# 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  
With the observations of the B1I/B3I IF combination, the results of B2b-RTPPP time transfer using the post-processing PPP solution with GBM final product (GBM-PPP) were taken as a reference. As we concluded in Section 3.2.3, the quality of DCB products needs to be further improved in the near future. The performance of B2b-RTPPP time transfer with/without DCB corrections was investigated to analyze the contribution of DCB to B2b-RTPPP. The average STDs of the difference between B2b-RTPPP time link solutions and GBM-PPP time link solutions were calculated at each time link during the test period at a cut-off elevation angle of 10\({}^{\circ}\), as shown in Figure 11. Note that the DCB of GPS was not provided by PPP-B2b. We have two key findings here. First, the STD values of the B2b-RTPPP solutions were all less than 1 ns. The STDs of the B2b-RTPPP solution without DCB correction were the largest (0.20-0.80 ns), approximately two to three times greater compared with those of the B2b-RTPPP solutions with DCB corrections (0.05-0.20 ns). It should be noted that the STD of the GC B2b-RTPPP solution was slightly lower than that of the C B2b-RTPPP solution. The G B2b-RTPPP solution had the greatest STD. The following three reasons can be used to explain above findings. (1) This is possibly due to the fact that the visible satellites of GPS are fewer than that of BDS-3. (2) For PPP-B2b, the quality of BDS-3 satellite's clock outperforms that of GPS, as shown by previous findings. (3) The pseudorange residual of B2b-RTPPP solutions without DCB is greater than that with DCB. Figure 12 presents the pseudorange residual on DOY 92. The DCB corrected pseudorange residual fluctuates around zero, while there is a systematic bias for the DCB noncorrected pseudorange residual. Thus, the DCB must be corrected in time transfer, otherwise, a part of the DCB will be absorbed by the station clock offset, reducing the B2b-RTPPP time transfer performance [40]. Second, in conjunction with Table 2 and Figure 11, it is found that the STD values of B2b-RTPPP solutions exhibit a strong correlation with the geographical distance of the time link. Interestingly, the STD values gradually increased as the distance of the time link increased. These relationships may be partially explained as follows. For the short baseline, most of the satellites tracked by two stations are the same. The satellite orbits and clock offsets have a similar effect on two stations that are close to each other, and the effect on the B2b-RTPPP solutions can be eliminated. For the long baseline, however, most of the satellites tracked by the two stations are not the same. The difference cannot be eliminated [41]. This resulted in differences in the B2b-RTPPP solutions for different time links at different distances. Figure 10: The DCB biases of the PPP-B2b with respect to CAS. The reductions of the C and GC B2b-RTPPP solutions with DCB compared with that without DCB at the STDs of the clock difference are illustrated in Table 5. For the C B2b-RTPPP, the reductions of STD ranged from 49.3% to 73.6%, with a mean reduction of 64%. The reductions of GC B2b-RTPPP were almost the same as that of GC B2b-RTPPP. Therefore, the DCB had a great impact on the time transfer, which validated our previous findings. In addition, compared with G B2b-RTPPP, the reductions of the C and GC B2b-RTPPP solutions with DCB ranged from 69.0-80.1% and 70.6-81.3%, with a mean reduction of 76.7% and 77.7%, respectively. For the B2b-RTPPP without DCB, STDs were reduced by at least 14.5%, and by a maximum of 60.1% for C B2b-RTPPP. STDs were reduced by at least 14.6%, and by a maximum of 60.4% for GC B2b-RTPPP. Thus, the C and GC B2b-RTPPP performed better than the G B2b-RTPPP. Frequency stability is another indicator of the time transfer performance. It should be noted that the frequency stability analysis is affected by both the device under test and the reference device.