Integrating multi-source altimetry observations for gravity anomaly derivation in the South China Sea

The Ka-band radar interferometer (KaRIn) on the Surface Water and Ocean Topography (SWOT) satellite provides wide-swath measurements, advancing satellite altimetry to enable high-resolution two-dimensional mapping of the sea surface. This capability presents novel opportunities for ocean gravity field modeling. This study aims to validate the importance of integrating wide-swath altimetry with conventional satellite data for gravity field recovery. By combining SWOT data with Geodetic Mission (GM) data from traditional satellites, we establish a deflection of the vertical model (SCSDOV) and subsequently derive a gravity anomaly model (SCSGRA) for the South China Sea region (104°E–124°E, 5°N–25°N). For each common ground track, SWOT Sea Surface Height (SSH) measurements from repeat passes were averaged. The accuracy of the averaged SSH at each location depends on the number of cycles included in the average. Leveraging SWOT's cross-track measurements, a multi-directional geoid gradient inversion method improves accuracy compared to traditional along-track-only approaches. The least squares collocation method further refines the results by combining both datasets. Validation shows that combining altimetry data from various observation modes—including SWOT and traditional missions—is effective for deriving high-accuracy, high-resolution DOV grids. The final SCSGRA model achieves a root mean square error (RMSE) of 4.872 mGal against shipborne gravity data. Compared to the gravity field inverted from SWOT, the accuracy of the inverted SCSGRA model is improved by 0.082 mGal (about 1.7%) overall, with a further improvement of 0.218 mGal (about 3.2%) observed within SWOT coverage gaps. This demonstrates the effectiveness of multi-source altimetry data fusion in marine gravity field modeling.