# Calibrating the SAR SSH of Sentinel-3A and CryoSat-2 over the Corsica Facilities
## 3 Results
### Sentinel-3A SSH Calibration and Corrections Validation  
The Sentinel-3A ascending pass #741 overflies the Senetosa site and \(\sim\)5 s later the Ajaccio site. For the first time this allows us to compare the SSH biases, which can be independently determined at both locations, and then to assess geodetic references together with in-situ measurements. Launched on February 16 th 2016, Sentinel-3A was in a ground-segment ramp-up phase during the first months with different versions of the IPF (instrument processing facilities) making the different cycles difficult to compare. Thanks to a reprocessing campaign [10] completed in September 2017 by the respective ESA and EUMETSAT agencies for cycles 5-19 (June 2016 to July 2017) with the same Processing Baseline 2.15, it is now possible to analyze a homogeneous set of data using the Non-Time Critical (NTC) product. We thus analyzed and compared the two modes which are used to derive the sea surface height from the radar measurements: (i) the SAR (synthetic aperture radar) mode and (ii) the PLRM (pseudo LRM) mode that mimics the classic LRM. A complete description of the products used and the processing options is given in Section 0. Concerning the ionospheric correction, both the dual-frequency altimeter derived and the GIM model were averaged over a distance of \(\sim\)140 km centered on the geoid area of Senetosa and Ajaccio (slightly shifted by \(\sim\)14 km for Senetosa to avoid measurements over Sardinia). The results presented in Figure 2 show good consistency between the instrument and GIM model in terms of bias (+6 mm) and stability (5-6 mm). Moreover, the results from either the Senetosa or Ajaccio sites show good consistency. Concerning the wet tropospheric correction deduced from the satellite radiometer, we have reduced the land contamination (as the radiometer has a large footprint) by filtering to keep the non-contaminated signal within our area of study. The selected corrections are linearly interpolated with an outlier rejection at 3-\(\sigma\) up to the edges of the study area. They are then considered as a constant for both Senetosa and Ajaccio processing. The Senetosa interpolation area was used for \begin{table}
\begin{tabular}{c c c} \hline
**Year** & **Reference Used** & **AJAC-G** & **Number of Days** \\ \hline
2005 & 50.0188 \(\pm\) 0.0100 & 2 \\
2013 & 50.0386 \(\pm\) 0.0011 & 3 \\
2015 & 50.0370 \(\pm\) 0.0002 & 7 \\ Average & 50.0351 \(\pm\) 0.0004 & \\ \hline \end{tabular}
\end{table} 
Table 1: Height differences between permanent Global Positioning System (GPS) (AJAC) and tide gauge (G) at Ajaccio (in m). both the Senetosa and Ajaccio sites (Figure 3) because in this area the radiometer is almost not contaminated by land. The permanent GPS receiver at Senetosa and Ajaccio also allows us to compute the wet tropospheric corrections at these locations. Recently, Collecte Localisation Satellites (CLS) have developed an algorithm to reduce the land contamination in coastal conditions (available for cycle five to 15) [16]. The three wet tropospheric corrections were compared and the differences are shown in Figure 2. We note that the radiometer wet tropospheric correction provided in the products appears to be biased on average (Senetosa and Ajaccio) by \(-17.5\) mm compared to the model and \(-20.5\) mm compared to the GPS. The differences between the two sites are very small (3-5 mm). The \(-\)\(-20\) mm bias is probably due to a remaining land contamination in the radiometer footprint. The analysis of the new radiometer correction developed by CLS to limit this effect shows better agreement with the correction derived from GPS. On the same time series (cycle 5-15), the bias is not only reduced by 15 mm in average but the standard deviation is also improved by 2 mm in average (Table 2). Figure 3: Map of the Sentinel-3A configuration: the shaded areas correspond to the locations where the wet tropospheric correction (radiometer and model) is interpolated.