# Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes
###### Abstract  
Multi-mission satellite altimetry (e.g., ERS, Envisat, TOPEX/Poseidon, Jason) data have enabled a synoptic-scale view of ocean variations in past decades. Since 2016, the Sentinel-3 mission has provided better spatial and temporal sampling compared to its predecessors. The Sentinel-3 Ku/C Radar Altimeter (SRAL) is one of the synthetic aperture radar altimeters (SAR Altimeter) which is more precise for coastal and lake observations. The article studies the performance of the Sentinel-3 Level-2 sea level altimetry products in the coastal areas of the Baltic Sea and on two lakes of Estonia. The Sentinel-3 data were compared with (i) collocated Global Navigation Satellite System (GNSS) ship measurements, (ii) the Estonian geoid model (EST-GEOID2017) together with sea-level anomaly corrections from the tide gauges, and (iii) collocated buoy measurements. The comparisons were carried out along seven Sentinel-3A/B tracks across the Baltic Sea and Estonian lakes in 2019. In addition, the Copernicus Marine Environment Monitoring Service (CMEMS) Level-3 sea-level products and the Nucleus for European Modelling of the Ocean (NEMO) reanalysis outcomes were compared with measurements from Estonia's 21 tide gauges and the buoy deployed offshore. Our results showed that the uncertainty of the Sentinel-3 Level-2 altimetry product was below decimetre level for the seacoast and the selected lakes of Estonia. Results from CMEMS Level-3 altimetry products showed a correlation of 0.83 (RMSE 0.18 m) and 0.91 (RMSE 0.27 m) when compared against the tide gauge measurements and the NEMO model, respectively. The overall performance of the altimetry products was very good, except in the immediate vicinity of the coastline and for the lakes, where the accuracy was nearly three times lower than for the open sea, but still acceptably good. s 2020 xxxxxxxx-xxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxxxx-xxxxxxxxxxxx-xxxxxxxxxxxxxx-  
sparsely distributed along the shoreline and do not all cover the same time period. Aforenamed problems are the key limitations for their use. To avoid these influences and to transform sea surface heights (SSH) into a common height system, it is recommended to use the tide gauges connected to the Global Navigation Satellite System (GNSS) stations or satellite altimetry (SA) to calculate the absolute SSH above a reference ellipsoid [1; 2]. Radar altimeter is a precise ranging tool designed for measuring SSH over the open ocean at an uncertainty level of around 0.035-0.050 m [1; 3; 4]. Satellite altimetry has been used in sea-level variation studies for more than 30 years. The first satellite providing altimetry data was the experimental Seasat mission in the late 1970s. Satellite altimetry gained popularity in the 1990s when TOPEX/Poseidon and ERS-1 missions were launched. Nowadays, it is an important tool for understanding the topography of mesoscale eddies and the multidecadal trend of eustatic (global) sea-level rise. Sentinel altimetry has been successfully used in the open ocean for water level monitoring, determination of wave height, storm surge, tides, etc. However, it has increasingly used to estimate mean sea-level (MSL) trends in the coastal areas where satellite data complement in situ TG time series and hydrodynamic models. Major progress was made over the last 15 years thanks to several innovative retracking algorithms in the coastal zone. Success has been furthered by the development of the Synthetic Aperture Radar Altimeter (SAR Altimeter), which performs coherent processing of groups of transmitted pulses and exploits the Delay Doppler signal of the full bandwidth [5]. The progress of altimetry data processing methods has also been driven by the improvement of geophysical corrections, such as wet/dry troposphere corrections, ionospheric delay, tidal models, and the necessary waveform retracking algorithms [6].