# Calibrating the SAR SSH of Sentinel-3A and CryoSat-2 over the Corsica Facilities
## 3 Results
### Sentinel-3A SSH Calibration and Corrections Validation
Figure 2: Sentinel-3A corrections differences at the Senetosa (**left**) and Ajaccio (**right**) calibration sites. Ionospheric correction differences in the upper panels. Wet tropospheric correction differences in the lower panels. A second study of the wet tropospheric correction was conducted using the Ajaccio interpolation area (Figure 3) that should be more sensitive to land contamination compared to Senetosa. All the other parameters are identical to the standard processing performed for Ajaccio. Figure 4 shows that, in this case, the wet tropospheric correction is biased by \(-\)79 mm (light blue) using the Ajaccio GPS wet tropospheric correction as a reference. The 59 mm increase in the difference between using the Ajaccio and Senetosa interpolation areas illustrates the challenge to fully remove land contamination in the radiometer footprint in such complex coastal areas. In this case, the CLS correction clearly provides a great improvement even though a residual bias of \(-\)14 mm still exists (orange). In addition, the differences between the wet tropospheric corrections deduced from both the Senetosa and Ajaccio GPS data (\(\sim\)37 km separation distance and only five seconds time lag for a typical satellite overpass) are given. It is worth noting that, if the corrections are equivalent (+1 mm) on average, the dispersion of 12 mm certainly reflects different meteorological conditions at both sites at the same time. We conclude and confirm that it is very important to use GPS measurements as close as possible to satellite radiometer data when computing wet tropospheric correction terms. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Radiometer-GPS**} \\ \cline{2-3}  & **Mean** & \(\sigma\) \\ \hline Senetosa & & \\ Standard product & \(-\)23 & 16 \\ CLS & \(-\)8 & 13 \\ Ajaccio & & \\ Standard product & \(-\)20 & 17 \\ CLS & \(-\)5 & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Wet tropospheric correction differences on the common cycles (5–15) (in mm). Figure 4: Sentinel-3A wet tropospheric correction differences at Ajaccio (using the Ajaccio interpolation area). Wet tropospheric correction in the upper panel: derived from satellite microwave radiometer using the Senetosa interpolation area (dark blue), radiometer using the Ajaccio interpolation area (light blue), GPS from Ajaccio (light green), GPS from Senetosa (dark green), radiometer (from Collecte Localisation Satellites (CLS)) using the Senetosa interpolation area (red) and radiometer (from CLS) using the Ajaccio interpolation area (orange). Wet tropospheric correction differences in the lower panel: reference is always the wet tropospheric correction from the Ajaccio GPS and same color code is used for the corrections being compared. Finally, we also compared the corrections derived from the satellite instruments (radiometer and altimeter) using either SAR or PLRM processing. We found very small differences (<1 mm) that are caused by range differences between these two processing schemes. Finally, we compared the sea state bias corrections for SAR and PLRM that, for the moment, use the same correction tables: there are few millimeter differences originating from significant wave height (SWH) and wind speed differences, as the waveform shapes are different for SAR and PLRM. The SSH calibration was performed independently at Ajaccio and Senetosa using respectively a radar tide gauge and three pressure tide gauges. Figure 5 illustrates the process used that is detailed in [3]. We recall that by convention the SSH bias corresponds to the difference (SSH\({}_{\text{altitude}}\) - SSH\({}_{\text{tide gauge}}\)). This process was first carried out without taking into account the Ajaccio tide gauge SSH offset (see Section 0); a 26 mm offset clearly appears in the averaged results (\(-\)5 mm for Ajaccio and +21 mm for Senetosa) highlighting that the Ajaccio offset is clearly detectable by our absolute SSH calibration process. As a result, the Ajaccio tide gauge SSH time series were corrected by \(-\)30 mm, definitively.