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

Abstract

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 −7.72~2.79 mm with an RMS of 14.53~18.62 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 6~30 km, 16~28 km, and 18~30 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.

Altimeter Calibrations in the Preliminary Four Years’ Operation of Wanshan Calibration Site

“…The Generalized Gauss-Markov noise (GGM) and white noise + generalized Gauss-Markov noise (WN + GGM) models were used to analyze the noise of the PGS daily solution data. Details of the PGS processing can be found in [16].…”

“…The WSCS has been chosen to be the first specified site for Cal/Val of the satellite altimeters in China since 2013 by the National Satellite Ocean Application Service (NSOAS) [16]. They plan to accomplish Cal/Val for HY-2 series satellites.…”

“…Moreover, our research can provide a unified datum for multiple altimeters, and ensure the consistency and continuity of the measurements by long-term series altimeters. The WSCS has been chosen to be the first specified site for Cal/Val of the satellite altimeters in China since 2013 by the National Satellite Ocean Application Service (NSOAS) [16]. They plan to accomplish Cal/Val for HY-2 series satellites.…”

“…The WSCS is a favorable site for altimeter calibrations, as southeast of the Dan'gan, Zhi'wan, and Miao'wan islands extends the South China Sea, and there are no islands to contaminate the altimeter signals (Figure 2). There are four PGSs placed on Wai'ling'ding, Dan'gan, Zhi'wan, and Miao'wan islands, with a data reception period of 1 s [16]. The four PGSs were fixed with the bearing wall of the building or bedrock of the islands to ensure its stability.…”