## 5 RT POD Processing and Analysis
### Availability of PPP-B2b in the RT POD of the TJU-01 Satellite  
This study assumes that the TJU-01 satellite can receive PPP-B2b signals. The sampling interval for the TJU-01 satellite observations used in the visibility analysis is 30 s. The TJU-01 satellite observation onboard GNSS receiver can track 8 GPS satellites and 12 BDS satellites, including BDS-2 and BDS-3. Owing to the restricted distribution of monitoring stations, PPP-B2b can only provide corrections of flight arcs over the Asia-Pacific region for the BDS-3 and GPS satellites. Considering the high speed of LEO satellites, their geometric configuration with GEO will constantly change. When the Earth is located on the line connecting GEO and LEO satellites, it will obstruct the signal of GEO satellites, and LEO satellites will not be able to receive the PPP-B2b correction. The availability and visibility of PPP-B2b are computed and plotted for the TJU-01 satellite at each epoch on 31 January 2022, as shown in Figures 11 and 12 and Table 8 (GPS) and Table 9 (BDS-3). Figure 11 presents the number of satellites that can receive PPP-B2b corrections at each epoch corresponding to the sub-satellite point trajectory of the TJU-01 satellite. Up to eight GPS satellites can receive PPP-B2b corrections at a certain epoch, whereas up to seven BDS-3 satellites can receive the PPP-B2b correction. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Ephemeris**} & **Type** & **SISRE (ORB)/m** & **SISRE/m** \\ \hline GPS broadcast & 1.29 & 0.54 \\ Ephemeris & 0.81 & 0.22 \\ BPS-3 broadcast ephemeris & 1.77 & 0.14 \\ BDS-3 PPP-B2b & 1.24 & 0.12 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: The SISRE (ORB) and SISRE of the broadcast ephemeris and PPP-B2b. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Number of Satellites** & **0** & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** \\ \hline Available & & & & & & & & & & \\ epochs & 800 & 471 & 475 & 450 & 223 & 177 & 79 & 33 & 3 \\ Total number of GPS & & & & & & & & & \\ satellites & & & & & & & & & \\ Epoch & & & & & & & & & & \\ proportion & 29.5 & 17.4 & 17.5 & 16.5 & 8.2 & 6.5 & 2.9 & 1.2 & 0.1 \\ (\%) & & & & & & & & & \\ Quantity proportion & & & & & & & & & \\ (\%) & 0 & 12.5 & 25 & 37.5 & 50 & 67.3 & 75 & 87.3 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Statistical availability of the GPS PPP-B2b for the TJU-01 Satellite. Figure 10: Data availability rate of the GPS (**top**) and BDS (**bottom**) observations onboard the TJU-01 satellite. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Number of Satellites** & **0** & **1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline Available & & & & & & & & \\ epochs & 338 & 543 & 462 & 520 & 436 & 291 & 68 & 2 \\ Total number of BDS3 & & & & & & & & \\ satellites & 2628 & 4071 & 3493 & 3981 & 3334 & 2209 & 500 & 21 \\ Epoch & & & & & & & & \\ proportion (\%) & 12.7 & 20.3 & 17.3 & 19.5 & 16.3 & 10.9 & 2.5 & 0.1 \\ Quantity proportion (\%) & 0 & 13.3 & 26.5 & 39.2 & 52.3 & 65.9 & 81.6 & 66.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Statistical availability of PPP-B2b for the TJU-01 satellite. Figure 12 shows the percentages of the GPS and BDS-3 satellites receiving corrections relative to the number of satellites observed by the TJU-01 satellite at each epoch on a single day. It is seen that compared with the signals observed by ground stations, the PPP-B2b signals observed by BDS-3 and the GPS have a wider geographical range. The geographical range in which PPP-B2b is received by the GPS and BDS-3 navigation systems is much larger than the geographical range of ground stations, mainly because the TJU-01 satellite is much higher than the ground stations. The percentage GPS reception is smaller than the percentage BDS-3 reception for two main reasons. Firstly, fewer GPS satellites than BDS-3 MEO satellites receive the PPP-B2b signals. This mainly results from the different configurations of the GPS and BDS-3 satellite constellations.