## 3 POD Results
### Comparison among Different POD Strategies  
According to the four POD modes mentioned above, the precise orbits derived from three-day arcs are estimated. The two-day overlapping differences between two adjacent three-day solutions in along-track (A), radial (R), and cross-track (C) direction are used to evaluate the internal consistency of the orbits. The RMS results of MEO and IGSO satellites are presented in Figure 3, and the RMS value along with the errors of GEO satellites C59 are listed separately in Table 2 because they are much larger than those of MEO and IGSO satellites. Figure 2: Number of available data in BDS Week 715: (**a**) the number of dual one-way ISL measurements, where outliers and single one-way observations that cannot form a dual one-way link are discarded; and (**b**) the number of code/phase measurements, where outliers and observations of short arcs less than 1200 s are discarded. According to Figure 3 and Table 2, it can be found that the addition of ISL data has a positive effect on the orbit determination. For the ground-only POD, the overlapping accuracy of MEO satellites is similar to that of Joint 2 POD and higher than Joint 1 POD because there are currently enough globally distributed ground stations to observe MEO satellites. On the other hand, the overlapping errors as an indicator of internal coincidence precision reflect the internal consistency of the orbit estimates, not the \begin{table}
\begin{tabular}{c c c c c c} \hline
**Type** & **Strategy** & **Along-Track (cm)** & **Cross-Track(cm)** & **Radial (cm)** & **3D (cm)** \\ \hline \multirow{4}{*}{MEO} & Ground-only orbit & 5.8 & 3.1 & 1.4 & 6.7 \\  & ISL-only orbit & 13.7 & 14.0 & 2.1 & 19.8 \\  & Joint 1 orbit & 5.8 & 4.2 & 1.7 & 7.4 \\  & Joint 2 orbit & 5.3 & 3.9 & 1.5 & 6.7 \\ \hline \multirow{4}{*}{IGSO} & Ground-only orbit & 17.6 & 10.3 & 4.7 & 20.9 \\  & ISL-only orbit & 21.0 & 22.2 & 4.3 & 30.9 \\  & Joint 1 orbit & 8.4 & 7.6 & 3.2 & 11.8 \\  & Joint 2 orbit & 8.6 & 9.0 & 3.1 & 12.8 \\ \hline \multirow{4}{*}{GEO} & Ground-only orbit & 87.3 & 6.7 & 6.9 & 87.9 \\  & ISL-only orbit & 20.6 & 18.6 & 6.0 & 28.4 \\ \cline{1-1}  & Joint 1 orbit & 14.2 & 9.2 & 5.7 & 17.9 \\ \cline{1-1}  & Joint 2 orbit & 14.4 & 9.2 & 5.4 & 17.9 \\ \hline \end{tabular}
\end{table} 
Table 2: RMS of two-day arc overlapping difference of ground-only orbit, ISL-only orbit, Joint 1 orbit, and Joint 2 orbit based on three-day arcs solutions according to the satellite type. Figure 3: Comparison of two-day arc overlapping difference of ground-only orbit, ISL-only orbit, Joint 1 orbit, and Joint 2 orbit based on MEO/IGSO satellites three-day arcs solution. true accuracy. Besides, the improper weight of ISL data may also affect the contribution of ISL data on joint orbit determination, which needs further research in the future. Different from MEO satellites, the IGSO satellites can only be observed by iGMAS stations located in the Asia-Pacific region during the evaluation period. Therefore, the accuracy of Joint 2 has been significantly improved by 38.8% and 43.5% after adding ISL data for satellites C38 and C39, respectively. As a GEO satellite, satellite C59 has the characteristic of geostationary and high orbit height, and it can only be tracked by ground stations near the Asia-Pacific region similar to IGSO satellites. With the help of ISL, the weak geometry of the GEO satellite is strengthened, and the number of measurements is increased. Compared with ground-only results, the Joint 1/2 orbit accuracy of satellite C59 is greatly increased by 79.6%. As for ISL-only POD results, the orbit accuracy for the GEO satellite is greatly improved compared to the orbits obtained by ground-only POD strategy. It is noted that the ISL-only POD errors of the whole constellation in the along-track and cross-track direction are larger than those of the ground-only POD, except for the along-track direction of GEO satellites, which may be caused by the weaker orbital orientation constrain from only two anchor stations [12].