# Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes
## 5 Conclusions  
This study discussed applicability of the Sentinel-3A/B Level-2 altimetry product and the CMEMS Level-3 product in the Baltic Sea area. To validate the Level-2 product, the \(\mathit{SSH}_{\mathit{SA}}\) in Estonian coastal waters (Gulf of Finland and Gulf of Riga) and larger lakes (Lake Peipus and Lake Vortsjarrv) was compared with \(\mathit{SSH}_{\mathit{GNSS}}\), \(\mathit{SSH}_{\mathit{buoy}}\), and \(\mathit{SSH}_{\mathit{TG}}\). In addition, to validate the along-track Level-3 product (\(\mathit{SLA}_{\mathit{L3}}\)) and NEMO reanalysis outcomes (\(\mathit{SLA}_{\mathit{NEMO}}\)) from the CMEMS database, the water levels were compared with in situ measurements at 21 Estonian coastal tide gauge stations. The comparison was made with data from six satellite missions from 2014 to 2017. The validation results of the Sentinel-3 Level-2 product showed that the altimetry accuracy was nearly three times higher in the open sea than for the lakes. The average difference between \(\mathit{SSH}_{\mathit{SA}}\) and \(\mathit{SSH}_{\mathit{TG}}\) (residual difference \(\mathit{\Delta SSH}_{\mathit{TG}}\), Equation (5)) was 0.07-0.10 m in the Gulf of Finland and the Gulf of Riga. The corresponding numbers were 0.23 m and 0.27 m in Lakes Peipus and Vortsjarv, respectively. Based on the average residual differences, it was found that the altimetry SSH values in inland waters were systematically shifted: \(\mathit{SSH}_{\mathit{SA}}\) was 0.14 m and 0.13 m higher than \(\mathit{SSH}_{\mathit{TG}}\) values in Lake Peipus and Lake Vortsjarv, respectively. The systematic bias between altimetry and validation dataset has been detected by several authors [9; 38; 39]. [PERSON] et al. 2019 and [PERSON] et al. 2018 had noticed that the systematic bias in altimetry data was caused by variations in satellite orbit, errors in correction models, errors associated with sensors, and choice of reference datum and retracker. Figure 13: Sea-level anomaly frequency distribution (and their overlaps) calculated with a bin size of 0.05 m for (**a**) \(\mathit{SLA}_{\mathit{TG}}\) and \(\mathit{SLA}_{\mathit{L3}}\) and (**b**) \(\mathit{SLA}_{\mathit{TG}}\) and \(\mathit{SLA}_{\mathit{NEMO}}\) in the Baltic Sea. In the coastal waters, positive and negative residual differences were more equally distributed, i.e., average residual differences were close to zero. The best fit with the altimetry height was found using buoy measurements in the Gulf of Riga (average difference 0.01 \(\pm\) 0.09 m). In summary, considering all the validation methods, our results show that satellite altimetry can determine the height of the water level in Estonian coastal areas with an average accuracy of 0.08 \(\pm\) 0.07 m and in inland waters with accuracy of 0.20 \(\pm\) 0.26 m.  
The Level-3 along-track altimetry products from six active altimeter missions and the modelled results from the NEMO reanalysis product (available at the CMEMS database) were compared with in situ tide gauge measurements between 2014 and 2017. The sea-level anomaly comparison between the three data sources was carried out with concurrent measurements at 21 stations around the Estonian seacoast (from 2014-2017) and a buoy station in the open part of the Gulf of Riga (in 2019). The validation of the CMEMS Level-3 product showed that it has similar accuracy with the Sentinel-3A/B Level-2 product. The residual difference between the NEMO reanalysis model and the satellite data was approximately twofold near the coast compared with the open sea. The Pearson correlation coefficient on the other hand was better for the \(\mathit{SLA}_{\mathit{NEMO}}\) than the \(\mathit{SLA}_{\mathit{L3}}\) data near the coast. The weighted mean correlation between the NEMO fields and the in situ measurements was 0.91 and the mean difference was 0.26 m. The respective statistics between the satellite and the in-situ measurements were 0.83 and 0.12 m. This showed that satellite measurements near the coast have, on average, a ME twice as low as that of the NEMO model.