# Comprehensive Analysis of PPP-B2b Service and Its Impact on BDS-3/GPS Real-Time PPP Time Transfer
## 3 Results
### B2b-RTPPP Time Transfer under Constrained Sky-View  
As we know, the receiver may be blocked artificially or naturally, which means less sky-view. In this case, fewer satellites can be observed by the receiver. Additionally, the cut-off elevation angle is an important preset parameter of PPP. Since the atmospheric delay error and multipath error become larger as the angle decreases, the possibility of uncontrollable factors appearing on the signal propagation path is also higher, so a large cut-off elevation angle can weaken the influence of the above errors as much as possible [44; 45]. Considering that the PPP-B2b performance in terms of elevation angle is unknown, we conducted a time transfer experiment at different cut-off elevation angles to analyze its effect on B2b-RTPPP. The cut-off elevation angle ranged from 10\({}^{\circ}\) to 40\({}^{\circ}\), with a step of 10\({}^{\circ}\). On the basis of previous findings, we only carried out GC B2b-RTPPP time transfer to avoid frequent outliers and re-convergence of time transfer because of the lack of trackable satellites. Figure 16 displays the clock difference of the TLM2-USUD time link. The sequence of clock difference substantially overlaps at 10\({}^{\circ}\), 20\({}^{\circ}\), and 30\({}^{\circ}\), while the fluctuation range is larger at 40\({}^{\circ}\), which is mainly due to the poorer spatial geometric distribution of visible satellites. In addition, the PDOP values of TLM2 and USUD stations are shown in Figure 17. As we can see, the PDOP values increase gradually as the cut-off elevation angle rises. The average PDOPs are statistically 1.4, 1.8, 2.7, and 5.0 for the TLM2 station, and 1.3, 1.7, 2.7, and 4.8 for the USUD station at 10\({}^{\circ}\), 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\), respectively. The sequence of PDOP values for the TLM2 station has a larger fluctuation at a cut-off elevation angle of 40\({}^{\circ}\). Therefore, B2b-RTPPP can obtain a relatively great time transfer solution at a lower cut-off elevation angle in theory. To comprehensively investigate the performance of the B2b-RTPPP time transfer, the STDs of the difference between B2b-RTPPP solutions and GBM-PPP solutions are shown in Figure 18. The B2b-RTPPP time transfer performance decreased as the cut-off elevation angle increased. Moreover, the STD values of the clock difference were within 0.2 ns at the cut-off elevation angle from 10\({}^{\circ}\) to 30\({}^{\circ}\), with no significant difference. However, the STD values present a significant increase at a mask angle of 40\({}^{\circ}\), approximately two to four times greater than the STD values at a mask angle of 10\({}^{\circ}\). It is worth noting that the STD values were all within 1 ns with different B2b-RTPPP solutions. Furthermore,  
Figure 16: Clock difference of the TLM2-USUD time link. Table 6 shows the improvement of the B2b-RTPPP solution with the mask angles of 10\({}^{\circ}\), 20\({}^{\circ}\), and 30\({}^{\circ}\) compared with the B2b-RTPPP solution with the angle of 40\({}^{\circ}\) at the STDs for the clock difference. Compared with the B2b-RTPPP solution at the mask angle of 40\({}^{\circ}\), the B2b-RTPPP solutions at the mask angles of 10\({}^{\circ}\), 20\({}^{\circ}\), and 30\({}^{\circ}\) improved by 71.9-80.6%, 64.1-78.1%, and 58.3-74.5%, with a mean reduction of 76.1%, 70.8%, and 67.8%, respectively. Overall, the STDs of the B2b-RTPPP solution for all time links varied within an acceptable range for the B2b-RTPPP time transfer. Figure 19 depicts the MDEV of the TLM2-USUD time link. No significant difference in the MDEV was observed in the short term. The MDEV of 10\({}^{\circ}\), 20\({}^{\circ}\), 30\({}^{\circ}\), and 40\({}^{\circ}\) are 2.33 \(\times\) 10\({}^{-14}\), 2.14 \(\times\) 10\({}^{-14}\), 2.39 \(\times\) 10\({}^{-14}\), and 2.35 \(\times\) 10\({}^{-14}\) at 960s.