## 3 Experiments and Discussions
### Accuracy and Convergence of RT-PPP in MGEX Stations  
#### 3.3.1 Dual-Frequency PPP  
The time series of the position differences between the BII+B3I PPP solutions of the four MEGX stations and the SINEX solutions on the E, N, and U components are presented in Figure 6. The average RMSE of the static PPP at JFNG, ULAB, URAM, and WUH2 stations is shown in Figure 7. Accordingly, the positioning accuracy of BDS-3 dual-frequency PPP based on the PPP-B2b real-time products is within 4.6 cm in the three directions for the four stations. In contrast, the GPS dual-frequency PPP with B2b correction is about 5.3 cm in three directions. The lower accuracy of GPS PPP with B2b corrections is caused by the product accuracy of GPS orbits and clocks, which has been studied in the above section. Additionally, the positioning accuracy of PPP-B2b-based positioning in the N component is higher than in the E component, which is related to the satellite's observation geometry strength of the carrier phase ambiguity. Most BDS-3 satellites have a north-south ground track and, thus, provide strong observation geometry strength in N directions and better positioning accuracy. In addition, the fixing ambiguity may improve the positioning accuracy in the E components [39], and similar results can be obtained in [22, 40]. In general, the positioning accuracy of static RT-PPP with PPP-B2b is slightly lower than that of IGS real-time products, but higher than that of WHU ultra-rapid products and broadcast ephemeris. The statistics accuracy in terms of RMSE of static BII+B3I PPP using different real-time products is listed in Table 5. The positioning accuracies of BDS-3 PPP with PPP-B2b service are 4.8 cm and 5.4 cm in horizontal and 3D components, respectively. While using GPS, the positioning accuracy is slightly lower than that of BDS-3, with 5.4 cm and 5.9 cm in horizontal and 3D components, respectively. This demonstrates that the PPP-B2b service has the capability of providing centimeter-level positioning accuracy for users in China and neighboring countries while using the dual-frequency static PPP mode. However, the accuracy of PPP using PPP-B2B services is still lower than those solutions based on IGS real-time products. Among these solutions, the solution based on the products of WHU has the highest accuracy in horizontal components with 1.1 cm. In comparison, the solution based on the CAS products has the highest accuracy in 3D with 1.9 cm. The solutions based on DLR real-time products present the worst accuracy with 3.5 cm and 3.9 cm in horizontal and 3D components, respectively. Additionally, the horizontal and 3D positioning accuracies using WHU ultra-rapid products are 10.5 cm and 12.1 cm, respectively, while those based on broadcast ephemeris are 23.9 cm and 27.7 cm. Generally, the BII+B3I BDS-3 PPP based on PPP-B2b SSR corrections can provide users with centimeter-level static positioning solutions, which is close to these based on IGS RTS SSR corrections, especially in horizontal components. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Products**} & \multicolumn{4}{c}{**Orbit (cm)**} & \multirow{2}{*}{**Clock (ns)**} \\ \cline{2-2} \cline{4-5}  & **Radial** & & **Along-Track** & **Cross-Track** \\ \hline CAS & 6.73 & 8.82 & 5.79 & 0.31 \\ DLR & 4.37 & 11.95 & 7.08 & 0.27 \\ GFZ & 5.41 & 12.40 & 7.63 & 0.31 \\ WHU & 5.38 & 8.57 & 6.41 & 0.22 \\ WHU\_U & 5.39 & 7.25 & 5.54 & 0.35 \\ BRDC & 9.70 & 22.21 & 25.74 & 0.87 \\ B2b BDS & 9.42 & 21.26 & 28.65 & 0.18 \\ B2b GPS & 13.73 & 23.83 & 17.96 & 0.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The average accuracies of real-time orbit and clock products. vertical are continuously better than 10 cm and 20 cm for at least 10 min. The convergence times of static PPP for each MGEX station are presented in Figure 8. Since it is difficult to converge while using WHU ultra-rapid product and broadcast ephemeris, we only provide the convergence times of PPP using IGS real-time products and PPP-B2b products.