# Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes
## 3 Results
### Sentinel-3 Level-2 Product Validation with the Geoid-Based Sea Surface Height  
We studied the waterbodies separately for validation with the geoid model. Two transects were chosen for the Gulf of Finland (Figure 2) and the residual of differences for S3A and S3B (\(\Delta{SSH_{TG}}\), Equation (5)) are presented in Figure 7. The mean \(\Delta{SSH_{TG}}\) was calculated for each 25-km-long transect when the satellite passed over the gulf. The time difference between S3A and S3B overpasses is two days. It is observed that the validation results and trends for both passes (S3A pass 186 and S3B pass 625) are very similar in the Gulf of Finland, which implies a similar accuracy level for both missions. The average \(\Delta{SSH_{TG}}\) for S3A and S3B was \(0.02\pm 0.08\) m. Note that there is no \(\Delta{SSH_{TG}}\) value for S3B for May 2019 (Figure 7, cycle 25) because the \(\Delta{SSH_{TG}}\) at \(0.31\pm 0.55\) m in this cycle is clearly an outlier. Such a big residual difference could have been caused by a local wind-driven water pile-up against the coast. After removing \(\Delta{SSH_{TG}}\) from cycle 25, an average residual difference of \(0.01\pm 0.06\) m was obtained for the Gulf of Finland (Table 5).  
Three transects were chosen for comparison in the Gulf of Riga: S3A pass 186, S3A pass 397 and S3B pass 397 (Figure 2). The results of S3A and S3B fit quite well in the Gulf of Riga (Figure 8). S3A pass 186 showed a better fit due to the shorter transect length (54 km, from the mainland to the island). For S3A pass 397 and S3B pass 397, the transects were 133 km (from the mainland to the mainland). Larger deviations were observed in winter from December to April when stormy weather conditions prevailed. The wind set-up near the coast is proportional to tangential wind speed component squared and can reach up to \(ca\) 1 m in certain Estonian coastal locations during strong storms [35]. The average residual differences between \(SSH_{SA}\) and \(SSH_{TG}\) was \(0.01\pm 0.07\) m. After removing an outlier (S3A  
Figure 9: Residual differences (\(\Delta{SSH_{TG}}\)) between the sea surface height of Sentinel-3 (\(SSH_{SA}\)) and geoid-based water surface height (\(SSH_{TG}\)) in Lake Peipus (blue dots) and Lake Vortsjárv (red dots). The zero line denotes the reference \(SSH_{TG}\). Coloured numbers indicate the cycle’s number. Vertical bars represent the \(\pm 1\sigma\) standard deviations.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Statistics** & **Gulf of Finland** & **Gulf of Riga** & **Lake Peipus** & **Lake Vörtsjárv** \\ \hline MEAN (m) & 0.01 & 0.00 & 0.14 & 0.13 \\ STD (m) & 0.06 & 0.07 & 0.16 & 0.27 \\ RMSE (m) & 0.07 & 0.10 & 0.23 & 0.27 \\ MIN (m) & \(-\)0.32 & \(-\)0.98 & \(-\)0.56 & \(-\)1.13 \\ MAX (m) & 0.33 & 0.58 & 0.98 & 0.78 \\ Common measurements \({}^{\star}\) & 75 & 190 \({}^{\star}\)445 \({}^{\star}\) & 100 & 30 \\ \hline \hline \end{tabular} \({}^{\star}\)Number of common measurements per each cycle, Figures 7–9. ** Sentinel-3A pass 186. *** Sentinel-3A pass 397 and Sentinel-3B pass 397.  
\end{table}
Table 5: Statistics of differences between the sea surface heights derived from altimetry (\(SSH_{SA}\)) and geoid-based sea surface height (\(SSH_{TG}\)) according to the Equation (5).  
pass 397 cyc 25 (11.03.2019) \(\Delta\)\(SSH_{\rm TG}\) = 0.410 \(\pm\) 0.600 m), the average residual difference for the Gulf of Riga is 0.00 \(\pm\) 0.07 m (Table 5).