The Copernicus Sentinel-3 mission, whose constellation is already composed of two units A and B - the third and the fourth C and D units being built - is an unprecedented valuable Earth observation system, providing accurate measurements which are then converted in products usable for various applications. Its three payloads - two for the optical mission, the Ocean and Land Colour Imager (OLCI), the Sea and Land Surface Temperature Radiometer (SLSTR), and one for the altimetry mission, the Synthetic Aperture Radar Altimeter (SRAL) - deliver routine and systematic acquisitions worldwide, i.e. over both land and marine areas.
Many products and datasets are provided operationally. In particular, Integrated Water Vapor (IWV) and Land Surface Temperature (LST) are produced and distributed systematically since launch. IWV are part of the OLCI level 2 products whereas LST are computed from SLSTR L1 products acquired on the nadir view. On a contrary, the Aerosol Optical Depth (AOD) has been added in January 2020 in the S3 portfolio and take the opportunity of the SYNERGY processing branch to use both OLCI and SLSTR radiometric measurements.
A significant effort is currently made to validate the Sentinel-3 products by various teams and experts, coordinated under different frameworks. The S3 Validation Team (S3VT), managed by the agencies, gathers international experts who use and validate Sentinel-3 products. A workshop is held annually during when validation results are shared and discussed. The Sentinel-3 Mission Performance Centre (S3MPC) service contract aims to perform a routine quality control of all S3 products and to perform a quality assessment of the S3 mission in a whole. Calibration and validation activities, as well as the corrective and evolutive maintenance of the Instrument Processing Facilities (IPFs) are also part of the S3MPC.
Despite all these efforts, there is still a lot to do and the LAW project has been designed to perform a more extensive and systematic validation, independent and complementary to the work performed by S3MPC experts.
This validation has been done using ground-based measurements from selected networks. These networks have been analyzed and defined as relevant ones for the purpose of this project.
In addition, thanks to a gap analysis of the LST ground-based measurement performed during the phase 1 of this project, relevant locations have been defined for the deployment of five new LST stations. This choice was driven by the necessity of increasing the representativity of sampled biomes around the globe. The procurement, installation and operation of these new LST stations is preformed in the frame of the LAW project and data will be then used for the validation of LST products.
Along with the validation of these three S3 products, two more objectives have been pursued in LAW:
- The analysis of performance and limitations of S3 products and the definition of requested evolutions to be discussed with S3 experts.
- The availability to users of all matchups between ground-based measurements and S3 operational data, used for validation - and in particular, in-situ data from the new LST stations through this website.
LAW has been started in January 2020. Matchups database is openly available since October 2022. The final validation results for AOD and IWV datasets have been delivered to ESA in December 2021 whereas consolidated LST validation results are expected in December 2022.