## 3 Improvements in HY-2B POD
### Performance Analysis of the Refined PEA Model  
For a clear identification, a unique identifier (ID) was introduced based on the processing strategy described in Section 2.3. The solution IDs consider the differences in the direction and frequency of PEA (see Table 2). The first digit in the code represents the along-track direction, the second digit represents the cross-track direction, and the third digit is the radial direction. The number represents the frequency of empirical acceleration in that direction. For example, the 111 solution means that the empirical acceleration is set at 1 CPR in all three directions, whereas the 110 solution means that the empirical acceleration is set at 1 CPR in the along-track and cross-track directions. The letter \"C\" in 110 + C represents the addition of constant acceleration in the cross-track direction to the 110 solution. To facilitate the identification, the 5.5 CPR signal is referred to as a 6 CPR signal (see Table 2). In addition to the strategies mentioned in Section 2.2, the 110 and 111 solutions were used as regular strategies for comparison. Spectral analysis of the unmodeled acceleration provides effective prior information for setting the frequency of the periodic PEA. The approach taken to optimize the PEA model was to change the perturbation frequency to improve the orbit's accuracy. A frequency test based on spectral analysis was used to verify whether better orbit determination results could be obtained when the empirical acceleration was consistent with the characteristic signal of the spectrum results. The SLR technique is a powerful means of evaluating a GPS-based orbit [46] and it was used to evaluate the orbit accuracy of the PEA model with different solutions. The fitting curve of the SLR residual with elevation angle and azimuth angle was used [47, 48]. Eleven high-performance International Laser Ranging Service (ILRS) stations (i.e., Yarragadee (7090), Greenbelt (7105), Haleakala (7119), Hartebeest (7501), Zimmerwald (7810), Mt. Stromlo (7825), Graz (7839), Herstmonceux  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Solution** \\ **Type** \\ \end{tabular}  } & \multicolumn{2}{c}{**Along-Track**} & \multicolumn{2}{c}{**Cross-Track**} & \multicolumn{2}{c}{**Radial**} \\ \cline{2-7}  & **Periodic** & **Constant** & **Periodic** & **Constant** & **Periodic** & **Constant** \\  & **Acc. ** & **Acc. ** & **Acc. ** & **Acc. ** & **Acc. ** & **Acc. ** \\ \hline
110 & 1 CPR & No & 1 CPR & No & No & No \\ \hline
410 & 4 CPR & No & 1 CPR & No & No & No \\ \hline
460 + C & 4 CPR & No & 5.5 CPR & Yes & No & No \\ \hline
111 & 1 CPR & No & 1 CPR & No & 1 CPR & No \\ \hline
412 & 4 CPR & No & 1 CPR & No & 2 CPR & No \\ \hline
462 + C & 4 CPR & No & 5.5 CPR & Yes & 2 CPR & No \\ \hline \hline \end{tabular(7840), Potsdam (7841), Matera (7941), and Wettzel (8834)) [46] were used for SLR validation. The relevant models used in processing are listed in Table 1.  
The trend line for all stations' SLR residuals with respect to the elevation angle (see Figure 3) showed an overall dependency on the orbit solutions. The SLR residuals from the 110 orbit solution showed high dependency; much lower mean residuals were found for Solutions 410 and 460 + C. The 410 or 460 + C solutions probably represent reality well. It is worth mentioning that the empirical accelerations in the along-track direction compensated for the effect of the unmodeled part of the atmospheric drag. Both models deal with non-conservative forces in the along-track direction, and thus there is a process of mutual influence. The dependency between the SLR residuals and the azimuth angles from the stations to the satellite is shown in Figure 4. A large mean bias occurred for the 111 and 412 solutions, and a slight improvement was observed for the 462 + C solution.