## 4 PPP Accuracy Evaluation
### Static PPP Performance  
The mean convergence time for static PPP of six combinations for each station in the east, north, and up directions is presented in Figure 9, and the mean convergence time of each combination is listed in Table 6. It can be seen from Figure 9 and Table 6 that the convergence time of GPS is the shortest among six combinations, which are 6.01 min, 5.53 min, and 16.52 min for the east, north, and up directions, respectively. For the east and north directions, the convergence time of Galileo is faster than that of BDS, and the GLONASS show the longest convergence time, which may be that all selected stations are located in the region with low latitudes, and better positioning performance can be achieved at high latitudes than low latitudes for GLONASS [38]. In terms of up component, the convergence time of Galileo is shorter than that of GLONASS, while the BDS presents the poorest convergence performance in the up component among the six combinations, and its convergence time is 32.17 min. Furthermore, compared to the GPS-only solution, \begin{table}
\begin{tabular}{c c c} \hline \hline
**System** & **Average** & **RMS** \\ \hline GPS & 0.62 & 0.21 \\ BDS-2(MECO) & 2.19 & 0.65 \\ BDS-2(IGSO) & 3.73 & 0.90 \\ BDS-3(MECO) & 1.97 & 0.71 \\ BDS-3(IGSO) & 2.01 & 0.35 \\ Galileo & 0.62 & 0.79 \\ GLONASS & 2.48 & 1.04 \\ QZSS & 0.98 & 1.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The average accuracy and RMS of DOY 283–289 SISRE for Multi-GNSS satellites (unit: m). the convergence time of the GPS/QZSS combination can be shorted, and the improvements are 10.37%, 0.90%, and 1.15% in the east, north, and up directions, respectively. While compared to the BDS-only solution, the BDS/QZSS solutions only short the convergence time in the up direction, the improvement is 1.65%. The reason may be that the number of BDS satellites in Asia-Pacific is larger than 20, when conducting PPP, by adding QZSS satellites, the improvements in convergence time for static PPP are limited. The mean positioning accuracy for static PPP of six combinations for each station in the east, north, and up directions are presented in Figure 10, and the mean positioning accuracy of each combination is listed in Table 6. It can be clearly seen that the positioning accuracy is better than 3 cm in the east direction except for PIMO and SIN1 stations. Apart from the USUD station, the positioning accuracy of other stations is better than 2 cm in the north component. The positioning accuracy is outperformed 5 cm in the up direction except for YARR station. The positioning accuracy of GPS is the best, and it is 1.09 cm, 0.78 cm, and 1.68 cm in east, north, and up directions, respectively. For three components, the positioning accuracy using BDS is better than that of GLONASS, while Galileo shows the worst performance. Compared to the GPS-only solution, the positioning accuracy of GPS/QZSS solutions can be improved, and the improvement is 0.92%, 1.28%, and 1.19% in the east, north, and up directions, respectively. The few improvements may be caused by the limited number of QZSS satellites. The improvement of the BDS/QZSS solution in terms of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Convergence Time (min)**} & \multicolumn{3}{c}{**Positioning Accuracy (cm)**} \\ \cline{2-7}
**System** & **E** & **N** & **U** & **E** & **N** & **U** \\ \hline G & 16.01 & 5.53 & 16.52 & 1.09 & 0.78 & 1.68 \\ GJ & 14.35 & 5.48 & 16.33 & 1.08 & 0.77 & 1.66 \\ C & 27.47 & 13.58 & 32.17 & 1.40 & 1.03 & 2.08 \\ CJ & 28.29 & 13.59 & 31.64 & 1.38 & 1.01 & 2.04 \\ R & 28.44 & 15.44 & 26.26 & 1.93 & 1.32 & 3.07 \\ E & 23.93 & 10.78 & 20.79 & 2.24 & 1.12 & 2.65 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Convergence time and positioning accuracy of different combination static PPP. Figure 9: Mean convergence time for static PPP in each station. positioning accuracy is 1.43%, 1.94%, and 1.92% in east, north, and up directions compared to the single BDS-only solution.