# _remote sensing_
## 2 Introduction and Altimeter Calibration Method of WSCS  
### Introduction of WSCs  
The WSCS is located at the south of Guangdong Province, China (114.3\({}^{\circ}\)E, 22.0\({}^{\circ}\)N) (Figure 2). The climate is subtropical and displays clear seasonal variations. About 5.3 typhoons pass through this area every year, causing great economic losses. The typhoons also create difficulties for the fixation of the facilities on this site. However, the continuity of equipment measurement is good because the firmness and stability was considered carefully at the beginning of the construction.  
Figure 1: Satellite altimeter calibration sites and experiments. The red triangulars represent the operational in situ calibration sites. The blue dots represent the altimeter calibration campaigns.  
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. Each PGS is equipped with a SOUTH Net-S9 receiver and STHCR3-G3 Choke Ring antenna, and the antenna phase center stability is within 2 mm ([[http://www.ngs.noaa.gov/ANTCAL/](http://www.ngs.noaa.gov/ANTCAL/)]([http://www.ngs.noaa.gov/ANTCAL/](http://www.ngs.noaa.gov/ANTCAL/)), accessed on 10 August 2023). An XZC 6 meteorological instrument and a CE318 sun photometer were placed on the mountain top of Dan'gan island, together with the PGS.  
Three acoustic tide gauges (ATGs) were installed with a data reception of 30 s for Dan'gan and Wai'ling'ding islands and 6 min for Zhi'wan island, to measure the sea level, in August 2019. The ATGs are equipped with Aquatrak 5003 sensors, one calibration tube, several extension tubes, and one stainless-steel protective tube with a sea-level measurement accuracy of less than 3 mm. The stainless frame was installed to keep the vertical of the sensor and the tubes and protect them. The DGB was used to define the datum of the ATGs for three times during 2019-2023 [17].  
The above devices constitute the main facilities for altimeter calibrations. Moreover, an emergency monitoring center was established for equipment monitoring and maintenance, data collection, and initial processing on Gui'shan island (Figure 2). All the observation data are transmitted to the integration laboratory via BeiDou satellite communication, wireless bridge, and GPRS communication link, and then to NSOAS, China. Moreover, a  
Figure 2: Detailed view of the facilities of WSCS and nearby related stations. The pink triangles represent the combined stations of PGSs and ATGs. The yellow triangles represent the station of PGSs on Miao’wan island and Hong Kong (HKWS), respectively. The coral circle represents the laboratory of WSCS on Gui’shan island. The B329 tide gauge station of GCOS in Hong Kong were also included in this research (coral triangular).  
moored GNSS buoy, meteorological buoy, and seabed equipped with a pressure tide gauge were also planned to displace about 30-100 km at the southeast of this site.