po APB Co-po particular Part. Cte Co-po APB

X-po APB Xpo Cte X-po APB X-po total X-poX pAoT BCte X-po Signal X-po total X-po ATB X-po AMB X-po Signal X-po Signal X-po AMB X-po Signal Instrumental Atmospheric Instrumental Atmospheric AMBdepol << AMB// è post-treatment L2 values values AMBdepol << AMB// è post-treatment L2 values values In-flight On-ground Acquisition In-flight Treaments On-ground Acquisition Treaments FigureFigure 14. Illustration 14. Illustration of signal of signal cross talks cross and talks need and for need cross for crosstalk corrections talk corrections.. Figure 14. Illustration of signal cross talks and need for cross talk corrections.

6.2. Performance Summary Table1 shows the ATLID instrument radiometric characteristics predicted for ﬂight and Table2 shows the ATLID detection performance.

Atmosphere 2021, 12, 76 12 of 15

Table 1. ATLID radiometric characteristics.

Radiometric Characteristics ATLID Unit Receiver field of view 66.5 1 µrad Telescope diameter 620 mm Solar background filter equivalent bandwidth 710 pm Mie Co Polar transmission EOL 2 45 % Mie Cross Polar transmission EOL 2 43 % Rayleigh transmission EOL 2 43 % (molecular to molecular channel 3) C_mm 75 % (molecular to particle channel 3) C_mp 25 % (particular to molecular channel 3) C_pm 16 % (particular to particular channel 3) C_pp 84 % Laser pulse energy 35 mJ Pulse repetition frequency 51 Hz Transmitter field of view 36 µrad Transmitter EOL transmission 89 % Transmitter polarisation Linear - 1 value predicted in flight is different from ground measured value due to gravity focusing effects; 2 End of life value without etalon transmission; 3 photo flux ratio characteristics of HSRE.

Table 2. ATLID detection performance.

Detection Performance ATLID Unit Vertical resolution from 0 to 20 km 100 m Vertical resolution from 20 km to 40 km 500 m Vertical cross talk 4.5% on 500 m, 11% on 100 m Computed quantum efﬁciency 79/75/79 1 % <+/−1 2 Linearity % <+/−2 3 Noise worst case 2.2 e- rms Dynamic margin on channel vs. predicted 18/5/12 1 % worst signal 1 (MieCoPo/MieCross/Rayleigh); 2 from 10 e- to the top; 3 from 1 e- to 10 e-.

Table3 shows the derived ATLID LIDAR performance of backscatter absolute retrieval accuracy, based on the relative retrieval accuracy on each channel and the absolute calibration of the LIDAR constant. The accuracy is given at 10 km altitude, on a 10 km horizontal integration length, for three types of scenes: • a sub visible cirrus: faint cirrus at the limit of instrument detection, β = 8 × 10−7 sr−1 m−1 • a thin cirrus: β = 1.4 × 10−5sr−1m−1 −5 • a depolarisation cirrus: cirrus with 10% depolarisation ratio, βpar = 2.6 × 10 −1 −1 −6 −1 −1 sr m and βper = 2.6 × 10 sr m

Table 3. ATLID LIDAR retrieval accuracy.

LIDAR Absolute Backscatter Retrieval Accuracy 10 km Typical BOL Worst Case EOL Unit Horizontal Integration Mie Co Polar on sub visible cirrus: β = 8 × 10−7 sr−1 m−1 31 48 % Mie Co Polar on thin cirrus: β = 1.4 × 10−5 sr−1 m−1 6 8 % Mie Cross polar on depol. cirrus: β = 2.6 × 10−6 sr−1 m−1 19 23 % Rayleigh (above cirrus 10 km) 12 17 % Atmosphere 2021, 12, 76 13 of 15

7. ATLID Verification Closure and Lessons Learnt Instrument performance verification steps have been completed as shown above. Following the environmental qualification and calibration at instrument level, ATLID has been integrated onto the EarthCARE satellite in March 2020 and will undergo satellite level thermal vacuum testing in 2022. Lessons learnt from previous LIDAR experience on ALADIN have shown the im- portance of prediction of the long-term evolution of the laser properties. The lifetime elements of the laser are the 808 nm pump diode aging, the high fluence coatings and the laser housing pressure. The laser diodes have undergone life testing, to demonstrate their longevity in accordance with the requirements of ATLID flight unit ground testing and mission lifetime. In addition, one other lesson learnt from ALADIN is the verification of the CCD dark current map stability. A “hot pixel” phenomena is observed on ALADIN, whereby random pixels exhibit an elevated signal level, with the number of hot pixels increasing over time [16]; this forces Aeolus satellite to have to compensate the effect with an additional, regular calibration that was not foreseen before flight. ATLID pixel level CCD behaviour has been characterised under flight-like operational parameters and the dark current values do not present the same level of defect, with no hot pixels seen; this is due to a different CCD control sequence, which reduces the time that the charge resides in the CCD by a factor of 10. Nevertheless, the pixel responses will be monitored and checked again once in satellite thermal vacuum operation, where flight dark current value can be recorded with the nominal −30 ◦C operating temperature.

8. Conclusions Since the completion of the full instrument assembly in 2019 [9], ATLID has been subjected to an ambient performance test campaign [10,11], followed by a successful environmental qualification test campaign, including performance calibration and characterisation in thermal vacuum conditions. ATLID has been successfully operated as a self-standing instrument and all performance aspects, including transmitter and receiver characteristics, have been found to be in line with targeted expectations and compliant with all major science, in-flight, end of life, performance requirements. This includes under worst case scenarios, where is assumed instrument performance degradation as well as operation in less favourable orbital points regarding sun illumination conditions. The instrument has been delivered to the EarthCARE industrial Prime for assembly and integration onto EarthCARE satellite platform, a task that was completed in May 2020 (Figure 15). Instrument and platform interface verifications are now approaching an end, with the execution of the last performance checks and functional testing at system level. With the completion of the foreseen platform qualification test campaign and flight ac- ceptance review in 2022, the launch of EarthCARE satellite in March 2023, and the first 6 months of in-orbit commissioning phase, ATLID will be ready to start its 3 year long mission, to measure aerosols and thin clouds from space. Data from ATLID is expected to provide a major contribution to the earth observation science community’s understanding of the cloud-aerosol-radiation interaction and Earth radiative balance, and in development of more reliable climate and numerical weather prediction models. Atmosphere 2021, 12, 76 14 of 15 Atmosphere 2021, 12, x FOR PEER REVIEW 9 of 15

FigureFigure 15. 15. ATLID integrated on on EarthCARE EarthCARE satellite satellite platform platform.. Reproduced Reproduced with with permission permission of of Airbus © . Airbus ©.

AuthorAuthor Contributions:Contributions: FFormalormal ana analysis,lysis, investigation, investigation, and and data data curation curation,, T.K. T.K.,, B.C. B.C.,, T.B., T.B.,P.B.; P.B.;writ- writing—originaling—original draft draft preparation, preparation, J.P.d. J.P.d.C.,C., G.d. G.d.V.,V., P.B., P.B., T.B., T.B., B.C.; B.C.; writing writing—review—review and editing, and editing, K.W., K.W.,K.G.,K.G., T.K.;T.K.; supervision, supervision, G.d.V G.d.V.,., J.P. J.P.d.C.;d.C.; project project administration, administration, A.L. A.L.,, K.W. K.W.,, F.C F.C.. AllAll authorsauthors have have readread and and agreed agreed to to the the published published version version of of the the manuscript. manuscript. Funding:Funding:This This development development was was funded funded by by European European Space Space Agency, Agency Earth, Earth Explorers Explorers Programme. Programme. InstitutionalInstitutional Review Review Board Board Statement: Statement:Not Not applicable. applicable. InformedInformed Consent Consent Statement: Statement:Not Not applicable. applicable. DataData Availability Availability Statement: Statement:The The data data presented presented in in this this study study are are available available on on request request from from the the correspondingcorresponding author. author. The The data data are are not not publicly publicly available available due due to to commercial commercial reasons. reasons. Acknowledgments:Acknowledgments:First First of of all, all, the the authors authors want want to to thank thank their their teams teams for for the the outstanding outstanding dedication dedication and commitment over the last years. The permanent interest from the science community, as well and commitment over the last years. The permanent interest from the science community, as well as as the support from ESA member states, has been highly appreciated. the support from ESA member states, has been highly appreciated. Conflicts of Interest: The authors declare no conflict of interest. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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