# _remote sensing_
## 3 Related Data and Correction Models in Cal/Val of Altimeters
### Mass
#### 3.4.1 Campaigns of Sea Surface Measurements  
Two campaigns have been conducted in 2018 and 2022 to measure the sea surface between the ATG of Zhi'wan island and the footprints of the HY-2B. In 2018, a GPS towing-body was used to measure the sea surface of this area with a width of 6 km and about 26 km from the southeast of Zhi'wan island (Figure 5a). The temporal GNSS reference station was placed on Zhi'wan island to process the kinematic towing-body data. The precision of the GPS towing-body was evaluated using the DGB. The gradient of the measured sea surface is about 1.62 cm/km along the HY-2 orbit, which has a bias of \(-\)5\(\sim\)5 cm compared with CNES_CLS11 MSS [18]. Figure 5: Sea surface measurements along the ATG of Zhi’wan island and HY-2B orbits. Image **(a)** is the GPS towing-body used in the campaign of 2018. Image **(b)** is the principle of SSH measurements for HASMS. Image **(c)** is the displacement of the HASMS and the data acquisition and the GNSS receiver were placed in the cabin. Image **(d)** is the measured sea surface of this area using the ordinary kriging interpolation of the measured lines in ArcGIS 10.4. In 2022, an HASMS developed by the National Ocean Technology Center (NOTC) was used to measure the sea surface of this area. This system consists of a geodetic type GNSS receiver and Chock ring antenna, a liquids level measurement sensor (LLMS), and a data acquisition module (Figure 5b). It was installed at the front of the ship in the campaign (Figure 5c). The SSH measured by the HASMS (\(\mathrm{SSH}_{\mathrm{HASMS}}\)) can be estimated by Equation (7):  
\[\mathrm{SSH}_{\mathrm{HASMS}}=\mathrm{H}_{\mathrm{GNSSRef}}-\mathrm{h}_{\mathrm{ ref}}-\mathrm{h}_{\mathrm{LLMS}} \tag{7}\]  
where the \(\mathrm{H}_{\mathrm{GNSSRef}}\) is the height measured by the Chock ring antenna;  
\(\mathrm{h}_{\mathrm{ref}}\) is the height from the Chock ring antenna reference point to the LLMS, which was measured within 1 mm;  
\(\mathrm{h}_{\mathrm{LLMS}}\) is the height from the LLMS to the sea surface. The NMEA data of the GNSS receiver was collected together with the LLMS to keep the time consistency with an interval of 1 s. An experiment was also undertaken to evaluate the accuracy and precision of HASMS using the DGB and a pressure tide gauge. The pressure tide gauge was placed on the wharf, and the HASMS was moored less than 100 m together with the DGB. The bias was 1.33 mm compared between the HASMS and DGB [17]. The standard deviations were 12.73 mm and 36.66 mm compared by the HASMS with the DGB and the pressure tide gauge, respectively (Figure 6). The SSH measured by the HASMS and the DGB show very good consistency after eliminating the influence of the ocean [17]. The measured sea surface in the two campaigns was interpolated using the cubic interpolation method. It also includes the effects of average sea level, atmospheric pressure, sea state deviation, etc. Therefore, the measured sea surface is the result of a combination of multiple factors. The sea surface of this area is low in the northwest and high in the southeast, with a difference of about 1.05 cm/km along the orbit of HY-2B. A comparison has been made between the measured sea surface and the global MSS models such as the DTU 2021 (www.space.dtu.dk, accessed on 18 February 2023) [34], EGM 2008 ([[http://earth-info.nima.nim](http://earth-info.nima.nim)]([http://earth-info.nima.nim](http://earth-info.nima.nim)), accessed on 19 May 2019) plus CLS_MDT_2018 ([[ftp://ftp-access.aviso.altimetry.fr](ftp://ftp-access.aviso.altimetry.fr)]([ftp://ftp-access.aviso.altimetry.fr](ftp://ftp-access.aviso.altimetry.fr)),  
Figure 6: Comparisons between the DGB and the HAMSM. The distance between the two equipment is less than 100 m.  
accessed on 28 July 2019) [35; 36], and CLS_MSS2015 ([[ftp://ftp-access.aviso.altimetry.fr](ftp://ftp-access.aviso.altimetry.fr)]([ftp://ftp-access.aviso.altimetry.fr](ftp://ftp-access.aviso.altimetry.fr)), accessed on 28 July 2019) [37].