# Calibrating the SAR SSH of Sentinel-3A and CryoSat-2 over the Corsica Facilities
## 3 Results
### Sentinel-3A SSH Calibration and Corrections Validation
In Figure 5, the insert in green illustrates the improvement brought by SAR processed data. Indeed, when overflying the small Sanguinaires islands, only 2-3 data look bad and can be easily detected and rejected using a simple 3-\(\sigma\) criteria (circled crosses). For comparison with LRM in the same area, all the Envisat data that were at a distance less than 10 km from the coast were eliminated [6]. However, it is important to note that the areas used to perform the calibration (blue boxes in Figure 5) are far enough (>10 km) to avoid any land contamination for the altimeter, except maybe in PLRM mode in the Ajaccio area. Figure 5: Example of sea surface height (SSH) bias processing for Sentinel-3A in synthetic aperture radar (SAR) mode (pass 741, cycle 8). For each independent processing (Ajaccio, top/Senetosa, bottom) the upper plot shows \(\pm\)4 s of altimeter SSH series around the point of closest approach (PCA). The lower plots show the selected SSH (inside the geoid area) that are corrected from the geoid height differences between the SSH location and the tide gauge locations. The Figure 6 upper plots shows the SSH bias time series for SAR and PLRM respectively at Senetosa (left) and Ajaccio (right), while the lower plots show the differences (SAR-PLRM) either for \"orbit-range\" only or total SSH bias (\"orbit-range + corrections\"). The Ajaccio time series looks slightly noisier for SAR and much more so for PLRM compared to the Senetosa time series. This can be explained by more possible land contamination in the radar footprint in the Ajaccio area (e.g., the Capu di Muro and Sanguainaires islands, see Figure 5). The very good consistency of the \"orbit-range\" and \"SSH bias\" differences indicates that the correction differences between SAR and PLRM are very small, as explained previously. They are mainly due to SSB differences because SWH and wind speed estimations are different between SAR and PLRM processing. For the moment, the SSB model in the product is the same for SAR and PLRM. An improvement in terms of consistency is foreseen by the further tuning of SSB models for both SAR and PLRM, instead of using the same model. Thus, the main difference is due to range estimation that is different in SAR and PLRM processing. For each cycle, we have computed the averaged SSH bias from both Senetosa and Ajaccio independently (see the time series in Figure 7). First, it shows that the SSH bias for SAR is very stable with a standard deviation of 24 mm. Concerning the PLRM, as expected due to data processing issues (reconstruction of pseudo LRM waveforms from original SAR measurements that make the PLRM measurements noisier than real LRM), the standard deviation of 42 mm is higher than the classical LRM missions (e.g., 38 and 37 mm for Jason-1 and Jason-2 respectively [2]). Second, the 13 mm difference in the SSH bias between SAR and PLRM comes mainly from the differences in the altimeter range. From the entire data set (13 cycles), our best estimate of the SSH bias for Sentinel-3A is then:  
* Sentinel-3A SAR: +22 \(\pm\) 7 mm
* Sentinel-3A PLRM: +9 \(\pm\) 12 mm  
Recent studies using the transponder installed in Crete show that the SAR range bias is 0 \(\pm\) 12 mm [17] and 8 \(\pm\) 12 mm [18], so our results are in very good agreement because most of the SSH bias (+22 mm) originates from the radiometer wet tropospheric correction. Figure 6: Sentinel-3A SSH bias time series from Senetosa (**left**) and Ajaccio (**right**). The upper panels show the time series for SAR (black crosses) and Pseudo LRM (PLRM) (red diamonds). The lower panels show the differences between SAR and PLRM for the “orbit-range” (green circles) and the total SSH bias (blue squares).