# _remote sensing_
## 5 Discussion  
The WSCS has been in trial operation since August 2019 and has been in continuous operation so far. At the beginning of the construction of WSCS, it was designed for the Cal/Val of the HY-2 series altimeters. Fortunately, it still has the potential to calibrate other altimeters. In previous experiments, we completed the calibration of Jason-3 and S3A satellites using temporary GNSS reference stations, DGB, and tide gauge stations, and the calibration results were basically consistent with those in other calibration sites [10].  
At present, we calibrate the HY-2B, HY-2C, Jason-3, and S3A satellite altimeters using the ATGs placed on the islands of WSCS. The datum of the ATGs was regularly transmitted from the PGSs to the sensors during 2019 to 2023. The MSS and tide models used in this paper are global/regional models, and their accuracy is limited. We plan to use a combination of in-site data and remote sensing data to construct a regional MSS/sea surface and tidal model, in order to improve the calibration accuracy of satellite altimeters. The wet atmospheric delay of the altimeter is also an important correction for its distance measurement, using PGSs and GPS radiosonde data [1; 5; 10].  
The DGB together with the PGSs or temporary GNSS stations were also used for Cal/Val of HY-2B and HY-2C altimeters at WSCS. The DGB was moored less than 0.5 km from the altimeter footprints, and more than 2 h before and after the altimeter overflights. The baseline length between the PGSs and the DGB was less than 20 km, which could measure the SSH in an accuracy of less than 35 mm [17]. The HY-2B and HY-2C altimeters were calibrated for six and four times during the on-orbit test campaign in 2018 and 2020, respectively. The estimated accuracy of the HY-2B altimeter's SSH was a 21.2 mm bias with a standard bias of 20.8 mm, which was equivalent to the calibration results of the ATGs. The standard bias of the Cal/Val results was 7.9 mm, which demonstrated high precision of altimeter measurements.  
At the beginning of the founding of WSCS, three methods were planned to accomplish the Cal/Val of the altimeters, including the ATGs, moored GNSS buoys (MGB), and bottom pressure tide gauges (BPTG). The three methods complement and test each other and improve the accuracy of the Cal/Val of altimeters. This is incomparable to other international calibration sites. In addition to existing PGSs, ATGs, and meteorological equipment, there are also other planned equipment for the Cal/Val of WSCS, such as the MGB and BPTG, which will be placed in a timely manner. These devices will be directly placed at the footprint HY-2 altimeters (Figure 13), and the datum will be defined using the PGSs and DGB of WSCS. Such devices will ignore the MSS and tide model corrections and will also be used for SWH and WS measurement, thereby completing the validation of such products of the altimeter.  
In the future, it is necessary to establish more calibration sites to carry out Cal/Val for other satellite altimeters such as HY-2D and SWOT. More calibration verification campaigns for the equipment of the calibration sites should be conducted to ensure consistency in the benchmark, deviation, offset, and other information. Moreover, the connection of several islands in WSCS (blue lines in Figure 13) is similar to a triangle. If another station is set up near Hong Kong or the Da'wan'shan island area (yellow circles in Figure 13), it can achieve calibration of the wide swath satellite altimeter such as SWOT and Guanlan missions.