# Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes
## 2 Material and Methods
### Copernicus Marine Environment Monitoring Service (CMEMS) Data Products  
One of the purposes of this study was to compare the accuracy of CMEMS Level-3 (L3) along-track altimetry products (\(\mathit{SLA}_{\text{L3}}\)) [(29)] and NEMO reanalysis model (\(\mathit{SLA}_{\text{NEMO}}\)) [(30)] outcomes against \(\mathit{SLA}_{\text{TG}}\). The along-track \(\mathit{SLA}_{\text{L3}}\) is produced with the resolution of 7 km between measurements. The reported RMS of the differences between gridded Data Unification and Altimeter Combination System Delayed Time 2018 (DUACS DT2018) sea-level anomaly and independent along track measurements for the Baltic Sea vary approximately between 0.03 and 0.07 m [(31)]. The CMEMS Baltic Sea Physical Reanalysis product provides a physical reanalysis for the entire Baltic Sea area. It is produced using the ice-ocean model NEMO-Nordic [(30)]. The reported mean correlation between \(\mathit{SLA}_{\text{TG}}\) and \(\mathit{SLA}_{\text{NEMO}}\) for the entire Baltic Sea is 0.95 with the Root Mean Square Error (RMSE) of 0.07 m, with lower values in highly dynamic marine areas [(32)].  
The analysed \(\mathit{SLA}\) data covered the time interval from 2014 to 2017 when the sea-level measurements of all methods were available. The \(\mathit{SLA}_{\text{L3}}\) data along track was collected only from the altimeters that were operational for the specified timespan (Table 3). The specific time interval in Table 3. is set according to the first and last collocated measurement in the dataset. The number of measurements in Table 3 is dependent on satellite orbit and location of tide gauge.  
Figure 6: Pressure sensor-based buoy (**a**) and its deployment in the Gulf of Riga on 20 June 2019 (**b**).  
Altimetry \(SLA_{L3}\) along-track data were collected at a maximum distance of 10 km from the listed gauges (mean distance 7.9 km), and the maximum time difference between \(SLA_{TG}\) and \(SLA_{L3}\) measurements was one hour (mean time difference 18 min). For the NEMO, \(SLA_{NEMO}\) at the closest grid point to the station within an hour's time difference was extracted (mean distance 1.5 km). Considering seasonal component in the MSL variations in the Baltic Sea [34; 35], SLA data from all seasons were used in the comparison. Final filtering for \(SLA_{TG}\), \(SLA_{L3}\), and \(SLA_{NEMO}\) was made according to match in measurement time which resulted in the number of common, collocated measurements of 625.  
A separate comparison was carried out with the data from the buoy in the Gulf of Riga (see green circle on Figure 2) to measure the accuracy of \(SLA_{L3}\) along-track data as well as the reanalysis model outcomes. The \(SLA_{L3}\) data was collected for time period from June to November 2019 using the same scheme, i.e., maximum distance between along track measurement and \(SLA_{TG}\) of 10 km and maximum time difference of one hour. The final analysis consisted of 33 unique data points (mean distance: 4.3 km; mean time difference: 16 min) in all three sources.  
Data analysis results are presented as standard descriptive statistics: Pearson correlation coefficient (\(r\)), RMSE and Mean Error (ME). The statistics were calculated between SLA values since the accurate translation of modelled water levels into a common reference surface is an unresolved technical issue [34].