# Comprehensive Analysis of PPP-B2b Service and Its Impact on BDS-3/GPS Real-Time PPP Time Transfer
## 3 Results
### B2b-RTPPP Time Transfer with BDS-3/GPS Observations
Three-corner-hat (TCH) method can provide a better separation between the noise in the links and the noise in the station clocks. We first performed the frequency stability analysis for all the stations according to the literature [42] using the TCH and found that all the links except the TLM2-USUD link had a high percentage of noise in the time link due to the excessive noise of one of the stations. In this case, the frequency stability experiment is a failure. Therefore, we only evaluate the TLM2-USUD time link. In this paper, we define averaging time less than 1000 s as short-term and averaging time greater than 10,000 s as long-term. Figure 11: STD of difference between B2b-RTPPP solutions and GBM-PPP solutions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **MIZU-USUD** & **TLM2-USUD** & **JFNG-USUD** & **LCK3-USUD** \\ \hline BDS-3-only & 49.3 & 69.4 & 73.6 & 63.7 \\ GPS + BDS-3 & 50.9 & 69.5 & 75.0 & 65.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The reduction of C and GC B2b-RTPPP solutions with DCB compared to that without DCB at STD values for clock difference (%). Figure 12: Pseudorange residual of B2b-RTPPP without (**left**) and with (**right**) DCB corrections. Each color represents a satellite. Figure 13 shows the modified Allan deviation (MDEV) of the TLM2-USUD time link. For the five schemes, the G, C, and GC B2b-RTPPP solutions demonstrated a relatively high similarity at the same averaging time, while the G B2b-RTPPP solution had the poorest frequency stability in the long term. The MDEV of GC B2b-RTPPP solution with DCB performed better than the other solutions for most of the averaging time. In terms of short-term frequency stability, the MDEV of the G without DCB, C with DCB, C without DCB, GC with DCB, and GC without DCB are (1.13 \(\times\) 10\({}^{-13}\), 4.06 \(\times\) 10\({}^{-14}\), 7.86 \(\times\) 10\({}^{-14}\), 3.62 \(\times\) 10\({}^{-14}\), and 6.18 \(\times\) 10\({}^{-14}\)) at 960 s. In terms of long-term frequency stability for four time links, the MDEV of the G without DCB, C with DCB, C without DCB, GC with DCB, and GC without DCB are (6.52 \(\times\) 10\({}^{-15}\), 1.49 \(\times\) 10\({}^{-15}\), 6.17 \(\times\) 10\({}^{-15}\), 1.40 \(\times\) 10\({}^{-15}\), and 6.12 \(\times\) 10\({}^{-15}\)) at 122,880 s. These findings of frequency stability are similar to the STD values of B2b-RTPPP solutions. The enhancements of GPS satellites to BDS-3 PPP are limited the PPP-B2b corrections. As our previous evaluation of the PPP-B2b product concluded, the quality of the GPS satellites is inferior to that of the BDS-3 satellites. In addition, the frequency stability improved as the averaging time increased. It should be noted that compared with the MDEV without DCB correction, the MDEV with DCB correction was similar in the short-term, while the B2b-RTPPP solutions with DCB demonstrated better long-term MDEV. The main reason for the frequency stability without DCB correction is that the large pseudorange residuals without DCB correction led to additional errors absorbed by the receiver clock offset. The frequency stability of the B2b-RTPPP solutions without DCB was significantly poorer at the TLM2-USUD time link than that of the B2b-RTPPP solutions with DCB, which is analyzed below. To verify the previous findings, the number of satellites involved in B2b-RTPPP at a cut-off elevation angle of 10\({}^{\circ}\) for the TLM2 and USUD stations during the test period are shown in Figure 14. The average number of visible satellites is 7.8, 8.6, and 16.4 for the G, C, and GC at the TLM2 station; and 8.3, 8.6, and 16.9 for the G, C, and GC at the USUD station, respectively. In addition, there were relatively fewer GPS satellites at the TLM2 station; up to five satellites at some epochs, leading to the appearance of outliers and reconvergence [43], as shown in Figure 15, which presents the clock difference between the two station clock offsets. The uncorrected DCB solutions had a slower convergence rate, which reduced the B2b-RTPPP time transfer.