# Comprehensive Analysis of PPP-B2b Service and Its Impact on BDS-3/GPS Real-Time PPP Time Transfer
## 3 Results
### Quality Evaluation of PPP-B2b Corrections
#### 3.2.1 PPP-B2b Orbit Corrections  
The average root mean square error (RMSE) was employed to indicate the accuracy of the orbits. Figures 3-6 show the RMSEs of the PPP-B2b and CNES orbits in the R/A/C directions during the test period. The GBM final orbit product was used as the reference. The four figures show that the RMSEs of the CNES orbits are lower than those of the PPP-B2b orbits for both BDS-3 and GPS satellites, which is attributed to the fact that products from CNES are heavily reliant on global tracking stations, whereas those from PPP-B2b are primarily reliant on tracking stations in China [37]. As shown in Figures 4 and 6, GPS satellites have smaller RMSEs for CNES real-time orbits in the R/A/C directions \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Site** & **Receiver** & **Antenna** & **Clock** &
\begin{tabular}{c} **Distance from** \\ **USUD (km)** \\ \end{tabular} \\ \hline USUD & SEPT POLAR5 & AOAD/M\_T & H-MASER & – \\ MIZU & SEPT POLAR5 & SEPCHOKE\_B3E6 & CESIUM & 413.3 \\ TLM2 & SEPT POLAR5 TR & HEMA45 & H-MASER & 2060.2 \\ JFNG & TRIMBLE ALLOY & TRM59800.00 & – & 2289.4 \\ LCK3 & TRIMBLE ALLOY & LEIAR25.R3 & H-MASER & 5303.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Information of selected stations. \begin{table}
\begin{tabular}{l l} \hline \hline
**Items** & **Methods** \\ \hline Observations & GPS: L1/L2; BDS-3: B1I/B3I \\ Sampling rate & 30 s \\ Estimator & Kalman filter \\ Cut-off elevation & \(10^{\circ}\) \\ Phase windup & Model corrected [31] \\ Tidal effect & Model corrected [32] \\ Relativistic effect & Model corrected [32] \\ Satellite orbit and clock offset & Broadcast ephemeris + PPP-B2b corrections \\ Satellite DCB & PPP-B2b DCB corrections \\ Receiver clock offset & Estimated as white noise \\ Receiver DCB & Estimated together with receiver clock offset \\ Satellite antenna APC & Fixed to the values from igs14.atx \\  & ZHD: Saastamoinen model [33] \\ Troposphere & ZWD: random walk estimation + GMF [34] mapping function \\ Phase ambiguities & Float \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Processing strategies for the B2b-RTPPP. compared with those of the BDS-3 satellites. The RMSEs of GPS satellite orbits are within 0.10 m, and those of BDS-3 satellite orbits are within 0.30 m. Note that the CNES orbit has nearly the same accuracy in the three directions for GPS satellites. However, as shown in Figures 3 and 5, for the PPP-B2b orbit products, the RMSEs of the BDS-3 satellite orbits in the R/A/C directions perform better than those of the GPS satellite orbits, especially in the A component, which potentially benefits from the implementation of inter-satellite link (ISL) technology [35]. Because ISL can effectively improve the orbit determination, which can further provide a more accurate satellite position, especially in the A direction. Furthermore, we can see that the R direction has a minimum RMSE, while the RMSEs in the A and C directions are large, approximately two to four times that in the R direction. Therefore, the orbit error is primarily coming from the A component, which has the greatest impact on the orbit accuracy. Note that the C31, C38, C39, C40, G05, G11, G14, G23, G28, and G32 satellites were excluded from the comparison because the information of these satellites (except C38, C39, and C40) were not included in PPP-B2b products during the test period. Additionally, the quality of the orbit and clock offset for IGSO satellites (C38, C39, and C40) was poorer than that for MEO satellites [25]. Figure 4: RMSEs of the CNES orbits for BDS-3 satellites. Figure 3: RMSEs of the PPP-B2b orbits for BDS-3 satellites. To further evaluate the precision of the PPP-B2b and CNES orbits, their average RMSEs were calculated, as presented in Table 4. For the BDS-3, the RMSEs are 0.082, 0.288, and 0.262 m in the R/A/C directions for the PPP-B2b real-time orbit, and 0.073, 0.066, and 0.138 m for the CNES real-time orbit.