## 4 Maneuver Assessment and POD Results
### POD Performance of Single GNSS  
As an internal consistency assessment method for the POD, overlap comparison is utilized to evaluate the orbit precision of the GPS-based POD for the TH-2. Table 3 shows the average RMS of 6 h overlap comparisons in the radial, along-track, and normal direction for the GPS-based POD solutions. The 3D RMS statistics are given in the most right column. Since the two satellites are equipped with the same type of receiver, the RMS of the overlap comparisons for the TH-2A are close to that for the TH-2B. The along-track components of the overlap errors are larger than the radial and normal components, which is attributed to the weaker constraint of the reduced-dynamic model in this direction [49]. The result indicates that the GPS-based POD of the TH-2 has a good internal consistency, with 0.7 cm in 3D RMS.  
For the BDS2-based POD, the average 3D RMS of the overlap comparisons are 5.49 and 6.36 cm for the TH-2A and TH-2B, respectively. Compared with the GPS-based orbits, the internal consistency of the BDS2-based orbits is weaker due to the poor viewing geometry. Table 4 shows the statistics of the orbit comparisons between the BDS2- and GPS-based POD in different regions. It may be noted that the along-track  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Residual**} & \multirow{2}{*}{**Observation**} & \multicolumn{2}{c}{**TH-2A (without**} & \multicolumn{2}{c}{**TH-2A (with Maneuver**} & \multicolumn{2}{c}{**TH-2B**} \\  & & **Maneuver Estimation)** & **Estimation)** & **(Maneuver-Free)** \\ \hline \multirow{2}{*}{LC RMS (cm)} & GPS & 0.93 & 0.58 & 0.58 \\  & BDS2 & 1.53 & 0.58 & 0.58 \\ \hline \multirow{2}{*}{PC RMS (m)} & GPS & 0.84 & 0.81 & 0.81 \\  & BDS2 & 0.72 & 0.56 & 0.57 \\ \hline \end{tabular}
\end{table}
Table 2: Average RMS of PC and LC residuals of GPS- and BDS2-based POD for TH-2A (without maneuver estimation), TH-2A (with maneuver estimation), and maneuver-free TH-2B.  
Figure 10: Relative errors of velocity changes obtained by BDS2-based POD wrt those by GPS.  
the RMS errors in the Asia-Pacific region are smaller than those in the other areas. Since most BDS2 satellites are located in the Asia-Pacific region, the regional constellation results in a 3D RMS error of 7 cm in this region, which is slightly better than that of 9 cm in other regions.  
There are about 2.0 h with no available BDS2 IGSOs/MEOs per day. Figure 11 shows the orbit differences between the BDS2- and GPS-based POD during the arcs with and without the BDS2 data. The orbit accuracy with/without the BDS2 data in the radial and normal direction is 2-3 cm. However, the orbit accuracy in the arcs of no BDS2 data in the along-track direction significantly degrades by 42.1% on average.  
Figure 12 shows the daily RMS of the orbit differences between the BDS2- and GPS-based POD. The along-track components of the RMS errors are larger than the radial and normal components for each arc. The 3D RMS of orbit differences are all smaller than 15 cm, and the average values are 7.99 and 7.83 cm for the TH-2A and TH-2B, respectively. These results indicate that the precision of the BDS2-based orbits of the TH-2 can achieve a subdecimeter level and satisfies the InSAR mission requirement.