## 4 Discussion  
In this study, the BDS/GPS observation data collected by IGS/MGEX and the CNCORS are used to determine the precision orbit of the BeiDou satellites. The results show that a high-precision BeiDou satellite orbit can be obtained when the stations are well distributed. In the four strategies of this paper, the BDS/GPS joint orbit determination based on observation data from 124 stations yields the highest accuracy. The 3D overlapping arcs precision of the GEO, IGSO, and MEO satellites is 17.2, 4.9, and 5.6 cm, respectively, and the radial precisions are all better than 2.3 cm. Our results are quite encouraging when compared to those reported by previous studies. In the latest literature, publication [3] achieved a radial overlap precision at the 0.1 m level for both BeiDou GEO and IGSO satellites. The 3D overlap precision of the GEO, IGSO, and MEO satellites in literature [8] reached 155, 33, and 28 cm, respectively, and the radial overlapping arcs precision was basically better than 10 cm. The most precise orbit of IGSO and MEO satellites obtained in literature [12] has an accuracy of 13.2 and 11.9 cm, respectively. Consistent with previous studies [2; 6; 10; 11; 12], GEO satellites have the worst precision among the three types of satellites. The main reason for this phenomenon is the much smaller changes of the observation geometry of the GEO satellites compared to the IGSO and MEO satellites. As a result, strong correlations occur among the orbital elements, solar radiation pressure parameters, ambiguities, and ISBs, which resulted in large errors in the along-track direction [6]. However, the precision of the GEO satellites in the cross-track direction is better than that of the IGSO and MEO satellites. Further studies are needed to explain this result. We analyzed the beta angles (the elevation angles of the Sun above the satellite orbital planes) of BeiDou satellites in the considered period. The results show that the beta angels of all the GEO satellites are 17-19 degrees. For all the MEO satellites, the absolute value of beta angles is greater than 28 degrees. For IGSO satellites, the absolute values of the beta angle of C06, C08, C09, C15 are greater than 22 degrees. The beta angle of IGSO C07 ranges from \(-1.9\) to 10.4 degrees, and that of IGSO C10 ranges from \(-1.5\) to 10.9 degrees. It is noted that the absolute value of the beta angle of C07 is less than 4 degrees most of the time, so that the orbit precision is relatively low [37]. Take the results of the strategies 1 and 2 as an example, the 3 DRMS values of C07 are 18.4% and 13% higher than the average values of IGSO, respectively. The lack of ground stations is one of the reasons for the low precision of the BeiDou orbit in previous studies, especially in the current service regions such as in China. Literature [9; 12] show that the addition of ground stations in proper areas significantly improves the orbit determination accuracy  
Figure 6: Comparison with the GBM orbit from strategies 1 and 3 (unit: cm). of BeiDou satellites. This paper found that the addition of 24 stations in China greatly improved the accuracy of the BeiDou orbit determination, especially for the IGSO satellites. For the BeiDou/GPS joint orbit determination method and the single-system BeiDou orbit determination method, the precision of IGSO satellites is improved by 36.4% and 29.6%, respectively. BDS single-system orbit determination can achieve an acceptable accuracy, and the 3D overlapping arc precision of the GEO, IGSO, and MEO satellites is 19.0, 7.6, and 11.0 cm, respectively. However, the accuracy of BDS/GPS joint orbit determination is higher than that of the BeiDou single-system orbit determination, indicating that GPS can helps to strengthen the parameter estimation, such as the tropospheric delay and receiver clock error. The difference between BDS/GPS joint orbit determination (strategy 1) and BeiDou single-system orbit determination (strategy 3) was further analyzed.