## 4 Discussion  
In this paper, the refined PEA strategy is proposed for superior orbital accuracy of the HY-2B satellite. A spectral analysis was performed to identify the potential periodicity of empirical acceleration in HY-2B POD. Noticeable signals were identified in the along-track and radial directions, and a mixed spectrum was observed for the cross-track direction. According to the spectral analysis of HY-2B empirical accelerations, six sets of PEA strategies were employed to assess the impact of the PEA model on the POD performance. SLR, as a completely independent space-geodetic technology, was used to evaluate the accuracy of GPS-based orbit solutions [46; 52]. In order to deal with the complexity of the cross-track direction, two comprehensive PEA strategies were adopted for HY2B POD. Moreover, we found that the PEA model with empirical parameters in the along-track and cross-track directions provided satisfying orbit determination accuracy. This indicates that the forces in the radial direction of the satellite were well described by the background model. The 14 months of GPS observations are used for the reprocessing to evaluate different PEA solutions. It is worth mentioning that the optimized PEA strategy in this article is only applicable to HY2B satellites. Future work can consider investigating the POD performance of the refined PEA model for other LEO satellites and can also focus on the refinement of non-gravitational force modeling that might further reduce the uncertainty in LEO POD [13; 20; 24].  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Station (ID)**} & \multirow{2}{*}{\(N_{np}\)} & \multicolumn{2}{c}{**Float Amb.**} & \multicolumn{2}{c}{**Amb. Fixed**} \\ \cline{3-6}  & & **Mean** & **STD** & **Mean** & **STD** \\  & & **(mm)** & **(mm)** & **(mm)** & **(mm)** \\ \hline \hline \multicolumn{1}{l}{Yarragadee (7090)} & 10,496 & \(-\)3.3 & 16.0 & \(-\)3.3 & 13.7 \\ \hline \hline \multicolumn{1}{l}{Greenbelt (7105)} & 3102 & \(-\)4.2 & 17.3 & \(-\)4.0 & 14.8 \\ \hline \hline \multicolumn{1}{l}{Haleakala (7119)} & 1010 & 3.2 & 11.8 & 3.3 & 10.2 \\ \hline \hline \multicolumn{1}{l}{Hartebeest (7501)} & 2265 & 3.4 & 15.6 & 1.9 & 11.3 \\ \hline \hline \multicolumn{1}{l}{Zimmerwald (7810)} & 3990 & \(-\)0.4 & 12.4 & \(-\)1.1 & 10.2 \\ \hline \hline \multicolumn{1}{l}{Mt. Stromlo (7825)} & 5447 & \(-\)5.1 & 13.3 & \(-\)5.5 & 9.9 \\ \hline \hline \multicolumn{1}{l}{Graz (7839)} & 2224 & 7.2 & 16.2 & 6.2 & 13.7 \\ \hline \hline \multicolumn{1}{l}{Hertsmonceux (7840)} & 2726 & \(-\)0.2 & 13.8 & \(-\)1.4 & 12.2 \\ \hline \hline \multicolumn{1}{l}{Potsdam (7841)} & 1212 & \(-\)8.8 & 13.6 & \(-\)9.3 & 11.7 \\ \hline \hline \multicolumn{1}{l}{Matera (7941)} & 2024 & \(-\)9.8 & 13.8 & \(-\)9.7 & 11.4 \\ \hline \hline \multicolumn{1}{l}{Wettzel (8834)} & 1730 & \(-\)8.2 & 17.5 & \(-\)8.8 & 13.7 \\ \hline \hline \multicolumn{1}{l}{Total} & 36,226 & \(-\)2.3 & 15.7 & \(-\)2.7 & 13.3 \\ \hline \hline \end{tabular}
\end{table}
Table 5: SLR residuals and number of normal points for \(11\) high-performance ILRS stations used for the validation of the HY-2B orbit.  
Similar to other LEO missions, inconsistencies at a level of 2 cm were confirmed for the HY-2B GPS antenna phase center [16; 25; 26]. More than one year's data were used for PCO estimation, and the change trend in the PCO z-component could be inferred from the time series of the results. The GRG provided by the CNES/CLS analysis center was used to support the implementation of HY-2B ambiguity-fixed POD solutions, which resulted in a 15% improvement of SLR residuals compared to the float solution. Similar results based on OBS-based products provided by CODE were also obtained by other LEO satellite missions [13; 30]. The improvement of these GNSS products will benefit the LEO POD community.