Enabling planetary science across light-years. Ariel Definition Study Report

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution

TRISHNA In-orbit Radiometric Calibration

TRISHNA is an Indian-French cooperative mission to be launched in 2025. Its principal focus will be on evapo-transpiration, ecosystem stress, and coastal oceans sea surface temperature. The satellite will host two payloads: one instrument in the VSWIR range – 7 bands from 485 to 1600 nm under ISRO’s responsibility, and one instrument in the TIR range – 4 bands from 8.5 to 11.5 μm. The resolution of both instruments is 57m at nadir and their swath is about 1000 km. TRISHNA’s TIR and VSWIR instruments will be extensively calibrated and characterized in on-ground facilities to ensure that their performance will meet the specifications throughout TRISHNA’s lifetime. Yet, the operation in space after the launch and during several years can lead to different behaviors as the ones expected from ground calibration, justifying a validation and if necessary a calibration of some parameters of the ground processing. The TRISHNA TIR instrument radiometric performances rely on an on-board calibration device (blackbody) and cold space viewing. Some so-called vicarious calibration methods are being developed in the TIR using natural targets in order to validate and monitor the calibration performed using the blackbody, and to potentially be used as a backup in case of hardware failure. These natural targets include the oceans, the Moon, snow deserts in Antarctica or Greenland and instrumented sites. The TRISHNA VSWIR instrument on the other hand will not benefit from an on-board calibration and will exclusively rely on vicarious calibration. CNES has experience in vicarious calibration in the VSWIR and will use some methods which have been already proven for other sensors (Rayleigh scattering, deserts, clouds, Moon and instrumented sites). The presentation will focus on the description of the radiometric calibration strategy and the associated methods

VENμS: Mission Specificities, Products Features and In-orbit Absolute Calibration

Earth observation satellites like Sentinel-2 or Landsat 8 have already demonstrated the importance of a global coverage associated with high resolution (about 10 m) for regional and country scales applications. These applications, such as detailed land-cover mapping, agri-environment policies, water management, vegetation primary productivity and yield estimates, are crucial for defining global change mitigation or adaptation policies. To prepare the future earth observations systems, users raised one question about the increasing of the revisit period in order to limit the impact of cloud-coverage on the applications and to capture rapid phenomena. In this context, VENµS products offer an undeniable added value to explore the benefit of expanding the time rate of high resolution acquisition in visible and near infrared spectral bands. VENµS is a joint space system venture of Israeli and French governments for Earth observation (EO). The scientific mission focuses on vegetation and land surface monitoring. VENµS was launched on August 1st, 2017. It provides 5 and 10 m resolution images in 12 shortwave spectral bands every two days over a set of 110 scientific sites, with constant viewing angle and overpass time. This article presents the objectives of the mission, its main characteristics and available products. A special focus is made on the in-orbit absolute calibration, based on vicarious techniques, including specific capabilities such as calibration using Moon images. The process of inter-calibration with Sentinel-2 through simultaneous nadir observation will be explained, and the results detailed. VENµS data are freely available to everybody for peaceful and non-commercial uses on the French Theia land data center: http://www.theia-land.fr. Continuous observations will be performed all along the scientific mission duration, until mid-2020

A Software Tool for Atmospheric Correction and Surface Temperature Estimation of Landsat Infrared Thermal Data

International audienceLand surface temperature (LST) is an important variable involved in the Earth's surface energy and water budgets and a key component in many aspects of environmental research. The Landsat program, jointly carried out by NASA and the USGS, has been recording thermal infrared data for the past 40 years. Nevertheless, LST data products for Landsat remain unavailable. The atmospheric correction (AC) method commonly used for mono-window Landsat thermal data requires detailed information concerning the vertical structure (temperature, pressure) and the composition (water vapor, ozone) of the atmosphere. For a given coordinate, this information is generally obtained through either radio-sounding or atmospheric model simulations and is passed to the radiative transfer model (RTM) to estimate the local atmospheric correction parameters. Although this approach yields accurate LST data, results are relevant only near this given coordinate. To meet the scientific community's demand for high-resolution LST maps, we developed a new software tool dedicated to processing Landsat thermal data. The proposed tool improves on the commonly-used AC algorithm by incorporating spatial variations occurring in the Earth's atmosphere composition. The ERA-Interim dataset (ECMWFmeteorological organization) was used to retrieve vertical atmospheric conditions, which are available at a global scale with a resolution of 0.125 degrees and a temporal resolution of 6 h. A temporal and spatial linear interpolation of meteorological variables was performed to match the acquisition dates and coordinates of the Landsat images. The atmospheric correction parameters were then estimated on the basis of this reconstructed atmospheric grid using the commercial RTMsoftware MODTRAN. The needed surface emissivity was derived from the common vegetation index NDVI, obtained from the red and near-infrared (NIR) bands of the same Landsat image. This permitted an estimation of LST for the entire image without degradation in resolution. The software tool, named LANDARTs, which stands for Landsat automatic retrieval of surface temperatures, is fully automatic and coded in the programming language Python. In the present paper, LANDARTs was used for the local and spatial validation of surface temperature obtained from a Landsat dataset covering two climatically contrasting zones: southwestern France and central Tunisia. Long-term datasets of in situ surface temperature measurements for both locations were compared to corresponding Landsat LST data. This temporal comparison yielded RMSE values ranging from 1.84 • C–2.55 • C. Landsat surface temperature data obtained with LANDARTs were then spatially compared using the ASTER data products of kinetic surface temperatures (AST08) for both geographical zones. This comparison yielded a satisfactory RMSE of about 2.55 • C. Finally, a sensitivity analysis for the effect of spatial validation on the LST correction process showed a variability of up to 2 • C for an entire Landsat image, confirming that the proposed spatial approach improved the accuracy of Landsat LST estimations

New RadCalNet Instrumented Site at Gobabeb, Namibia: Installation Field Campaign and First Absolute Calibration Results

A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellite sensors in the visible/near-IR has been set up in Namibia, near the Gobabeb Research and Training Centre on the edge of the Namib Desert. This site is the European contribution to the Committee on Earth Observation Satellites (CEOS) initiative RadCalNet (Radiometric Calibration Network). The Gobabeb area has been selected based on the analysis of different datasets to estimate surface spatial homogeneity, cloud coverage, temporal variability, atmospheric turbidity and flatness. A field campaign took place in November 2015 in order to find the precise location of the future permanent instrumentation in the area identified from satellite data (Modis, Landsat8, Sentinel2, Pleiades). This location is the one with the best spatial homogeneity at different scales: instrument field-of-view (tens of centimeters), extent of the instrument-monitored area (tens of meters) and satellite sensor resolution (tens to hundreds of meters). The field campaign also focused on the characterization of the surface reflectance, the surface hemispherical directional reflectance factor and the atmospheric turbidity. Following this campaign, the permanent instrumentation (CIMEL photometer) has been installed in June 2017 and has the ability to measure atmosphere (aerosol optical thickness etc.) and surface conditions (reflectance). The data has been processed by the ROSAS processing software at CNES in order to obtain first results of surface reflectance and to perform the vicarious absolute calibration of optical sensors (Sentinel2A and Sentinel2B, Landsat8 etc.) The presentation will focus on the installation field campaign and the analysis of the data produced by this new calibration station

New RadCalNet Instrumented Site at Gobabeb, Namibia: Field Campaign Conclusions and First Absolute Calibration Results

A new permanently instrumented radiometric calibration site for high/medium resolution imaging satellite sensors in the visible/near-IR has been set up in Namibia, near the Gobabeb Research and Training Centre on the edge of the Namib Desert. This site is the European contribution to the Committee on Earth Observation Satellites (CEOS) initiative RadCalNet (Radiometric Calibration Network). The Gobabeb area has been selected based on the analysis of different datasets to estimate surface spatial homogeneity, cloud coverage, temporal variability, atmospheric turbidity and flatness. A field campaign took place in November 2015 in order to find the precise location of the future permanent instrumentation in the area identified from satellite data (Modis, Landsat8, Sentinel2, Pleiades). This location is the one with the best spatial homogeneity at different scales: instrument field-of-view (tens of centimeters), extent of the instrument-monitored area (tens of meters) and satellite sensor resolution (tens to hundreds of meters). The field campaign also focused on the characterization of the surface reflectance, the surface hemispherical directional reflectance factor and the atmospheric turbidity. Following this campaign, the permanent instrumentation (CIMEL photometer) has been installed in May/June 2016. The data has been processed by the ROSAS processing software at CNES in order to obtain first results of surface reflectance and to perform the vicarious absolute calibration of optical sensors (Sentinel2, Sentinel3 etc.). The presentation will focus on the field campaign conclusions and the analysis of the data produced by this new calibration station

RadCalNet: A Radiometric Calibration Network for Earth Observing Imagers Operating in the Visible to Shortwave Infrared Spectral Range

Vicarious calibration approaches using in situ measurements saw first use in the early 1980s and have since improved to keep pace with the evolution of the radiometric requirements of the sensors that are being calibrated. The advantage of in situ measurements for vicarious calibration is that they can be carried out with traceable and quantifiable accuracy, making them ideal for interconsistency studies of on-orbit sensors. The recent development of automated sites to collect the in situ data has led to an increase in the available number of datasets for sensor calibration. The current work describes the Radiometric Calibration Network (RadCalNet) that is an effort to provide automated surface and atmosphere in situ data as part of a network including multiple sites for the purpose of optical imager radiometric calibration in the visible to shortwave infrared spectral range. The key goals of RadCalNet are to standardize protocols for collecting data, process to top-of-atmosphere reflectance, and provide uncertainty budgets for automated sites traceable to the international system of units. RadCalNet is the result of efforts by the RadCalNet Working Group under the umbrella of the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV) and the Infrared Visible Optical Sensors (IVOS). Four radiometric calibration instrumented sites located in the USA, France, China, and Namibia are presented here that were used as initial sites for prototyping and demonstrating RadCalNet. All four sites rely on collection of data for assessing the surface reflectance as well as atmospheric data over that site. The data are converted to top-of-atmosphere reflectance within RadCalNet and provided through a web portal to allow users to either radiometrically calibrate or verify the calibration of their sensors of interest. Top-of-atmosphere reflectance data with associated uncertainties are available at 10 nm intervals over the 400 nm to 1000 nm spectral range at 30 min intervals for a nadir-viewing geometry. An example is shown demonstrating how top-of-atmosphere data from RadCalNet can be used to determine the interconsistency between two sensors.European Space Agency Technology and Research Programme [4000110704]; European Space Agency Earthnet Programme [CCN5 4000110704]; Metrology for Earth Observation and Climate project (MetEOC-2) within the EMRP programme [ENV55 532]; EURAMET; European Union's FP7 programme; European Union's H2020 programme; European Space Agency Technology and Research Programme through the ACTION project; UK Government's Department for Business, Energy and Industrial Strategy (BEIS) through the UK's National Measurement System programmes; Bureau of International Co-operation Chinese Academy of SciencesChinese Academy of Sciences [181811KYSB20160040]; NASANational Aeronautics & Space Administration (NASA) [NNX14AE20G, NNX15AM86G, NNX16AL25G]; USGSUnited States Geological Survey [G14AC00371]; MetEOC-3 project under the EMPIR programme [16ENV03]Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Ariel: Enabling planetary science across light-years

Ariel Definition Study ReportAriel Definition Study Report, 147 pages. Reviewed by ESA Science Advisory Structure in November 2020. Original document available at: https://www.cosmos.esa.int/documents/1783156/3267291/Ariel_RedBook_Nov2020.pdf/Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution