# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 5 Results
### Inter-Comparisons of SA-Based ASL Trends
#### 5.4.2 Sea-Level Trends of Different SA Orbit Inclination Categories  
As mentioned earlier, the sea-level trend can be estimated by merging multi-mission SA data over different time frames. However, some studies have shown that various SA missions could lead to different data quality depending on their constellation across different regions, which could also affect the subsequent trend estimation. For instance, over sub-arctic areas, low-inclination SA mission orbits (e.g., the Jason series) are able to provide better data than high-inclination orbit missions (e.g., Envisat, Sentinel series, etc.) that align more closely with ground-truth observations [33], although the high-inclination SA missions have better coverage and larger quantities of data [82]. Therefore, in this study, the SA missions were divided into two distinct groups based on their constellation: high-inclination (SA1) and low-inclination (SA2) orbit satellites (cf. Figure 3). This division aimed to investigate whether these distinctions could impact the SA-derived sea-level trend. The sea-level trend of each orbit category was estimated for the period of 2003-2022 (when the overlapped satellite mission data existed) and compared with the ground truth, the hourly TG-derived trend for the same time period (number of measurements is not normalized). These results are illustrated in Figure 11.  
Figure 10: SA-derived sea-level trend (red) moving away from the coast compared to the TG trend (blue) at six stations. The whiskers represent the uncertainties using 95% CI. The number of initial (non-averaged) SA data points within the distance are denoted in the right-hand-side vertical axis with numbers in thousand (K) format. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**TG** & **Dist [km]** & **5** & **10** & **15** & **20** & **25** & **30** & **35** & **40** & **45** & **50** \\ \hline
1 & 1.1 & 1.7 & 1.4 & 1.1 & 0.9 & 0.8 & 0.8 & 0.8 & 0.8 & 0.8 & 0.8 \\
2 & 1.4 & 0.9 & 0.8 & 0.5 & 0.4 & 0.1 & 0.1 & 0.1 & 0.1 & 0.1 \\
3 & 2.9 & 3.3 & 3.1 & 3.2 & 3.4 & 3.4 & 3.5 & 3.5 & 3.5 & 3.4 \\
4 & 17 & 1.7 & 1.8 & 1.7 & 1.6 & 1.6 & 1.6 & 1.6 & 1.6 & 1.7 \\
5 & 0.6 & 0.6 & 0.5 & 0.5 & 0.7 & 0.9 & 1.0 & 1.0 & 1.0 & 1.0 \\
6 & 0.4 & 0.9 & 0.9 & 0.9 & 0.9 & 0.8 & 0.8 & 0.7 & 0.7 \\
7 & 1.0 & 0.5 & 0.2 & 0.2 & 0.2 & 0.2 & 0.1 & 0.1 & 0.3 & 0.5 \\
8 & 2.8 & 2.9 & 2.7 & 2.6 & 2.6 & 2.5 & 2.5 & 2.5 & 2.5 & 2.5 \\
9 & 0.2 & 0.1 & 0.2 & 0.4 & 0.5 & 0.5 & 0.6 & 0.5 & 0.5 & 0.5 \\
10 & 1.1 & 1.1 & 1.0 & 0.6 & 0.3 & 0.2 & 0.3 & 0.6 & 0.7 & 0.7 \\
11 & 0.5 & 1.1 & 0.7 & 0.5 & 0.4 & 0.4 & 0.4 & 0.4 & 0.3 & 0.3 \\
12 & 1.8 & 2.1 & 2.1 & 2.1 & 2.0 & 1.9 & 1.9 & 1.9 & 1.9 & 1.9 \\
13 & 0.6 & 0.5 & 0.8 & 0.8 & 0.9 & 0.9 & 0.9 & 0.8 & 0.8 & 0.8 \\ RMSE & 1.49 & 1.63 & 1.53 & 1.47 & 1.48 & 1.46 & 1.48 & 1.48 & 1.48 & 1.47 \\ MEAN & 0.28 & 0.16 & 0.11 & 0.03 & \(-0.06\) & \(-0.12\) & \(-0.13\) & \(-0.09\) & \(-0.08\) & \(-0.06\) \\ \hline \end{tabular}
\end{table} 
Table 6: Absolute difference [mm/yr] between TG-derived and SA-derived trends at different distances (Dist) from the coast. The values are highlighted based on colour (i.e., the smallest absolute difference is in green and the largest is in red). The corresponding RMSE and arithmetic average (MEAN) of differences are computed at the bottom of the table [mm/yr]. The results of the comparison of each SA group with the TG data are represented in Table 7 (the best match with ground truth is highlighted in bold font). In general, the SA2 group (low inclination) with an RMSE of 3.15 mm/yr had a better match with the TG-derived trend compared to the SA1 group, which has an RMSE of 3.73 mm/yr. Over the 13 stations, the average discrepancy of the SA2 group is 2.5 mm/yr, with a maximum discrepancy of 6.9 mm/yr at Kungsholmsfort (#10), while the average discrepancy between the SA1 group and TG trends is 3.2 mm/yr, with the maximum discrepancy of 6.3 mm/yr at Swinoujiscie (#6).