## 4 Conclusions  
In the literature, many studies expounded the BDS-3 POD based on B1I/B3I combination, and few works analyzed BDS-3 POD based on all of the new signals. In this contribution, the observation quality of BDS-3 MEOs and IGSOs is analyzed. This indicates that C/N0 of MEO satellite is 2-3 dB higher than that of IGSO satellite at the same elevation angle. Meanwhile, the pseudo-range MP and noise for all operational signals of BDS-3 satellites are better than IGSO due to the lower satellite transmission power and the low elevation angle observation period. The overall observation quality of BDS-3 MEOs and IGSOs in each frequency is B1I \(<\) B1C \(<\) B3I \(<\) B2a \(\approx\) B2b.  
One month of observations from 129 MGEX stations are processed for BDS-3 POD based on three different combinations, i.e., B1 CB2a, B1 CB2b and B1 IB3I. The inter-agency consistency is assessed by orbit-only SISRE comparison between B1 CB2a combination, B1 CB2b combination, B1 IB3I combination, CODE and GFZ final orbits. The B1 CB2a and B1 CB2b combinations are superior to the B1 IB3I combination when compared with CODE or GFZ final orbits. Orbit-only SISRE for B1 CB2a/CODE, B1 CB2b/CODE, B1 CB2a/GFZ, B1 CB2b/GFZ and CODE/GFZ is approximately 5 cm for MEOs and 9 cm for IGSOs, respectively. Meanwhile, B1 IB3I/CODE and B1 IB3I/GFZ are about 1-2 cm worse. Comparison between B1 CB2a, B1 CB2b and B1 IB3I also indicates that B1 CB2a and B1 CB2b have good consistency, while B1 IB3I shows poor performance. Moreover, the performance of IGSOs is significantly worse than that of MEOs due to the geographical distribution of ground tracking stations and SRP modelling problems which should be further investigated. As for the orbit accuracy investigation by the SLR technique, result is in good agreement with the orbit-only SISRE assessment. For CODE orbit, GFZ orbit, B1 CB2a and B1 CB2b solutions, the mean RMS is 3-4 cm and the mean values are approximately 2 cm and \(-\)4 cm for CAST and SECM satellites, respectively. As for B1 IB3I solution, the mean RMS value is a few millimeters worse, which is approximately 4-5 cm. Moreover, mean values of SLR residuals for BDS-3 CAST and SECM satellites also demonstrate that deficiencies still exist in BDS-3 satellite empirical SRP modelling, i.e., ECOM and ECOM-2.  
In summary, the BDS-3 new signals such as B1C, B2a, and B2b are significantly better than the old B1I and B3I signals not only in observation quality but also in POD performance. We recommend using B1 CB2a or B1 CB2b combinations for BDS-3 POD, and BDS-2 can be handled as an individual constellation using the B1 IB3I combination.  
Figure 8: SLR residuals of BDS-3 four MEOs for different solutions. (**a**) for M2/C20, (**b**) for M3/C21, (**c**) for M9/C29, and (**d**) for M10/C30, respectively.  
**Author Contributions:** Conceptualization, writing--original draft preparation, writing--review and editing and methodology, B.T., Y.Y. and Q.A.; software, B.T.; validation, Q.A.; data curation, B.T. and Q.A.; formal analysis, Y.Y. and Q.A.; funding acquisition, B.T. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No. 42004022).  
**Data Availability Statement:** Our sincere thanks go to the International GNSS Monitoring and Assessment System (iGMAS) and the Crustal Dynamics Data Information System (CDDIS), Chinese Meridian Project for providing GNSS data; Center for Orbit Determination in Europe (CODE) and Deutsches GeoForschungsZentrum (GFZ) for providing multi-GNSS final orbit products; EUROLAS Data Center (EDC) for providing SLR tracking data.  
**Acknowledgments:** We thank the Astronomical Institute of the University of Bern (AIUB) for providing Bernese GNSS Software version 5.2 and School of Ocean and Earth Science and Technology at University of Hawai'i System for providing GMT Software.  
**Conflicts of Interest:** The authors declare no conflict of interest.