## 4 Conclusions  
The evaluation of the BDS-3 joint orbit determination using the ground-tracking and ISL observations is presented by using an improved joint POD strategy, which is referred to as the Joint 2 POD strategy. This strategy makes full use of the derived ISL geometry-free observables and clock-free observables, thus the performance of computed satellite orbits and clock offsets is improved. The experimental results show that the accuracy of the Joint 2 orbits is higher than ground-only orbits, ISL-only orbits, and Joint 1 POD orbits. For the two joint POD strategies with ISL data, the 3D accuracy of MEO satellites estimated using the Joint 2 strategy is improved by about 9%. In terms of IGSO and GEO satellites, the addition of ISL geometric-free observables increases the errors in the cross-track or along-track direction and results in a slight decrease of 8.5% in the 3D accuracy of the IGSO satellites. However, the results of the independent SLR check and the performance of satellite clock offset estimated with orbit parameters demonstrate that the orbit determination accuracy of the Joint 2 POD strategy is higher than that of Joint 1 POD regardless of the types of orbits.  
We also prove that the addition of ISL data can reduce the dependence on the ground stations. The results show that the accuracy of the orbit improves as the number of stations increases. When the number of stations reaches 50, the corresponding orbits achieve similar accuracy as the results using 80 stations. However, as the number of stations is larger than 20, each additional 10 stations contributes to a maximum of 7.5%, 3.9%, and 2.8% improvement for MEO, IGSO, and GEO satellites 3D accuracy, respectively.  
Further, the Joint 2 orbits estimated from only 10 regional stations or 10 global stations are analyzed to verify ISL's enhancement to POD. With the use of ISL measurements, the orbit accuracy of all three types of BDS satellites is greatly improved regardless of the distribution of stations, and the orbits using the global stations are slightly better than those of regional stations with a difference of only 0.9 cm in 3D errors. For the orbits estimated using 10 regional stations, the 3D error of MEO satellites is decreased by 69.7%. In the case of IGSO satellites, the 3D error using regional stations is reduced to 15.2 cm. For GEO satellites, the addition of ISL effectively reduces the along-track error of regional station POD orbits by 78%.  
Based on the above analysis, we conclude that the impact of ISL for MEO satellites is rather minor when there are enough ground tracking stations. As for GEO satellites, it is obvious that any improvement of geometry resulting from ISL ranging will have a positive impact on POD. Therefore, an appropriate ground tracking network should be selected for the three types of BDS-3 satellites according to the required accuracy and calculation efficiency.  
**Author Contributions:** Conceptualization, Y.L. and T.G.; methodology, Y.L. and T.G.; software, Y.L.; validation, Y.L. and T.G.; formal analysis, Y.L., T.G. and X.X.; resources, X.W. and F.Z.; writing--original draft preparation, Y.L.; writing--review and editing, T.G. and X.X.; and supervision, Q.Z. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was financially supported by the National Natural Science Foundation of China (Nos. 41674004 and 41974036) and Natural Science Foundation of Hubei Province (No. 2019 CFA051).  
**Acknowledgments:** International GNSS Monitoring and Assessment System (iGMAS) and IGS Multi-GNSS Experiment (MGEX) are greatly acknowledged for providing BeiDou data.  
**Conflicts of Interest:** The authors declare no conflict of interest.