# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 4 Datasets
### Satellite Altimetry  
In this study, ten available SA missions' data (Figure 3), including European Remote-Sensing Satellite-2 (ERS2), Envisat (ENV), Jason-1 (JA1), Jason-2 (JA2), Cryosat-2 (CS2), SARAL/AltiKa (SRL), Sentinel-3A (S3A), Jason-3 (JA3), Sentinel-3B (S3B), and Sentinel-6A Michael Freilich Jason-CS (S6A), are examined to determine the instantaneous \(DT_{SA}\). These SA missions have different temporal coverage and resolution; see Figure 3 and Table 1. The highest-frequency SA data were used, i.e., distributed at a 20 Hz rate for most of the missions, except for 40 Hz for SARAL, and 18 Hz for Envisat. Compared to the open ocean, the SA data quality is degraded near the shoreline due to land contamination, seasonal sea ice conditions, and, possibly, erroneous corrections (e.g., wet tropospheric correction). Therefore, from the trend estimates of this study, the SA data points closer than 5 km to the shoreline were removed. Also, an iterative data screening approach [33] was applied to remove the outliers from the remaining SA data. Firstly, the gross errors (\(|DT_{SA\#TG}|>1.5\) m) were removed from the SA datasets, followed by using median absolute deviation (MAD) for outlier detection. Figure 4 demonstrates the spatial distribution of the selected TG stations, which are mostly around the open parts of the sea (Baltic Proper) where more SA tracks can be acquired at these locations and has the smallest effect on ice coverage is apparent during the winter seasons (cf. [33]). High-quality SA along-track data were extracted up to 50 km from each TG station (see Figure 4). Equation (5) was then applied to obtain the averaged SA-derived DT estimates, whereas the number of SA data (\(m\)) varied from 1 to 326. In Figure 4, the selection of SA data at the location of the TG stations near the coast is illustrated, where the data points within 5-50 km from the shoreline of each SA mission are represented by different colours. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**ID**} & \multirow{2}{*}{**SA**} & \multirow{2}{*}{**Altitude** [km]**} & \multirow{2}{*}{**Inclination** [°]**} & \multirow{2}{*}{**Cycle [Days]**} & \multicolumn{2}{c}{**Data**} & \multirow{2}{*}{**Peroquers**} & \multirow{2}{*}{**Data Period (Baltic + SEAL)**} & \multirow{2}{*}{**Data Period (Standard Data)**} \\  & & & & & & & & [Hz] & \\ \hline
1 & ERS2 & 785 & 98.5 & 35 & 20 & 16 May 1995 & 01 Jul 2003 & - & - \\ \hline
2 & ENV & 800 & 98.5 & 35 & 18 & 18 Jun 2002 & 18 Oct 2010 & - & - \\ \hline
3 & JA1 & 1324 & 66 & 10 & 20 & 15 Jan 2002 & 20 Jun 2013 & - & - \\ \hline
4 & JA2 & 1324 & 66 & 10 & 20 & 12 Jul 2008 & 17 May 2017 & - & - \\ \hline
5 & CS2 & 717 & 92 &
\begin{tabular}{c} w/o exact \\ revisit cycle \\ \end{tabular} & 20 & 19 Jul 2010 & 04 May 2019 & - & - \\ \hline
6 & SRL & 800 & 98.5 & 35 & 40 & 14 Mar 2013 & 04 Jul 2016 & - & - \\ \hline
7 & S3A & 814 & 98.6 & 27 & 20 & 04 Jan 2017 & 13 Jun 2019 & 13 Jun 2019 & 31 Dec 2022 \\ \hline
8 & JA3 & 1336 & 66 & 10 & 20 & 17 Feb 2016 & 31 May 2019 & 31 May 2019 & 31 Dec 2022 \\ \hline
9 & S3B & 814 & 98.6 & 27 & 20 & 01 Nov 2018 & 19 May 2019 & 19 May 2019 & 31 Dec 2022 \\ \hline
10 & S6A & 1336 & 66 & 10 & 20 & - & - & 1 Apr 2022 & 31 Dec 2022 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the used SA missions (ordered by their launch dates) and related data spans. Figure 3: Available satellite altimeter missions are used in this study. The coloured (corresponding to the orbit inclination; \(\sim\)66°: yellow, \(\sim\)98°: red, \(\sim\)92°: green) portion of bars demonstrate the duration (arrow for the presently ongoing missions) of the SA missions. The SA revisit cycles in days (d) are also indicated for each mission in the brackets. The European Space Agency's Baltic + Sea Level (ESA Baltic + SEAL) Project dataset is an enhanced sea-level product that is specifically developed for the Baltic Sea [68].