Ole Baltazar Andersen (DTU Space, Technical University of Denmark,, Denmark)

The coastal mean sea surface (MSS) has a wide range of applications within geodesy and oceanography, such as geodetic mapping of ocean currents, calculations of marine gravity anomalies and bathymetry estimates. Moreover, it connects the open-ocean MSS from altimetry and tide gauge measurements at or close to the land and serves as a crucial factor in unifying vertical reference frames. Nowadays, available MSS models are mostly based on observations from radar altimetry, which enables the determination of the mean sea surface to centimeter accuracy in the open sea. Although the new generation of radar altimetry missions equipped with Synthetic Aperture Radar (SAR) are able to provide measurements every 300 m in the along-track direction, its performance is degraded in coastal areas due to different factors, such as land contamination, erroneous tropospheric corrections or complex tidal patterns. The Norwegian coastline, known as the second longest and one of the most rugged coastline in the world, introduces many challenges for using radar satellite altimetry, which in the end results in reduced number of valid altimetry observations. In 2018, the National Aeronautics and Space Administration launched ICESat-2, a laser altimetry mission equipped with the Advanced Topographic Laser Altimeter System, providing measurements every 0.7 m in the along-track direction. Due to its small footprint, ICESat-2 has the potential to fill the observation gap between tide gauges and the open sea. Therefore, we have determined an updated MSS model by combining observations from radar altimetry missions (Sentinel-3, CryoSat-2 and SARAL/AltiKa) used to create an existing MSS for Norway, NMBU18, with ICESat-2 observations. Finally, we have used NMBU18 and observations from the Norwegian network of permanent and temporary tide gauges inside Norway’s largest fjord, Sognefjorden, to validate the updated MSS model.