Calibrating the SAR SSH of Sentinel-3A and CryoSat-2 over the Corsica Facilities

Initially developed to monitor the performance of TOPEX/Poseidon and to follow the Jason legacy satellite altimetersat Senetosa Cape, Corsica, this calibration/validation site has been extended to include a new location at Ajaccio. This addition enables the site to monitor Envisat and ERS missions, CryoSat-2 and, more recently, the SARAL/AltiKa mission and Sentinel-3A satellites. Sentinel-3A and CryoSat-2 carry altimetersthat use a synthetic aperture radar (SAR) mode that is different to the conventional pulse-bandwidth limited altimetersoften termed “low resolution mode” (LRM). The aim of this study is to characterize the sea surface height(SSH) bias of the new SAR altimeterinstruments and to demonstrate the improvement of data quality close to the coast. Moreover, some passes of Sentinel-3A and CryoSat-2 overfly both Senetosa and Ajaccio with only a few seconds time difference, allowing us to evaluate the reliability and homogeneity of both ground sites in term of geodetic datum. The Sentinel-3A and CryoSat-2 SSH biases for the SAR mode are respectively +22 ± 7 mm and −73 ± 5 mm (for CryoSat-2 baseline C products). The results show that the stability of the SAR SSH bias time series is better than standard LRM altimetry. Moreover, compared to standard LRM data, for which the measurements closer than ~10 km from the coast were generally unusable, SAR mode altimetersprovide measurements that are reliable at less than few hundred meters from the coast.