Pierre Exertier

Initially developed to monitor the performance of TOPEX/Poseidon and to follow the Jason legacy satellite altimeters at 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 altimeters that use a synthetic aperture radar (SAR) mode that is different to the conventional pulse-bandwidth limited altimeters often termed “low resolution mode” (LRM). The aim of this study is to characterize the sea surface height (SSH) bias of the new SAR altimeter instruments 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 altimeters provide measurements that are reliable at less than few hundred meters from the coast.

This paper presents a direct comparison between two satellite time transfer techniques: common-view (CV) of satellites from the global positioning system (GPS) constellation, and time transfer by laser link (T2L2) through the low orbiting satellite Jason-2. We describe briefly both techniques, together with two independent relative calibration campaigns of the links involving four European laboratories. Between the same remote time scale reference points, the mean values of the calibrated differences between GPS CV and T2L2 are below 240 ps, with standard deviations below 500 ps, mostly due to GPS CV. Almost all sample deviations from 0 ns are within the combined uncertainty estimates. Despite the relatively small number of common points obtained, due to the fact that T2L2 is weather dependent, these results provide an unprecedented sub-ns consistency between two independently calibrated microwave and optical satellite time transfer techniques.