Obtaining accurate measurements of the sea surface height from a GPS buoy

A dedicated GPS buoy is designed for calibration and validation (Cal/Val) of satellite altimeters since 2014. In order to evaluate the accuracy of the sea surface height (SSH) measured by the GPS buoy, twelve campaigns have been done within China sea area between 2014 and 2021. In six of these campaigns, two static Global Navigation Satellite System stations were installed at distances of <1 km and 19 km from the buoy to assess how the baseline length influenced the derived SSH from the buoy solutions. The GPS buoy data was processed using the GAMIT/GLOBK software+TRACK module and CSRS-PPP tool to achieve the SSH. The SSH was compared with conventionally tide gauge (TG) data to evaluate the accuracy of the buoy with the standard deviation of the height element. The results showed that the difference in the standard deviation of the SSH from the buoy and the TG was less than 16 mm. The SSHs processed with different ephemeris (Ultra-Rapid, Rapid, Final) were not significantly different. When the baseline length was 19 km, the SSH solution of the GPS buoy performed well, with standard bias of less than 26 mm between the heights measured by the buoy and TG, meaning that the buoy could be used for Cal/Val of altimeters. The bias between the Canadian Spatial Reference System-precise point positioning tool and the TRACK varied a lot, and some of them were over 130 mm. This deemed too high to be useful for Cal/Val of satellite altimeters. Moreover, the GPS buoy solutions processed by GAMIT/GLOBK software+TRACK module were used for in-orbit Cal/Val of HY-2B/C satellites in ten campaigns. The SSH and significant wave height of the altimeters showed good agreements with the GPS buoy solutions.