# Comprehensive Analysis of PPP-B2b Service and Its Impact on BDS-3/GPS Real-Time PPP Time Transfer
## 5 Conclusions  
The BDS-3 began providing RTS to users in the Asia-Pacific region in 2020 using the PPP-B2b signal, offering promising opportunities for real-time PPP. In our contribution, the quality of PPP-B2b products was first evaluated with reference to GBM final products. Furthermore, the B2b-RTPPP time transfer performance was assessed using observation data from five selected stations over a seven-day period (DOY 92-98 in 2022). The conclusions are as follows.  
First, compared with the PPP-B2b GPS orbit, the PPP-B2b BDS-3 orbit has higher accuracy. The accuracy of the BDS-3 and GPS orbit in the R direction is centimeter-level. The accuracy of the BDS-3 orbit was better than that of the GPS orbit in the A direction owing to the additional ISL observations. Due to the limitations of the few regional observation stations, the PPP-B2b orbit accuracy performs worse than the CNES orbit for both GPS and BDS-3 satellites. For the BDS-3 satellites, the accuracy of the PPP-B2b clock offset outperforms that of the CNES. For the GPS satellites, the PPP-B2b clock's accuracy was lower than the CNES clock's. In terms of the PPP-B2b DCB, the biases of the PPP-B2b DCB are almost the same for all satellites with the CAS DCB as a reference. The average biases of the PPP-B2b DCB for B1I-, B1 Cp-, and B2 ap-B3I are 0.91 ns, 1.36 ns, and 10.31 ns, with STDs of 0.68 ns, 0.54 ns, and 0.48 ns, respectively.  
Second, B2b-RTPPP time transfer experiments using G, C, and GC observations were carried out. The accuracy of G, C, and GC B2b-RTPPP time transfer is sub-nanosecond level. The GC B2b-RTPPP solution exhibited the best performance, while the G B2b-RTPPP solution had the worst performance due to fewer satellites and the DCB corrections. Compared with the accuracy of uncorrected DCB solutions, the accuracy with DCB correction was improved by 64%. For the STD with DCB correction, the mean reductions in the STD are approximately 76.7% and 77.7%, respectively, compared with the G B2b-RTPPP solution. In addition, the performance of the B2b-RTPPP time transfer may suffer as a result of the increase in the baseline of the time link. Time transfer experiments at different cut-off elevation angles showed that time transfer performance decreased as the cut-off elevation angle increased, especially at large angles. To sum up, excitingly, a subnanosecond-level accuracy can be achieved by the B2b-RTPPP time transfer.  
Finally, the BIC/B2a IF combination has the smallest noise amplification factor among all dual-frequency combinations, which enables better PPP performance. Our further work will be conducted to investigate the B2b-RTPPP time transfer performance using observations of B1C/B2a IF combination. Considering the quality of the PPP-B2b DCB, analyzing the effect of the DCB on B2b-RTPPP time transfer will also be a focus in future work.  
J.T.: methodology, software, investigation, writing--original draft preparation. D.L.: conceptualization, methodology, validation, data curation, writing--review and editing. F.Z.: validation, resources, supervision, project administration. Y.G.: formal analysis, project administration, funding acquisition. R.Z.: visualization, formal analysis. All authors have read and agreed to the published version of the manuscript.  
The research was funded by National Natural Science Foundation of China (No. 42204039).  
The datasets in this study are available by contacting the corresponding author.  
We appreciate MGEX, GFZ, CAS, and CSNO for providing experimental data.  
The authors declare no conflict of interest.