## 4 BDS-3 Orbit Determination and Its Accuracy Analysis
In addition, the SLR residuals of the two methods are shown in Figure 11. The corresponding mean and RMS values of SLR residuals are presented in Table 6 based on the one-year BDS-3 orbit determination to analyze the orbit accuracy by the NES and SS methods. Since the SLR data points of BDS-3 were insufficient, the trend of the SLR residuals could not be acquired accurately. Figure 11: The SLR residuals of BDS-3 orbits based on NES and SS methods. (**a**) NES method, (**b**) SS method. Figure 10: The 3D RMS of BDS-3 one-day overlapping arc errors based on SS (**(a)** C31; **(b)** C32; **(c)** C33; and **(d)** C34). In the experiments, the 3D RMS of one-day overlapping arcs of BDS-3 (C31, C32, C33, and C34) based on the NES and SS are 31.0, 36.0, 40.3, and 50.1 cm and 34.6, 39.4, 43.4, and 55.5 cm, respectively. The RMS of SLR residuals of the NES and SS are 55.1, 49.6, 61.5, and 70.9 cm and 60.5, 53.6, 65.8, and 73.9 cm, respectively. In addition, the mean values of SLR residuals for the two methods are approximately equal, which could be caused by the different laser retroreflector arrays between BDS-2 and BDS-3. The experimental results showed that the NES is slightly better than the SS in BDS-3 orbit determination. Furthermore, the reason why the NES offers more accurate BDS-3 orbits can be explained as follows: (1) Due to the restriction of the number and distribution of BDS-3 tracking stations, the correlations between the BDS-3 orbit parameters and iGMAS stations related parameters are clearer. However, the SS method ignores the stochastic information of iGMAS parameters in the final parameter determination step; (2) The biases between different types satellites cannot be eliminated in SS, while the NES method takes the spatial seven-parameter transformation to reduce the difference of two normal equation. Moreover, the results in Table 6 show that the mean and RMS values of SLR residuals are larger than the initial assessment results conducted in [13]. It is suggested that a system error caused by the different SLR-related parameters (SLR retroreflectors) is included in the final results. Then, one-month of observations (DOY 228-259, 2016) were selected to conduct the BDS-3 orbit determination with four schemes and further illustrate the reliability and advantages of the proposed methods. This study calculated the orbit accuracy of BDS-3 in four schemes according to orbit overlapping arc errors. The specific schemes are as follows:  
* Based on the BDS-3 orbit determination using iGMAS observations, the traditional Turboedit was used, and the BDS-3 orbits were determined by one step (compared with SS, one step is widely used by GNSS data analysis centers [35], which estimates all parameters in a single-step solution). * Based on iGMAS observations, the improved cycle-slip detection and repair algorithm was used, and the BDS-3 orbits were acquired through one step. * Based on iGMAS and MGEX observations, the improved cycle-slip detection and repair algorithm was used to determine the BDS-3 orbits through SS. * Based on iGMAS and MGEX observations, NES based on the improved cycle-slip detection and repair proposed in this study was used to determine the BDS-3 orbits. The SLR residuals of the BDS-3 orbit cannot be accurately determined because the limited SLR data within the experimental period. Therefore, this study only counted the discrepancy of overlapping arcs as illustrated in Figure 8. Similarly, Table 7 lists the corresponding orbit 3D RMS and improvement (compared with Scheme 1), in which Scheme 4 was considered as the optimal strategy. Moreover, the accuracy of the BDS-3 orbits obtained by Scheme 4 improved by 34.07%, 41.05%, 72.29%, and 74.33% for C31, C32, C33, and C34, respectively.