Development of geoid-based offshore reference surfaces using SWOT altimetry observations

High-quality and -resolution marine geoid models provide a physically meaningful reference for water-level information and support navigation and bathymetric mapping. However, the realisation of accurate geoid-based reference surfaces in offshore regions is often limited by a lack of reliable gravity observations, due to factors ranging from significant time and cost constraints to geopolitical restrictions. The high-resolution KaRIn sea surface height observations from the SWOT (Surface Water and Ocean Topography) altimetry mission have the potential to offer an alternative approach to global marine geoid determination. Discrete offshore geoidal heights can be derived by removing modelled instantaneous estimates of dynamic ocean topography from the observed sea surface heights. A unified geoid surface model, together with its associated standard errors, can then be determined following least-squares collocation principles. Employing the geodetic infrastructure of the Baltic Sea region for the development and validation of the method, an agreement of just a few centimetres (standard deviation of 2.2 cm for the entire Baltic Sea) was achieved with the recently developed high-resolution gravimetric BSCD2000 geoid model. Since the SWOT-based geoid surface maintains high quality even in the near-shore zone, a seamless geoid model for land and offshore can be established by blending, with the land portion determined using the conventional gravimetric approach. Beyond the Baltic Sea region, the method's applicability is demonstrated in other parts of the world, showcasing how SWOT altimetry observations can facilitate an alternative to conventional gravimetric marine geoid determination.