Towards an Error Budget for SAR Altimeters over the Ocean: from Cryosat-2 SAR to Sentinel-3 STM and Jason-CS
Gommenginger, Christine¹; Martin-Puig, Cristina²; Thibaut, Pierre³; Cotton, David⁴; Bonekamp, Hans⁵; Benveniste, Jérôme^6
1National Oceanography Centre, UNITED KINGDOM; ²isardSAT, SPAIN; ³CLS, FRANCE; ⁴Satellite Oceanographic Consultants (SatOC), UNITED KINGDOM; ⁵EUMETSAT, GERMANY; ⁶ESA, ITALY

The Cryosat-2 SIRAL altimeter has been providing measurements in SAR mode over a number of ocean sites since July 2010, delivering the first SAR altimeter data from space for scientists to determine the real capabilities of SAR altimetry for oceanography and coastal monitoring. Among a number of attractive features, observational evidence from Cryosat-2 SAR data confirms that SAR altimetry offers important improvements in performance compared with conventional pulse-limited altimeters, including reduced noise in sea surface height (by a factor of 1.5 compared to Jason-2) and finer along-track spatial resolution (down to 300m).

The Cryosat-2 SAR altimeter is a precursor of the SAR altimeters to be flown on two major future altimeter missions: the GMES Sentinel-3 Surface Topography Mission (STM) and the Jason-CS mission (follow-on to Jason-3). With SAR altimeters potentially replacing the long-established conventional pulse-limited altimeters, it is important to rapidly establish how well SAR altimeters can satisfy existing user requirements for ocean altimetry, be it for ocean mesoscale mapping or to continue the long-term climate-quality sea surface height time series needed for sea level change detection and monitoring.

In this paper, we will present the latest experimental results based on Cryosat-2 SAR data over the ocean and review the state of knowledge about the performance of SAR altimetry for oceanography. The paper will seek to update the error budget for SAR altimeter missions insofar as is possible with the current state of knowledge. The paper will highlight those issues that remain outstanding and what action may be taken to resolve them, leading to direct recommendations for the SAR altimeters on the Sentinel-3 STM and Jason-CS missions.