## 4 Maneuver Assessment and POD Results
### GPS and BDS2 Fused Results  
The combination of multi-GNSS provides a better observation geometry and achieves a more robust positioning ability than a single navigation system. Therefore, it is necessary to analyze the results of the GC combined processing for the TH-2. In addition, as with the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sat**} & \multirow{2}{*}{**Region**} & \multicolumn{5}{c}{**Orbit Difference (cm)**} \\ \cline{3-6}  & & **Radial** & **Along-Track** & **Normal** & **3D** \\ \hline \multirow{2}{*}{TH-2A} & Asia-Pacific & 2.75 & 5.51 & 2.30 & 6.57 \\  & Beyond Asia-Pacific & 2.59 & 7.91 & 3.21 & 8.91 \\ \hline \multirow{2}{*}{TH-2B} & Asia-Pacific & 2.98 & 5.36 & 2.38 & 6.58 \\  & Beyond Asia-Pacific & 2.53 & 7.65 & 3.12 & 8.64 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average RMS of orbit differences between BDS2- and GPS-based POD in different regions. Figure 11: Orbit differences between BDS2- and GPS-based POD during the arcs with and without BDS2 data. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sat**} & \multicolumn{5}{c}{**Overlap Comparison (cm)**} \\ \cline{2-5}  & **Radial** & **Along-Track** & **Normal** & **3D** \\ \hline TH-2A & 0.32 & 0.46 & 0.35 & 0.68 \\ TH-2B & 0.31 & 0.46 & 0.33 & 0.66 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average RMS of overlap comparisons for GPS-based orbit solutions for TH-2A and TH-2B. BDS2-based POD, we exclude the BDS2 GEO observations so to ensure the precision of the combined processing results. In multi-GNSS data processing, the characteristics of the receiver must be taken into account when estimating the ISB. As introduced in Section 2.1, the time tag corrections of the receiver to the raw GPS and BDS2 observations are different when the GPS is used for positioning, which are a time-varying value and a constant, respectively. Thus, the clock offsets derived by the GPS- and BDS2-based POD are different from each other, as shown in Figure 13. For the GPS, most clock offsets are within 40 ns. However, several large deviations can be noticed due to the suspension of time calibration during the radar operating period. As the velocity of the GPS satellite is about 4 km/s, \(-\)150 ns of receiver clock offset will lead to about 0.6 mm error in the distance when calculating the satellite position from the ephemeris. Such a position error will cause a similar magnitude of deviation for the receiver positioning [50]. Therefore, the influence of the deviations on the precision of the GPS-based POD is insignificant. For the BDS2, the clock offsets are accumulated, which is attributed to the fact that the differences between the pseudorange offsets and constant corrections are accumulated in the observations. In addition, the limited available BDS2 satellites results in the periodic gaps of the clock offsets. Since the time tag corrections cannot be obtained, and the accumulation of the BDS2 receiver clock offset increases the orbit errors, we correct the raw BDS2 data by using clock offsets obtained from the first BDS2-based POD. Due to the discrepancies in the data processing strategy, the ISB between GPS and BDS2 is not a constant. Therefore, the independent clock offsets for GPS and BDS2 should be estimated respectively in the GC combined processing of the TH-2. The observation geometry has a significant impact on the orbit accuracy, whose characteristics can be reflected by the PDOP values. Figure 14 shows that about 86% of the epochs with the PDOP values of GPS less than three along the trajectories of the TH-2. When the BDS2 IGSO and MEO observations are added, more than 90% of the epochs have PDOP values of GPS + BDS2 less than three. The average PDOP values of GPS + BDS2 and GPS-only are 2.1 and 2.5, respectively, which means a better observation geometry for the GC combined processing. Figure 12: Orbit differences between BDS2- and GPS-based POD in radial, along-track, normal, and 3D directions for TH-2A (**top panel**) and TH-2B (**bottom panel**).