## 2 Methods
### Availability Analysis of GNSS Constellations  
Figure 2 depicts the global distribution of the average number of visible satellites for single systems and combinations of different orbital types of BDS-3, with an elevation cutoff of 7\({}^{\circ}\), over a day (1 September 2022). As shown in Figure 2, the globally average number of GPS reaches more than 10, and specifically in low- and high-latitude regions, it rises  
to more than 12. However, there are fewer than 10 for Galileo. For BDS-3, it is combined with MEO, IGSO, and GEO satellites. It is apparent that in the combination of BDS-3, MEO, and IGSO, there are more than 12 visible satellites in the area between longitude 45-180E and latitude 205-20N. Compared with BDS-3 MEO, it is clear that in high-latitude regions, the number of visible satellites for BDS-3 MEO+IGSO satellites increases greatly. Thus, it has more visible satellites in the Asia-Pacific region than GPS and Galileo. In contrast, the number of BDS-3 satellites in the western hemisphere at low and middle latitudes is under 8. The DPOP values are shown in Figure 3. It is clear that the value for each system corresponds to the distribution of their number of visible satellites.  
The distribution of the number of visible satellites and the value of PDOP for the single systems and combinations of different orbital types of BDS-3 over a day (1 September 2022), with an elevation cut-off of 7\({}^{\circ}\), is shown in Figure 4. It is clear that the range for GPS is from 9 to 12, which is the largest number among all single schemes, while it is only from 7 to 9 for Galileo, which has the least visible satellites in all the single systems. There are a range of 7-11 globally visible satellites for the BDS-3 MEO+IGSO, which is the widest distribution. The extensive distribution of BDS-3 MEO+IGSO indicates that there are fewer satellites for some stations, which may reduce the positioning accuracy of those stations. Although the visible satellite number can meet the basic requirement, it is worth noting that there are not many discernible satellites in a single system. For PDOP, BDS-3 MEO has the smallest  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Process Options** & **Strategy** \\ \hline Cutoff elevation angle & 7\({}^{\circ}\) \\ Sampling rate & 30 s \\ PCV and PCO & Corrected with IGS14 xt file \\ Satellite orbit & Fixed with GEF MGEX final products \\ Satellite clock & Fixed with GEF MGEX final products \\ Estimator & Kalman filter \\ Phase ambiguity & Estimated as float constants \\ Ionospheric delay & First-order effects removed with IF combination \\ Tropospheric delay & Mapping function: GMF, Dry part: corrected using \\ Saastamoinen model \\ Tides correction & Solid earth tide, pole tide, ocean tide loading \\ \hline \hline \end{tabular}
\end{table}
Table 1: Processing strategy for system combination PPP.  
Figure 1: Geographical distribution of the selected 143 worldwide MGEX tracking stations.  
number of visible satellites, so its PDOP is also the largest of the four systems. However, GPS has the smallest PDOP of the three systems because of its centralized distribution of the number of visible satellites. BDS-3 MEO+IGSO has the largest PDOP value among the four systems because of its wide distribution of the number of visible satellites.  
Figure 3: Average PDOP value of GPS, Galileo, BDS-3 MEO, and BDS-3 MEO+IGSO over a day (1 September 2022) with an elevation cut-off of 7\({}^{\circ}\).  
Figure 2: Average visible satellite numbers of GPS, Galileo, BDS-3 MEO, and BDS-3 MEO+IGSO over a day (1 September 2022) with an elevation cut-off of 7\({}^{\circ}\).