## 5 RT POD Processing and Analysis
### Results and Analysis of the RT POD of the TJU-01 Satellite  
In this study, the RT POD of the TJU-01 satellite is performed in a simulated mode based on real onboard observations considering PPP-B2b signals. The post-POD solutions with a precise orbit and clock offset from GFZ are used for reference to assess our RT POD performance. The post-processed orbits overlap differences of TJU-01 are 2-3 cm in the radial, along, and cross directions from January 28 to 5 February 2022. In an effort to test the enhancement effect of PPP-B2b on the RT POD of LEO satellites, the results of the RT POD of the TJU-01 satellite on 31 January 2022 are compared, as presented in Figure 13, for the GPS PPP-B2b versus the GPS broadcast ephemeris, the BDS-3 PPP-B2b versus the BDS-3 broadcast ephemeris, and the GPS and BDS-3 PPP-B2b versus the GPS and BDS-3 broadcast ephemeris.  
Figure 13 reveals that PPP-B2b improves the RT POD accuracy of the TJU-01 satellite. The RT POD errors when using PPP-B2b are smaller than those when using the broadcast ephemeris. Meanwhile, the navigation satellite orbit errors and TJU-01 satellite's RT POD errors in the along direction are the largest among the three directions. It is thus concluded that the orbit errors of the navigation satellites and the RT POD errors of the LEO satellites are positively correlated. To further assess the improvement effect of PPP-B2b on the RT  
Figure 13: Comparison of error series of the RT POD of the TJU-01 satellite using the PPP-B2b and broadcast ephemeris.  
POD of the TJU-01 satellite, the 9-day results are compared and averaged in Figure 14 and Table 10. Figure 14 shows that the TJU-01 satellite RT POD errors decrease the most in the along direction among the three directions, as PPP-B2b has the greatest correction in the along direction for the navigation satellite orbit errors.  
Table 10 shows that the accuracy of the RT POD of the TJU-01 satellite is higher when using the BDS-3 broadcast ephemeris than when using the GPS broadcast ephemeris and higher when using the BDS-3 PPP-B2b than when using the GPS PPP-B2b. The reasons are  
Figure 14: Comparison of the RMS of the orbit errors in radial, along, and cross directions for the RT POD of the TJU-01 satellite.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Direction**} & \multicolumn{2}{c}{**Radial (RMS/m)**} & \multicolumn{2}{c}{**Along (RMS/m)**} & \multicolumn{2}{c}{**Cross (RMS/m)**} \\ \cline{2-7}  & **Not** & **B2b** & **Not** & **B2b** & **Not** & **B2b** \\
**Ephemeris** & **Corrected** & **Corrected** & **Corrected** & **Corrected** & **Corrected** & **Corrected** \\ \hline GPS broadcast ephemeris & 0.15 & 0.12 & 0.28 & 0.19 & 0.19 & 0.14 \\ BDS-3 broadcast ephemeris & 0.12 & 0.11 & 0.23 & 0.20 & 0.16 & 0.13 \\ GPS + BDS-3 broadcast ephemeris & 0.11 & 0.10 & 0.21 & 0.13 & 0.12 & 0.09 \\ \hline \end{tabular}
\end{table}
Table 10: Comparison of the results of the RT POD of the TJU-01 satellite between the use of the broadcast ephemeris and PPP-B2b.  
as follows. (1) Owing to the inter-satellite links, the BDS-3 broadcast orbit accuracy is higher than the GPS broadcast orbit accuracy. (2) More BDS-3 satellites than GPS satellites receive PPP-B2b. (3) The integrity of GPS observations is worse than that of BDS-3 observations. Furthermore, the most accurate scheme is using GPS and BDS-3 PPP-B2b for the RT POD of the TJU-01 satellite, as there are more onboard observation GNSS data and more navigation satellites receiving PPP-B2b corrections.