# Long-Term and Decadal Sea-Level Trends of the Baltic Sea Using Along-Track Satellite Altimetry
## 4 Datasets
### Satellite Altimetry
However, this dataset contains the data only up to 2019; hence, in the present study, the \"standard\" EUMETSAT data were used to extend the trend estimation period from 2019 up to 31 December 2022. The Baltic + SEAL retracked data are available on request from [[http://balticseal.eu/outputs](http://balticseal.eu/outputs)]([http://balticseal.eu/outputs](http://balticseal.eu/outputs)) (accessed on 23 March 2021), whereas the standard data originated from the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT), which can be downloaded from the EUMETSAT Earth Observation Portal at [[https://eoportal.eumetsat.int/](https://eoportal.eumetsat.int/)]([https://eoportal.eumetsat.int/](https://eoportal.eumetsat.int/)) (accessed in 23 March 2023). The consistency of sea-level data from these two sources during the tandem phase of both datasets has been examined prior to our study. It should be noted that the former dataset includes a homogeneous estimate of multi-mission ASL, which has shown better results (as compared to the TG observations), rather than standard products [33]. Also, the necessary corrections (denoted by \(\Delta R\)) applied to all SA data consistently:  
\[\Delta R=(\text{wet tropospheric}+\text{dry tropospheric}+\text{ ionospheric}+\text{sea state bias}+\text{solid Earth tide}+\text{pole tide}+\text{radial orbit error}+\text{inverse barometric height}+\text{DAC})-\text{DAC} \tag{10}\]  
Note that the dynamic atmospheric correction (DAC) on the right-hand side of Equation (10) denotes the removal of the DAC from the used SA datasets. Also, the ocean tide corrections are not included in the instantaneous SA data, considering that the TG data were extracted at the exact time instants of SA cycles. This makes the SA-derived DT compatible with TG records, which inherently contain the atmospheric pressure and the Baltic micro-tide contribution to the sea level. To achieve a consistent SA dataset, all the corrections were processed and applied uniformly, even though certain corrections may use different models (e.g., the wet tropospheric correction model varies depending on the  
Figure 4: Spatial distribution of SA data at locations of selected TG stations within the 50 km radius. The white points represent the removed SA data within 5 km distance from the coastline. Baltic + SEAL mission data). It was determined that the impact due to differences between these models is negligible for the SA-derived trend estimation. The quality of SA data over the Baltic Sea is affected by two main features that limit the use of SA [33]: (i) the presence of sea ice and the proximity of the coast. For example, the northern part of the sea (62.1\({}^{\circ}\)N-65.8\({}^{\circ}\)N) is covered 110-190 days of the year by sea ice during the winter-spring seasons [69]. (ii) Also, the Baltic Sea contains the largest archipelagos in the world, with 30,500 islands and a 23 m average water depth only near Finnish coast [70]. Hence, this study focuses on the study area region (around the Baltic Proper), where these undesirable effects have the minimum impact on the SA sea-level trend estimates. The SA data need to account for other adjustments, including the permanent tide system [71] and the terrestrial reference frame unification. The ITRF transformation was carried out using the method explained by [72] in which the SA terrestrial reference frame is transferred from ITRF\({}_{2008}\) or ITRF\({}_{2014}\) to ITRF\({}_{\gamma YYY}\), which is an ETRF realization of the year \"\(\gamma YYY\)\" (e.g., 1995) related to the time epoch (\(\gamma YYY\)) of the observations [33]. This includes marginal intra-plate velocity deformations and the transformation between ITRF and ETRF.