## 4 Maneuver Assessment and POD Results
### GPS and BDS2 Fused Results
As an important navigation method in real-time satellite orbit determination, single point positioning (SPP) can be used to evaluate the contribution of BDS2 to the LEO orbit determination. By using the GFZ precise ephemeris, the SPP solutions are obtained based on the ionosphere-free pseudorange observations. Table 5 shows the statistics of the orbit differences of the GPS-only and GC combined SPP solutions with respect to the GPS-based POD. The SPP error in the radial is larger than that in the along-track or normal direction due to the uneven distribution of the viewing GNSS satellites in space (i.e., GNSS satellites in the nadir direction cannot be observed). Compared with the GPS SPP solutions, the precision of the GC combined SPP solutions is improved by more than 6% in 3D RMS. For a single GNSS system, the PDOP values usually amount to 2-3 under open sky [51]. A larger PDOP value means a weaker observation geometry, thereby degrading the precision of the orbits. Figure 15 shows the precision of the GPS and GC combined SPP solutions when the PDOP values of GPS exceed three. Compared with the GPS-only, the precision of the GC combined SPP solutions is improved by 14.4% and 12.4% for the TH-2A and TH-2B in 3D RMS, respectively. Since the PDOP values of GPS + BDS2 are smaller than those of GPS-only, the GC combined solutions can achieve a better accuracy in those regions with poor GPS viewing geometry. Figure 14: Cumulative percentages of the PDOP values of GPS-only and GPS + BDS2 along the trajectories of TH-2A. Figure 13: Clock offsets of TH-2A (on DOY 308, 2019) obtained from POD based on GPS observations (**top panel**) and BDS2 observations without correction (**bottom panel**). Blue and red points show the estimated clock offsets when radar is in operation and powered off, respectively. The contribution of the BDS observations for the combined POD is analyzed in this section. The residuals and overlap comparisons are used to assess the orbit precision. Table 6 shows that the values of the residual RMS of the combined POD are close to those obtained by the GPS- and BDS2-only POD, as listed in Table 2. The overlap comparisons of the GC combined POD are smaller than those of the single BDS- and GPS-based POD. Figure 16 indicates that the daily 3D RMS of the overlap differences of the GC combined POD are less than those of the BDS2-only. The overlap comparisons of the GC combined solutions are on average 0.65 and 0.64 cm (3D RMS) for the TH-2A and TH-2B, respectively. Compared with the GPS-based orbits, the precision of the GC combined solutions is only improved by 3-4% in 3D, which is hampered by the precision of the BDS2 precise orbit and clock products [36]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Residual** & **Observation** & **TH-2A** & **TH-2B** \\ \hline LC RMS (cm) & GPS & 0.59 & 0.58 \\  & BDS2 & 0.64 & 0.66 \\ \hline PC RMS (m) & GPS & 0.81 & 0.81 \\  & BDS2 & 0.53 & 0.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Average RMS of PC and LC residuals of GC combined POD. Figure 15: Differences of GPS and GC combined SPP solutions with respect to GPS-based POD when the PDOP values of GPS exceed 3. (**top panel**) is TH-2A and (**bottom panel**) is TH-2B. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sat**} & \multirow{2}{*}{**Solution**} & \multicolumn{4}{c}{**Orbit Difference (m)**} \\ \cline{3-6}  & & **Radial** & **Along-Track** & **Normal** & **3D** \\ \hline \multirow{2}{*}{TH-2A} & GPS SPP & 1.86 & 0.74 & 0.59 & 2.09 \\  & GC combined SPP & 1.71 & 0.69 & 0.57 & 1.93 \\ \hline \multirow{2}{*}{TH-2B} & GPS SPP & 1.84 & 0.74 & 0.59 & 2.07 \\  & GC combined SPP & 1.72 & 0.70 & 0.57 & 1.94 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Average RMS of orbit differences of GPS-only and GC combined SPP solutions with respect to GPS-based POD for TH-2A and TH-2B.