## 3 Results
### Impact of CNCORS on BDS/GPS Joint POD
#### 3.1.1 Overlapping Arcs Precision  
The BeiDou POD is conducted according to strategies 1 and 2 based on the BDS/GPS joint orbit determination method. The mean RMS values of the overlapping arcs are shown in Table 4 and  
Figure 1: Ground traces of BeiDou satellites and tracking stations. Note 1: The red points indicate the ground stations with BDS/GPS tracking capability. The five yellow points near the equator are the five BeiDou GEO satellites; Note 2: The PRN of the original C15 satellite was changed to C13 on 11 December 2016. Given that this change was not made when this paper was written, C15 is still used to represent the PRN of this IGSO satellite. This figure is drawn using GMT software [29]. Figure 2. Table 4 and Figure 2 show that when MGEX and CNCORS are used as ground stations, the precision of the orbit overlapping arcs (3 DRMS) is as follows: the average precision of GEO satellites is 17.2 cm, that of IGSO satellites is 4.9 cm, and that of MEO satellites is 5.6 cm. The IGSO satellites have the best internal precision, and the reason for this is that more ground stations exist in the IGSO satellite coverage area and these evenly distributed stations provide observations for the IGSO satellite for a long time. Among the three directions of the orbit, all the radial direction precisions of the three types of satellites are better than 2.3 cm, the cross-track direction is better than 3.3 cm, and the along-track direction for IGSO and MEO satellites is better than 4.5 cm. The along-track direction for GEO satellites is poor, and the mean RMS value reaches 16.6 cm. Thus, it can be seen that the precision of the BeiDou satellite orbit can reach a high level if the ground stations are evenly distributed. GEO satellites have a large along-track error due to their stationary nature, and the along-track component of the GEO satellite orbit is strongly correlated with other parameters. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Satellite Type**} & \multirow{2}{*}{**PRN**} & \multicolumn{2}{c}{**Along-Track**} & \multicolumn{2}{c}{**Cross-Track**} & \multicolumn{2}{c}{**Radial**} & \multicolumn{2}{c}{**3D**} \\ \cline{3-10}  & & **S1** & **S2** & **S1** & **S2** & **S1** & **S2** & **S1** & **S2** \\ \hline \multirow{6}{*}{GEO} & C01 & 21.6 & 21.5 & 1.4 & 1.5 & 1.0 & 1.1 & 21.7 & 21.8 \\  & C02 & 20.9 & 24.5 & 2.0 & 2.3 & 1.6 & 1.7 & 21.4 & 24.8 \\  & C03 & 11.1 & 16.0 & 1.5 & 2.2 & 2.3 & 2.4 & 12.2 & 16.8 \\  & C04 & 19.2 & 21.0 & 1.3 & 1.4 & 1.5 & 1.7 & 19.6 & 21.4 \\  & C05 & 10.4 & 14.0 & 2.3 & 2.2 & 1.2 & 1.4 & 11.1 & 14.4 \\  & **Mean** & **16.6** & **19.4** & **1.7** & **1.9** & **1.5** & **1.7** & **17.2** & **19.8** \\ \hline \multirow{6}{*}{IGSO} & C06 & 3.2 & 5.2 & 2.8 & 3.9 & 1.6 & 2.1 & 4.8 & 7.0 \\  & C07 & 3.6 & 5.8 & 2.7 & 4.4 & 2.3 & 2.8 & 5.8 & 8.7 \\  & C08 & 3.0 & 4.7 & 2.7 & 3.8 & 1.8 & 2.3 & 4.5 & 6.6 \\  & C09 & 2.7 & 6.5 & 3.1 & 5.5 & 1.7 & 2.7 & 4.6 & 8.6 \\  & C10 & 2.8 & 5.5 & 2.6 & 4.3 & 2.0 & 3.0 & 4.5 & 7.9 \\  & C15 & 3.3 & 4.6 & 3.3 & 4.1 & 1.9 & 2.2 & 5.2 & 7.4 \\  & **Mean** & **3.1** & **5.4** & **2.9** & **4.3** & **1.9** & **2.5** & **4.9** & **7.7** \\ \hline \multirow{6}{*}{MEO} & C11 & 4.1 & 4.4 & 2.4 & 2.0 & 1.3 & 1.3 & 5.7 & 5.1 \\  & C12 & 4.5 & 5.7 & 2.7 & 2.6 & 1.7 & 1.7 & 5.7 & 6.7 \\ \cline{1-1}  & C14 & 3.3 & 4.6 & 2.4 & 3.1 & 1.5 & 1.6 & 5.4 & 5.9 \\ \cline{1-1}  & **Mean** & **4.0** & **4.9** & **2.5** & **2.6** & **1.5** & **1.5** & **5.6** & **5.9** \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of overlapping arcs precision between strategies 1 and 2 (unit cm). Figure 2: _Cont._  
The influence of the CNCORS on the POD of the BeiDou satellite can be obtained according to the comparison of the orbit determination precision in strategies 1 and 2. Figure 2 shows that the orbit precision of GEO and IGSO satellites in strategy 1 is better than that in strategy 2. In addition, after the addition of the CNCORS, the 3D precision of GEO and IGSO satellites increased by 13.1% and 36.4%, respectively.