# Calibrating the SAR SSH of Sentinel-3A and CryoSat-2 over the Corsica Facilities
## 1 Introduction  
The Corsica geodetic facilities that are located both at Senetosa Cape for the TOPEX/Poseidon (T/P) and Jason satellite ground tracks and near Ajaccio for the Envisat satellite ground tracks have been developed to calibrate successive satellite altimeters in an absolute sense. Since 1998, the successful calibration process used to calibrate many oceanographic satellite altimeter missions has been regularly updated, see details in [1; 2; 3]. One of the main issues in comparing offshore altimetric sea surface height (SSH) measurements together with in-situ tide gauge data is taking into account the geoid slope (of several cm/km) over distances of tens of kilometers between both locations. As discussed in [4], other factors including ocean dynamics and tidal differences, impact the comparisons with independent coastal in situ measurements, but are negligible in such areas and within the short distance in our study (less than 20 km). A specific Global Positioning System (GPS) campaign was conducted in 1999 in order to determine a geoid map of about 20 km long and 5.4 km wide centered on the T/P and Jason satellite's ground track #085 at the Senetosa Cape site [4]. In addition, a second campaign was conducted in 2005 in the Ajaccio area under the Envisat ground track #130 (Figure 1) [5; 6]. Both local geoids are the key datum for the absolute calibration of the satellite altimeters. Indeed, any distortion or systematic errors between offshore and tide gauge locations will cause a systematic error in the absolute SSH bias. From this point of view, the calibration process also requires accurate vertical offsets to be determined for each individual tide gauge in use. In order to achieve and maintain accuracy at the level of just a few millimeters or even less, geodetic campaigns must be carried out regularly in order to check or to eventually update such offsets. The present paper focuses on the calibration of both the Sentinel-3A and CryoSat-2 missions using the same facilities and similar data processing. Sentinel-3A has a 27-day repeat cycle, where CryoSat-2 has a repeat cycle of 369 days and a 30-day sub-cycle; this repetition allows them to perform the absolute calibration over the Senetosa and Ajaccio area on a monthly basis for both satellites. Due to the configuration of Sentinel-3A repeat ground track and some CryoSat-2 passes, each altimeter overflies both the Senetosa and Ajaccio sites with a time delay of about five seconds corresponding  
Figure 1: **Left**: General configuration of the Corsica calibration site with all of the satellite altimeter missions ground tracks that have been monitored since 1998 (the black frame on the left bottom corresponds to the zoom on the right map). **Right**: zoom on Ajaccio and Senetosa sites with Sentinel-3A (red) and CryoSat-2 (grey) tracks. The red rectangles highlight the CryoSat-2 pass #4794 that crosses the Senetosa and Ajaccio sites (see Section 0). The yellow rectangle shows the area that is discussed in Section 0 for pass #2426 (see Figure 8c). to a distance of about 37 km (Figure 1). As a consequence, it allows us to determine two SSH biases for each mission that should be equivalent considering that, most of the time, the sea state conditions at both sites are almost identical due to the short time delay and the short distance between sites. From the analysis of all available calibration passes, systematic differences can be related to errors in the geodetic references or in the situ data. For T/P-Jason and ERS-Envisat-SARAL/AltiKa this was difficult to determine as each altimeter mission overflow only a single site, Senetosa or Ajaccio respectively. Since July 2016, the SARAL/AltiKa mission is in a drifting orbit phase allowing a similar study to the one presented here that is discussed in [7]. In this paper we first present the data and the methodology (Section 0).