## 3 POD Results
### Improvement on POD Using Regional Stations  
At present, the ground facilities of the BDS are all located in China. To verify the ISL improvement on POD when using only regional stations, which is crucial to the generation of broadcast ephemeris, a regional tracking network consisting of seven stations in China and three stations in Australia is selected. The results with or without ISL data are compared to precise orbits obtained from 10 globally distributed stations. The distribution of regional and global tracking stations, as well as the subsatellite points of three types of satellites, are plotted in Figure 8 in which the red dots represent regionally distributed stations and the green dots represent globally distributed stations. Figure 9 shows the comparison between the orbits estimated with different tracking networks, based on the two-day overlapping differences between two consecutive three-day orbit arcs. Obviously, after adding ISL data, the joint POD accuracy of all three types of BDS-3 satellites is significantly improved regardless of the distribution of stations. With the help of ISL, the orbits of the whole constellation obtained using 10 regionally distributed stations are comparable to the orbits obtained using 10 globally distributed stations with a difference of only 0.9 cm. For MEO satellites, due to the reasonable station distribution, the orbit accuracy based on global ground stations alone is higher than the results of using only regional ground stations. However, this difference resulting from the different distributions of stations is greatly reduced after adding ISL data. The 3D error using regional stations is decreased by 69.7% to 10.7 cm. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Types**} & \multirow{2}{*}{**Directions**} & \multicolumn{5}{c}{**Percentage Improvement**} \\ \cline{3-6}  & & **10 to 20** & **20 to 30** & **30 to 40** & **40 to 50** \\ \hline \multirow{4}{*}{MEO} & A & 18.1\% & \(-\)0.8\% & 3.0\% & 10.4\% \\  & C & 24.0\% & 10.5\% & 6.4\% & 3.3\% \\  & R & 4.5\% & 0.0\% & 1.0\% & 4.3\% \\  & 3D & 19.7\% & 3.3\% & 4.4\% & 7.5\% \\ \hline \multirow{4}{*}{IGSO C38} & A & 25.4\% & 3.6\% & \(-\)0.2\% & 2.7\% \\  & C & 15.0\% & 4.6\% & \(-\)0.3\% & 1.5\% \\ \cline{1-1}  & R & 6.9\% & 0.0\% & 4.0\% & 3.6\% \\ \cline{1-1}  & 3D & 20.0\% & 3.9\% & 0.0\% & 2.1\% \\ \hline \multirow{4}{*}{IGSO C39} & A & 9.7\% & 3.2\% & 2.3\% & \(-\)1.2\% \\  & C & 11.7\% & 2.3\% & 1.8\% & \(-\)0.7\% \\ \cline{1-1}  & R & 3.3\% & 2.5\% & 1.2\% & \(-\)1.2\% \\ \cline{1-1}  & 3D & 10.1\% & 2.7\% & 2.0\% & \(-\)1.0\% \\ \hline \multirow{4}{*}{GEO C59} & A & 14.5\% & 3.3\% & 1.5\% & 3.4\% \\  & C & 6.4\% & 2.0\% & \(-\)0.3\% & 1.3\% \\ \cline{1-1}  & R & 3.1\% & 1.8\% & 0.8\% & 2.8\% \\ \cline{1-1}  & 3D & 11.5\% & 2.8\% & 0.9\% & 2.7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Percentage improvement when increasing the number of tracking stations. Figure 8: Distribution of 10 regional stations (red dots) and 10 global stations (green dots). The subsatellite points of three types of satellites are marked as orange, blue, and purple, respectively. Figure 9: BDS-3 orbit accuracy obtained by regional (R) or global stations (G) with or without ISL measurements for: MEO satellites (**a**); IGSO satellite C38 (**b**); IGSO satellite C39 (**c**); and GEO satellite C59 (**d**). In terms of IGSO satellites, subject to the insufficient number of stations located in the Asia-Pacific region in our selected global tracking network, the orbit accuracy using the 10 regional stations (the green bars) in Figure 9b,c shows better performance than that using the 10 global stations (the yellow bars). Especially in the along-track direction, the RMS of IGSO satellites using the regional stations is 41.8% smaller than that using the global stations. After the addition of ISL data to POD, the accuracy obtained by the two strategies is comparable and the 3D error using regional stations is reduced to 15.2 cm.