GNSS Data Processing and Validation of the Altimeter Zenith Wet Delay around the Wanshan Calibration Site

The Wanshan calibration site (WSCS) is the first in-situ field for calibration and validation (Cal/Val) of HY-2 satellite series in China. It was built in December, 2018 and began business operation in 2020. In order to define an accurate datum for Cal/Val of altimeters, the permanent GNSS station (PGS) data of the WSCS observed on Zhiwan (ZWAN) and Wailingding (WLDD) islands were processed using GAMIT/GLOBK software in a regional solution, combined with 61 GNSS stations distributed nearby, collected from the GNSS Research Center, Wuhan University (GRC). The Hector software was used to analyze the trend of North (N), East (E), and Up (U) directions using six different noise models with criteria of maximum likelihood estimation (MLE), Akaike Information Criteria (AIC), and the Bayesian Information Criteria (BIC). We found that the favorite noise models were white noise plus generalized Gauss-Markov noise (WN + GGM), followed by generalized Gauss-Markov noise (GGM). Then, we compared the PGS velocities of each direction with the Scripps Orbit and Permanent Array Center (SOPAC) output parameters and found that there was good agreement between them. The PGSs in the WSCS had velocities in the N, E, and U directions of -10.20 +/- 0.39 mm/year, 31.09 +/- 0.36 mm/year, and -2.24 +/- 0.66 mm/year for WLDD, and -10.85 +/- 0.38 mm/year, 30.67 +/- 0.30 mm/year, and -3.81 +/- 0.66 mm/year for ZWAN, respectively. The accurate datum was defined for Cal/Val of altimeters for WSCS as a professional in-situ site. Moreover, the zenith wet delay (ZWD) of the coastal PGSs in the regional and sub-regional solutions was calculated and used to validate the microwave radiometers (MWRs) of Jason-3, Haiyang-2B (HY-2B), and Haiyang-2C (HY-2C). A sub-regional PGS solution was processed using 19 continuous operational reference stations (CORS) of Hong Kong Geodetic Survey Services to derive the ZWD and validate the MWRs of the altimeters. The ZWD of the PGSs were compared with the radiosonde-derived data in the regional and sub-regional solutions. The difference between them was -7.72 similar to 2.79 mm with an RMS of 14.53 similar to 18.62 mm, which showed good consistency between the two. Then, the PGSs' ZWD was used to validate the MWRs. To reduce the land contamination of the MWR, we determined validation distances of 6 similar to 30 km, 16 similar to 28 km, and 18 similar to 30 km for Jason-3, HY-2B, and HY-2C, respectively. The ZWD differences between PGSs and the Jason-3, HY-2B, and HY-2C altimeters were -2.30 +/- 16.13 mm, 9.22 +/- 22.73 mm, and -3.02 +/- 22.07 mm, respectively.