## 3 POD Results
### Comparison among Different POD Strategies
For the two different joint POD results, the Joint 2 orbits show a radial improvement of 10%, 2.5%, and 1.4% for MEO, IGSO, and GEO satellites, respectively, compared to the Joint 1 orbit. As a result, the 3D accuracy of MEO has increased by 9%. However, in the case of IGSO and GEO satellites, the addition of ISL clock information increases the errors in the cross-track or along-track direction and even leads to a decrease of 8.5% in the 3D accuracy of the IGSO satellites. Considering the fact that the IGSO and GEO satellites are still in the test phase and there is no measurement observed from stations other than iGMAS stations, a further study on the orbit differences between the two joint POD method needs to be carried out in the future. The SLR data can be used as a reliable independent tool to evaluate orbit accuracy. At present, there are only four MEO satellites that are regularly tracked by International Laser Ranging Service (ILRS) [29; 30]. The residuals (observed minus computed) are presented in Table 3. It follows that the ISL-only orbits have the worst standard deviations (STDs) while the ground-only orbits have the largest biases in the SLR residuals. When using both L-band and Ka-band data, the SLR residuals are significantly reduced. The RMS of the Joint 2 results is 8% smaller than that of the Joint 1 results. Since the orbit radial component is highly correlated with the clock offset parameters, the performance of the satellite clock offset estimated with the orbit parameters at the same time can \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Strategy** & **PRN** & **No. of Normal Points** & **Mean (cm)** & **STD (cm)** & **RMS (cm)** \\ \hline \multirow{4}{*}{Ground-only orbit} & C20 & 797 & 1.0 & 5.5 & 5.6 \\  & C21 & 766 & 3.6 & 5.6 & 6.6 \\  & C29 & 568 & \(-\)3.3 & 3.8 & 5.0 \\  & C30 & 572 & \(-\)3.5 & 3.9 & 5.2 \\ \hline \multirow{4}{*}{ISL-only orbit} & C20 & 797 & \(-\)0.2 & 7.8 & 7.8 \\  & C21 & 766 & 2.5 & 7.6 & 8.0 \\  & C29 & 568 & \(-\)1.2 & 10.9 & 10.9 \\  & C30 & 572 & 0.7 & 10.6 & 10.6 \\ \hline \multirow{4}{*}{Joint 1 orbit} & C20 & 797 & \(-\)1.1 & 3.3 & 3.5 \\  & C21 & 766 & 1.9 & 3.5 & 4.0 \\ \cline{1-1}  & C29 & 568 & \(-\)3.5 & 3.5 & 5.0 \\ \cline{1-1}  & C30 & 572 & \(-\)2.2 & 3.4 & 4.1 \\ \hline \multirow{4}{*}{Joint 2 orbit} & C20 & 797 & \(-\)0.9 & 3.0 & 3.1 \\ \cline{1-1}  & C21 & 766 & 2.1 & 3.1 & 3.7 \\ \cline{1-1}  & C29 & 568 & \(-\)3.0 & 2.8 & 4.1 \\ \cline{1-1}  & C30 & 572 & \(-\)3.0 & 3.1 & 4.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: SLR residuals to BDS-3 MEO satellites derived from ground-only orbit, ISL-only orbit, Joint 1 orbit, and Joint 2 orbit based on three-day arcs. also be used as an indicator to evaluate the accuracy of the orbits. Note that the ISL-only strategy does not solve the clock offset parameter. The clock overlapping differences of the two-day arc are computed based on two adjacent three-day arcs, and the RMS value is plotted in Figure 4. It can be found that the performance of the MEO satellite clock offsets became worse when applying the Joint 1 strategy, but the accuracy of clock offsets computed by the Joint 2 strategy shows a better performance than that of the ground-only strategy with an improvement of 27.0%, 49.5% and 122.3% for MEO, IGSO, and GEO satellites, respectively. Existing studies have shown that the orbit error contained in the clock offsets estimated by L-band observations can be observed in the Allan variance [31], while the application of ISL-only measurements can reduce this error in clock offsets [32]. Further analysis of the stability characteristic of the satellite clock is shown in Figure 5. The overlapping Allan deviation values are computed by the satellite clock offsets that have been detrended by removing a daily quadratic trend. In the case of MEO satellites, the obtained clock stability of the two joint POD with ISL is improved compared to the ground-only clock.