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The French space agency CNES (Centre National d'Etudes Spatiales) has proposed and is studying an instrument concept for SIFTI based on a static interferometer, where the needed optical path are generated by a pair of crossed staircase fixed mirrors (replacing the moving reflector of dynamic Fourier transform interferometers like IASI or MIPAS). With the SIFTI design, a very high spectral resolution (~0.1 cm-1 apodised) is achieved in a very compact optical setup, allowing a large throughput, hence a high SNR. The measurements are performed in the 9.5 μm band for 03 and in the 4.6 μm band for C0.
The science return of the sounder can be further increased if an "intelligent pointing" process is implemented. This consists in combining the TIR sounder with a companion TIR imager, providing information on the cloud coverage in the next observed scene. 0nboard, real-time analysis of the IR image is used to command the sounder staring mirror to cloud free areas, which will maximize the probability for probing down to the surface. After the first part of the phase A, the architecture of SIFTI was studied as a trade-off between performance and resource budget. We review the main architecture and functional choices, and their advantages. The preliminary instrument concept is then presented in its main aspects and in terms of main subsystem functions.
The preliminary budgets of mass, volume, size and power are also evaluated. Eventually the science performances are estimated, at instrument level and at mission level, and are compared to the specifications. To finish, the ways forward are discussed.
The Sentinel-2 mission is based on a satellites constellation deployed in polar sun-synchronous orbits. Sentinel-2 will offer a unique combination of global coverage with a wide field of view (290km), a high revisit (5 days with two satellites), a high resolution (10m, 20m and 60m) and multi-spectral imagery (13 spectral bands in visible and shortwave infra-red domains). The first sentinel 2A has been launched on June 22nd, 2015, from Kourou, French Guyana.
In this context, the Centre National d’Etudes Spatiales (CNES) supports ESA to insure the cal/val commissioning phase, for Image Quality aspects.
This paper provides first, an overview of the Sentinel-2 system after the launch.
Then the articles focuses on the means implemented and activated in CNES to perform the In Orbit Commissioning, the availability and performances of the different devices involved in the ground segment : the GPP in charge of producing the level 1 files, the “radiometric unit” that processes sensitivity parameters, the “geometric unit” in charge of fitting the images on a reference map, MACCS that will produce Level 2A files (computing reflectances at the Bottom of Atmosphere) and the TEC-S2 that will coordinate all the previous software and drive a database in which will be gather the incoming Level 0 files and the processed Level 1 files.
In this framework, the French Space Agency (CNES: Centre National d’Etudes Spatiales) supports ESA on the activities related to Image Quality, defining the image products and prototyping the processing techniques.
Scope of this paper is to present the Ground Prototype Processor (GPP) that will be in charge of Level-1 production during Sentinel-2 In Orbit Acceptance phase. GPP has been developed by a European industrial consortium composed of Advanced Computer Systems (ACS), Magellium and DLR on the basis of CNES technical specification of Sentinel-2 data processing and under the joint management of ESA-ESTEC and CNES. It will assure the generation of the products used for Calibration and Validation activities and it will provide the reference data for Sentinel-2 Payload Data Ground Segment Validation.
At first, Sentinel-2 end-users products definition is recalled with the associated radiometric and geometric performances; secondly the methods implemented will be presented with an overview of the Ground Image Processing Parameters that need to be tuned during the In Orbit Acceptance phase to assure the required performance of the products. Finally, the complexity of the processing having been showed, the challenges of the production in terms of data volume and processing time will be highlighted. The first Sentinel-2 Level-1 products are shown.
Desert sites have been widely used for sensor calibration since they have a stable spectral response over time. Because of their high reflectances, the atmospheric effect on the upward radiance is relatively minimal. In addition, they are spatially uniform. Their temporal instability without atmospheric correction has been determined to be less than 1-2% over a year. Very-high-altitude (10 km) bright clouds are good validation targets in the visible and near-infrared spectra because of their high spectrally consistent reflectance. If the clouds are very high, there is no need to correct aerosol scattering and water vapor absorption as both aerosol and water vapor are distributed near the surface. Only Rayleigh scattering and ozone absorption need to be considered. This method has been found to give a 4% uncertainty.
Radiometric cross calibration of Earth observation sensors is a crucial need to guarantee or quantify the consistency of measurements from different sensors. ScaRaB is compatible with CERES mission. Two main spectral bands are measured by the radiometer: A short-wave channel (0.2 to 4 μm) dedicated to solar fluxes and a Total channel (0.2 to 200 μm) for fluxes combining the infrared earth radiance and the albedo. The earth long-wave radiance is isolated by subtracting the short-wave channel to the Total channel.
Both Earth Radiation Budget missions (CERES and ScaRaB) have the same specification: to provide an accuracy of ~1% in the measurement of short-wave and long-wave radiances and an estimation of the short-wave and long-wave fluxes less than 10 W/m2. We use the CERES PAPS and Cross-Track SSF datasets for direct radiances and fluxes comparisons during two validation phases. The first one occurred during April 17th to June 8th (51 days) in 2012 and the second one occurred between March 22th and May 31st 2015. The first validation campaign has been held with the CERES team using the Terra FM2 data. The CERES PAPS mode was used to align the swath scan, in order to increase the collocated pixels between the two instruments. This campaign allowed us to validate the ScaRaB radiances and to refine the error budget. The second validation campaign aims to provide a temporal monitoring of ScaRab calibration.
Sentinel-2 is a constellation of 2 satellites on a polar sun-synchronous orbit with a revisit time of 5 days (with both satellites), a high field of view - 290km, 13 spectral bands in visible and shortwave infrared, and high spatial resolution - 10m, 20m and 60m. The Sentinel-2 mission offers a global coverage over terrestrial surfaces. The satellites acquire systematically terrestrial surfaces under the same viewing conditions in order to have temporal images stacks. The first satellite has been launched in June 2015. Following the launch, the CAL/VAL commissioning phase will then last during 6 months for geometrical calibration.
This paper first provides explanations about Sentinel-2 products delivered with geometric corrections. Then this paper details calibration sites, and the methods used for geometrical parameters calibration and presents the first linked results. The following topics are presented: viewing frames orientation assessment, focal plane mapping for all spectral bands, first results on geolocation assessment, and multispectral registration. There is a systematic images recalibration over a same reference which will be a set of S2 images produced during the 6 months of CAL/VAL. As it takes time to have all needed images, the geolocation performance with ground control points and the multitemporal performance are only first results and will be improved during the last phase of the CAL/VAL. So this paper mainly shows the system performances, the preliminary product performances and the way to perform them.
So as to ensure for the multi-temporal registration of the products, specified to be better than 0.3 pixels at 2σ, a Global Reference Image (GRI) will be produced during the CAL/VAL period. This GRI is composed of a set of Sentinel-2 acquisitions, which geometry has been corrected by bundle block adjustment. During L1B processing, Ground Control Points will be taken between this reference image and the sentinel-2 acquisition processed and the geometric model of the image corrected, so as to ensure the good multi-temporal registration.
This paper first details the production of the reference during the CALVAL period, and then details the qualification and geolocation performance assessment of the GRI. It finally presents its use in the Level-1 processing chain and gives a first assessment of the multi-temporal registration.
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