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) mis- sion, 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 thethe verticalvertical distributionsdistributions ofof atmosphericatmospheric liquidliquid waterwater andand iceice onon aa 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-combi- nation 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.