A Coastal Mean Sea Surface with Associated Errors along the Norwegian Coast Based n New- Generation Altimetry

The coastal mean sea surface (MSS) has applications within oceanography as well as geodesy. Together with a geoid model, it forms an important component for geodetic mapping of ocean surface currents that are in geostrophic balance. Furthermore, it forms a bridge between open ocean MSS and in situ measurements of mean sea level at or close to land, it  contributes to the mapping of the geoid and the marine gravity field, and it is essential for connecting tidal nautical chart datums to physical height systems or global geodetic reference frames. In this study, we determine a coastal MSS with an associated error field for Norway. The MSS is solely based on new-generation altimetry data, i.e., SAR(In) data from Sentinel-3A and CryoSat-2, as well as Ka-band data from Saral/AltiKa. The data sets partly overlap in time and cover the time period from 2010 to 2017 inclusive. We have chosen these altimeters  because they represent evolutions of conventional altimetry, with reduced footprint diameters as a main benefit. This is especially advantageous in the coastal zone, as a smaller footprint reduces the probability of radar pulses being contaminated by energy backscattered from land areas. The satellite missions were harmonized by applying inter-mission biases calculated in a regional crossoveranalysis. Furthermore, in a zone closer to land than 25 km, we have replaced the global ocean tide model with a regional ocean tide model provided by the Norwegian
Mapping Authority. We explore different data editing strategies, compare two methods for optimal interpolation of the along-track data to a regular grid, and discuss these in the context of the estimated error field. We assess our coastal MSS by comparison to existing state-of-the-art MSS products, as well as ellipsoidal mean sea level as observed by an array of tide gauges within the study area. The Norwegian coast is characterized by thousands of small islands, narrow fjords, rough topography, and complex tidal patterns, making the altimetric
measurement of the sea surface height particularly demanding in this area.