## 4 Assessment and Analysis of LeGNSS Precise Positioning
### Assessment and Analysis of LeGNSS PPP Performance
As shown in Figure 6, though there are three to four BDS satellites tracked by station P1, the low elevation and short arc of BDS satellites make the geometric distribution of G/C almost the same as GPS-only. For station P2, since it is out of the  
Figure 4: Satellite visibility of GPS (_blue_) and BDS (_red_) at four stations on one day. BDS service region, the GDOP values of G/C are unchanged compared to GPS-only. For stations P1 and P4, 5-8 LEO satellites can be tracked while only 1-2 can be tracked for stations P2 and P3 since the higher latitude of P1 and P4. The GDOP values of G/C/L are smaller than those of G/C for stations P1 and P4, which attribute to the increased number of LEO satellites. Figure 5: Satellite visibility of LEO satellites at four stations on one day. The different colors denote the different orbital planes (there are six colors here corresponding to six orbital planes). Figure 6: _Cont._  
With the aforementioned characteristics of the LeGNSS, including the time span, sky plot, and GDOP values, Figure 8 presents the kinematic PPP results for GPS-only, G/C, G/L, and G/C/L, respectively. Here we show the hourly results just in order to clearly see the convergence time. The convergence times of different combinations for all stations are listed in Table 2. As we can see, the GPS-only PPP needs nearly 30 min to converge when the positioning errors of three components are all smaller than 0.1 m. Except for  
Figure 6: Sky plots (azimuth vs. elevation) of GPS (_blue_), BDS (_green_), and LEO (_red_) for four stations over one hour. Figure 7: Number of satellites and the corresponding GDOP values at four stations for different system combinations. station P2, the G/C PPP significantly shortens the convergence time against the GPS-only case. Station P2 is not within the BDS service region and only one BDS MEO satellite is tracked, as shown in Figure 7, which has nearly no contribution to the convergence time. For the other stations, the convergence time is 10 min faster than the GPS-only case, where the G/C GDOP values become smaller. The convergence time of the G/L combination can be dramatically decreased to around 5 min at stations P1 and P4, although their GDOP values are not as small as those of the G/C combination. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **GPS-Only** & **GPS/BDS** & **GPS/LEO** & **GPS/BDS/LEO** \\ \hline P1 & 34.9 & 22.8 & 5.2 & 5.2 \\ P2 & 36.8 & 36.8 & 30.0 & 30.0 \\ P3 & 38.3 & 17.1 & 12.3 & 8.2 \\ P4 & 26.0 & 14.0 & 5.3 & 5.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Convergence time of different combinations for all stations. Unit: min. Figure 8: Comparisons of GPS-only, G/C, G/L, and G/C/L kinematic PPP solutions at four stations. With P4, for example, the G/C convergence time is about 15 min, while for G/L it is only 5 min. However, the G/C GDOP is about 2 while G/L GDOP is between 2 and 4. This much shorter convergence time of G/L is attributed mainly to the fast motion of LEO satellites with respect to the ground stations, which is observed from the quick variation of GDOP values. For stations P2 and P3, there are not as many LEO satellites observed as at stations P1 and P4, which generates the relative larger G/L convergence time for about 30 and 12 min. However, for the P3 case, though, there are only 2 to 3 LEO satellites in G/L combination, the G/L convergence time is shorter than that of G/C convergence time, which has nine BDS satellites in G/C combination. This reassures the contribution of fast geometric change of LEO to the PPP. For the G/C/L PPP, the convergence time is further shortened to several minutes, which is much shorter compared to that of multi-GNSS PPP. Actually, with the same simulated situation, the convergence time of combined GPS, GLONASS, BDS, and Galileo PPP is also about ten minutes which is close to that of the real situation. This, again, verifies the fact that the fast geometric change of LEO with respect to the ground stations can indeed greatly shorten the convergence time.