Centimeter-level land-sea geoid for Wanshan Radar Altimeter Calibration Site: Modeling and validation

High-precision geoid modeling for the Wanshan Radar Altimeter Calibration Site (the world's fifth operational base and China's first) is fundamental to satellite calibration and ocean dynamic research. Addressing the lack of traditional GNSS/leveling validation methods in marine areas, this paper proposes a remove-restore technique framework that integrates land-sea gravity data to construct a consistent geoid model. It innovatively proposes a virtual observation point mechanism to evaluate land-sea gravity data discrepancies quantitatively. Focusing on the test area (121.6°N–22.7°N, 113.8°E–114.6°E), the study integrates 55 GNSS/leveling points, shipborne gravimetry lines, and terrestrial gravity data. Virtual points assess land-sea gravity differences, and the remove-restore technique constructs a 1′ × 1′ land-sea geoid model. The model is validated on land using GNSS/leveling height anomalies, while at sea, an independent benchmark is built by subtracting the DTU mean dynamic topography (MDT22) from DTU mean sea surface (MSS21). Results show that no systematic bias between land-sea gravity anomalies, with a random error difference of only 1 mGal. Model accuracy is better than 4 cm (3.7 cm on land/3.6 cm at sea), meeting the absolute bias calibration requirements for Jason-3/Sentinel-6 satellite altimeters. The MSS21 and MDT22 based marine validation method achieves centimeter-level verification in the South China Sea for the first time, offering a novel solution for geoid evaluation in low-latitude complex tidal zones. This achievement supports establishing a centimeter-level height datum for the Wanshan Calibration Site, providing a unified vertical reference for sea-level monitoring and marine engineering in the Guangdong-Hong Kong-Macao Greater Bay Area.