## 4 Vertical Deflections  
### Directional Components at Crossover Points  
[PERSON] presented a method for determining the directional component of vertical deflection at crossover points [3]. The latitude velocities along ascending and descending passes are symmetrical and, thus, have the same numerical value but the opposite sign. In contrast, the longitude velocities along the ascending and descending passes are approximately equal. Following Section 3.2, we extracted latitude, longitude, measuring time and geoid height information from all series of HY-2 SDR products. The derivative of geoid height with respect to time was calculated by a first-order difference formula. Considering the approximate relationship between velocities along the ascending and descending passes, the meridional (north-south) and prime (east-west) components of vertical deflections at crossover points were calculated. The results were then validated with an XGM2019 gravity field model, as shown in Table 6.  
It showed the accuracy of meridional components at the crossover points' directional vertical deflection ranges from 0.94 to 1.93 arcsec. In contrast, the accuracy of the prime component was slightly lower, between 2.11 and 6.08 arcsec. The meridional component accuracy for HY-2A GM was about 1.7 arcsec, while the SD for HY-2B, HY-2C and HY-2D ERM was about 1.4 arcsec. The accuracy discrepancies of the prime component were more obvious. The accuracy for HY-2C and HY-2D was about 2.4 arcsec and HY-2B was about 3.7 arcsec; HY-2A was the worst, at 5.7 arcsec. Of these, the accuracy of the meridional component for HY-2D was the best and HY-2C had the best performance in calculating the prime component. The obvious accuracy discrepancies between the meridional and prime components were mainly due to the approximate north-south distribution of continuous measurements. The \(\sim\)45\({}^{\circ}\) orbital inclination was ideal for obtaining perpendicular ground tracks and determining the equal accuracy level of directional components. From the perspective of spatial coverage, the inclinations for these altimetry missions were designed to be from 66\({}^{\circ}\) to 108\({}^{\circ}\). For HY-2A and HY-2B with an inclination of 99\({}^{\circ}\), the accuracy of the prime component was limited. In contrast, HY-2C and HY-2D, with an inclination of 66\({}^{\circ}\), had obviously improved east-west signal-tracking ability. Hence, the accuracy of the prime \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Experimental Region**} & \multicolumn{2}{c}{**HY-2A**} & \multicolumn{2}{c}{**HY-2B**} & \multicolumn{2}{c}{**HY-2C**} & \multicolumn{2}{c}{**HY-2D**} \\ \cline{2-9}  & **NS-SD** & **EW-SD** & **NS-SD** & **EW-SD** & **NS-SD** & **EW-SD** & **EW-SD** \\ \hline
1 & 1.8158 & 5.8641 & 1.8111 & 3.6730 & 1.4160 & 2.4234 & 1.4950 & 2.8535 \\
2 & 1.8060 & 6.0823 & 1.3033 & 3.8435 & 1.3650 & 2.6606 & 1.4224 & 2.6351 \\
3 & 1.2695 & 4.8377 & 0.9614 & 3.0489 & 0.9961 & 2.3396 & 0.9435 & 2.1194 \\
4 & 1.8012 & 6.0917 & 1.4912 & 4.3776 & 1.5263 & 2.3788 & 1.3558 & 2.4246 \\
5 & 1.9329 & 5.8834 & 1.5815 & 3.3820 & 1.7654 & 2.4545 & 1.5086 & 2.2948 \\
6 & 1.8720 & 5.6527 & 1.4436 & 3.7008 & 1.5509 & 2.1746 & 1.6584 & 2.1735 \\ Average & 1.7495 & 5.7353 & 1.4320 & 3.6709 & 1.4366 & 2.4052 & 1.3972 & 2.4168 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Validation for directional components of vertical deflection at crossover points with XGM2019 model (unit: arcsec): Region 1, North Pacific; Region 2, South Pacific; Region 3, Mid-Pacific; Region 4, Indian Ocean; Region 5, North Atlantic; Region 6, South Atlantic.