The Copernicus Sentinel-3 mission, whose constellation is already composed of two units A and B - the third C units is about to be launched in Q3 2026 and the fourth D unit 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.
For the quality monitoring of all Sentinel-3 optical products and the measurement characterization in terms of accuracy and temporal stability, in-situ measurements are needed. But, as the quality assessment of acquisitions covering vast areas is a quite difficult task to be performed in a systematic way, the solution is to perform the validation over a carefully selected sub-set of sites covering the entire range of situations encountered by the satellite. These sites are called Fiducial Reference Measurements (FRM) sites and are intensively validated on the ground.
The Land and Water (LAW) project is being performed under a European Space Agency (ESA) contract with the French Space Agency (CNES). It provides accurate in-situ measurements over several long-term, instrumented FRM sites for Sentinel-3 OLCI and SLSTR radiometric products to support validation activities performed by ESA and its partners in relation to the Copernicus program.