# Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes
## 3 Results  
### Sentinel-3 Level-2 Product Validation with GNSS Campaigns  
The statistics of the residuals from the Equation (6) along altimetry tracks are presented in Table 4. Note that the number of common measurements vary in different waterbodies. It is caused by the length of the GNSS transect (Table 2). According to the results in multiple locations, the two datasets in the Baltic Sea correspond better than those of inland cases; the mean (MEAN) and STD were \(0.11\pm 0.08\) m and \(0.14\pm 0.05\) m in the Gulf of Finland and the Gulf of Riga, respectively. These results also match well with the validation by the geoid model for S3B pass 625 in the Gulf of Finland on 25 April 2019 (0.11 \(\pm\) 0.06 m) (Figure 7) and for S3A pass 186 in the Gulf of Riga on 20 June 2019 (\(0.09\pm 0.05\) m) (Figure 8). In Lake Peipus and Lake Vortsjarv, the MEAN was \(0.16\pm 0.13\) m and \(0.31\pm 0.39\) m, respectively. Again, these GNSS validation results agree with the validation results using the geoid-based water surface height, showing a discrepancy of \(0.15\pm 0.14\) m and \(0.28\pm 0.30\) m for the Lake Peipus on 19 June 2019 and for Lake Vortsjarv on 13 July 2019, respectively (Figure 9).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mission** & **Instrument** &
\begin{tabular}{c} **Ground Track** \\ **Repeat Cycle** \\ **(Days)** \\ \end{tabular} & **Time Interval** & **Measurements** \\ \hline SARAL & AltiKa & 35 & 13 January 2014–31 December 2017 & 167 \\ CryoSat-2 & SIRAL-2 & 369 & 5 January 2014–31 December 2017 & 225 \\ HY-2A & ALT & 14 & 24 April 2014–31 December 2017 & 111 \\ Jason-2 & Poseidon-3 & 10 & 9 January 2014–31 December 2017 & 50 \\ Jason-3 & Poseidon-3B & 10 & 30 May 2016–31 December 2017 & 24 \\ Sentinel-3A & SRAL & 27 & 1 July 2016–31 December 2017 & 48 \\ \hline \hline \end{tabular}  
* Detailed overview about the different missions’ parameters can be found from Table 1 in [33].  
\end{table}
Table 3: Overview of the satellite altimetry missions used *.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Statistics** & **Gulf of Finland** & **Gulf of Riga** & **Lake Peipus** & **Lake Vortsjärv** \\ \hline MEAN (m) & 0.11 & 0.14 & 0.16 & 0.31 \\ STD (m) & 0.08 & 0.05 & 0.13 & 0.39 \\ RMSE (m) & 0.13 & 0.15 & 0.21 & 0.49 \\ MIN (m) & \(-0.11\) & 0.00 & \(-0.33\) & \(-0.45\) \\ MAX (m) & 0.29 & 0.27 & 0.38 & 0.74 \\ Common measurements & 80 & 180 & 110 & 30 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Statistics of differences between the sea surface height (\(\Delta SSH_{\text{GNSS}}\)) derived from GNSS measurements (\(SSH_{\text{GNSS}}\)) and sea surface height derived from altimetry (\(SSH_{\text{SA}}\)) according to the Equation (6).  
Figure 8: Residual differences (\(\Delta SSH_{\text{TG}}\)) between the sea surface height of Sentinel-3 (\(SSH_{\text{SA}}\)) and the geoid-based sea surface height (\(SSH_{\text{TG}}\)) in the Gulf of Riga. The zero line denotes the reference \(SSH_{\text{TG}}\). Colored numbers indicate the corresponding cycle number for each satellite. Vertical bars represent the \(\pm 1\sigma\) standard deviations.  
Figure 7: Residual differences (\(\Delta SSH_{\text{TG}}\)) between the sea surface height of Sentinel-3 (\(SSH_{\text{SA}}\)) and the geoid-based sea surface height (\(SSH_{\text{TG}}\)) in the Gulf of Finland. The zero line denotes the reference \(SSH_{\text{TG}}\). Coloured numbers indicate the corresponding cycle number for each satellite. Vertical bars represent the \(\pm 1\sigma\) standard deviations.