## 3 Experiments and Discussions
### Accuracy of Real-Time Vehicle-Borne PPP  
The dynamic test is carried out on DOY 357 in 2021 from 06:00 to 08:30 on the fifth ring road with Inertial Navigation System (INS) (NovAtel SPAN EPSON G370) and NovAtel GNSS receiver, which receives the B1I and B2b signal from BDS-3, and L1 and L2 signal from GPS. To study the PPP-B2b performance in complex environments, especially the performance of new signal B2b, we equipped a GNSS jammer to interfere with the received GNSS signals. The jammer reduces the number of visible satellites rather than affecting the observed distances. In the dynamic test, the vehicle drove about 40 km in the north-south direction and 30 km in the east-west direction. The tight integration of GPS+BDS RTK and INS provided by Inertial Explorer software were used as reference values for the dynamic test, and the corresponding trajectory is presented in Figure 12.  
The positioning strategies are listed in Table 7. It should be noted that the satellite DCB products from both real-time SSR and IGS final products do not contain the satellite \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Direction** & **CAS** & **DLR** & **GFZ** & **WHU** & **WHU\_U** & **BRDC** & **B2b BDS** & **B2b GPS** \\ \hline E & 3.0 & 5.2 & 3.7 & 2.9 & 5.8 & 15.5 & 4.6 & 4.6 \\ N & 2.0 & 1.8 & 1.6 & 2.0 & 4.9 & 9.7 & 2.8 & 2.9 \\ U & 9.1 & 13.2 & 5.7 & 7.0 & 10.7 & 39.5 & 5.5 & 5.6 \\
2D & 3.6 & 5.5 & 4.0 & 3.5 & 7.6 & 18.3 & 5.4 & 5.4 \\
3D & 9.8 & 14.3 & 7.0 & 7.8 & 13.1 & 43.5 & 7.7 & 7.8 \\ \hline \end{tabular}
\end{table} 
Table 6: The average positioning accuracy at the four stations (unit: cm). Figure 11: Convergence time of single-frequency PPP using different SSR products. Figure 12: The trajectory of vehicle dynamic test. DCB value of C7D (B2b). Therefore, the DCB value of C7Z from CAS' was used to weaken the influence of satellite DCB on B2b frequency with ignoring the intra-frequency error. The number of visible satellites and corresponding PDOP values are presented in Figure 13. The average number of visible BDS-3 and GPS satellites during the 4-h dynamic test in Beijing are 8.9 and 2.0, respectively, and the average corresponding PDOP value of BDS-3 and GPS are 6.3 and 2.4, respectively. The sharp changes in the number of visible satellites are affected by the equipped GNSS jammer and further affect the PDOP values. Figure 14 presents the time series of position errors of vehicle-borne dual-frequencies (B1i+B2b and L1+L2) PPP, single frequency PPP (L1 and B2b) by comparing with the RTK/INS tight integration solutions in E, N, and U components. It is noted that the large fluctuations in the three components occur in dual-frequencies and single frequencies PPP, especially in the U components. Comparing to the visible satellite number presented in Figure 13, the presence of significant error components coincided with epochs of poor observations, which results in positioning discontinuity and further leads to PPP re-convergence. We chose dual-frequency PPP during epochs 3587 to 3588, red box in Figure 14, to further analyze the effect of GNSS signal blockage. At the period of epochs 3587 and 3588, the visible satellite is 0, and the BDS cannot provide continuous positioning results. At epoch 3589, the number of visible satellites is 8, and the positioning error at E, N, and U components are 4 m, \(-\)2 m, and \(-\)4 m, respectively. The average visible satellite number during epochs 3589 and 3900 is 8.9, and the B1I+B2b combination PPP with PPP-B2b converged to 0.5 and 1.0 m positioning accuracy in both horizontal and vertical components. The convergence time is 85 s in this period, and the average PDOP value is 2.2. The error components of single frequency PPP, L1 and B2b, present similar statuses.