The accuracy of sea surface height (SSH) merged maps is critical for monitoring oceanic small-scale variability, which underpins both oceanographic and meteorological applications. The integration of satellite radar altimeter data has substantially improved the quality of these maps. The deployment of China's autonomous Haiyang-2 (HY-2) satellite series has enhanced the country's independent ocean observation capacity while also contributing to the global ocean observing system. This study evaluates the impact of incorporating HY-2 data on the accuracy of SSH merged maps, a topic that has received limited attention to date. The combined contribution of indigenous satellites to SSH observations is of particular interest, as it offers potential improvements in the characterization of ocean dynamics and the accuracy of SSH predictions.
The primary objective of this study is to quantify the effect of HY-2 satellite data on SSH merged maps and to assess the resulting improvements in observational quality. Between 29 April 2022 and 3 February 2023, a two-dimensional variational merging method was applied to integrate SSH data from six international satellites and three HY-2 satellites. The resulting SSH merged maps, produced over the Northwest Pacific, had a spatial resolution of 0.12°×0.12° and daily temporal resolution. Accuracy was assessed through error statistics, correlation, and regression analyses, with cross-validation against geostrophic flow-corrected drifting buoy velocity data and tide gauge SSH observations. Results showed SSH errors of 2—5 cm, with flow velocity errors of 11 cm·s^-1 (zonal) and 13 cm·s^-1 (meridional), consistent with international merged products.
Inclusion of HY-2 satellites improved performance compared with buoy data, with vector direction errors reduced by nearly 0.1° and velocity root mean square error (RMSE) reduced by 0.31 cm·s^-1 (meridional) and 0.17 cm·s^-1 (zonal). Grid points with reduced RMSE accounted for 7% and 5% of the total buoy-covered area in the meridional and zonal directions, respectively. Against tide gauge observations, mean SSH error decreased by 0.1 cm and RMSE by 0.2 cm, while regression coefficients and correlation increased by 3%—5%. However, improvements plateaued once the number of merged satellites exceeded five, indicating a saturation point in observational density at approximately 50%. These results confirm that the inclusion of HY-2 satellites enhances the accuracy of SSH merged maps.
The HY-2 constellation plays a key role in complementing international data and supports diverse applications, including marine disaster prevention, transportation, resource development, environmental protection, scientific research, and national defense. Although traditional radar pulse technology has inherent limitations, merging altimetry data from three or more satellites effectively increases SSH map coverage and accuracy. Nevertheless, improvements diminish beyond a certain number of satellites, highlighting a point of diminishing returns. Future work should focus on optimizing merging algorithms and assessing the benefits of expanded satellite constellations for finer-resolution ocean monitoring. The findings provide valuable insights for strategic planning of satellite deployments and the utilization of merged SSH products in prediction models.