## 4 BDS-3 Orbit Determination and Its Accuracy Analysis  
In this study, the BDS-3 experimental satellite orbit determination was conducted from DOY 154, 2016 to DOY 149, 2017 with iGMAS and MGEX observations. The distribution of tracking stations is presented in Figure 7a, where the types of stations are represented by different colors. A total of 16 iGMAS (nine of them contain BDS-3 observations) and 54 MGEX stations were used at the beginning of the orbit determination. However, numbers of stations containing BDS-3 observations increased to 17 at the end of the experiment given the equipment update and network extension of the iGMAS. Figure 7b shows the distribution of stations, where the BDS-3 tracking stations are presented with magenta. The parameter configurations and related models of orbit determination refer to [15], in which the main parameters are listed in Table 5. Moreover, the model used in used GPS orbit determination, e.g., for solar radiation pressure, were considered here as a reference. This study assessed the BDS-3 experimental satellite orbit based on the following experiments to adequately analyze its accuracy: (1) the 3D RMS of one-day overlapping arc errors based on two \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter Name** & **Configuration** \\ \hline Observation & Undifferenced ionosphere-free code and phase combination B1 and B2 (BDS-2); \\ Elevation cut-off & B1 and B3 (BDS-3); L1 and L2 (GPS) \\ Weighing strategy & Elevation-dependent for the observation below 30\({}^{\circ}\) by 1/2 sin (E) \\ Observation sample interval & 30 s \\ Arcs length & three-days \\ Receiver ISBN and IFB Satellite phase center offset & Estimation: BDS-2 and GPSBDS-3: without consideration \\ Tide models & BeiDou [15]; GPS: gs\_08.atx \\ Relativity & International Earth Rotation and Reference Systems (IERS) 2010 \\ Gravity model & IERS 2010 \\ Satellite phase center variation & ECM 08 12 \(\times\) 12 \\ Satellite yaw models & BeiDou: without consideration; GPS: gs\_08.atx \\ Satellite yaw models & BDS-3 experimental satellites: Nominal model; BDS-2 and GPS: reference to [9] \\ Solar radiation pressure models & ECM \\ Ground antenna PCO and PCV & Not applied \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameter configurations for BDS-3 experimental satellite orbit determination. Figure 7: Stations distribution of BDS-3 experimental satellites orbit determination (black: iGMAS stations without BDS-3 observations; red: iGMAS stations with BDS-3 observations; blue: MGEX stations; and magenta: increased iGMAS stations with BDS-3 observations). (**a**) Stations distribution at the beginning, (**b**) Stations distribution at the end. adjacent orbit determination processes, the scheme of which is displayed in Figure 8; and (2) BDS-3 is equipped with laser retroreflector array, which can be used to check the orbit accuracy by SLR residuals. The overlapping arcs accuracy of two methods for BDS-3 (C31, C32, C33, and C34) orbit determinations are illustrated in Figures 9 and 10 for the period DOY 155, 2016 until DOY 149 in 2017. The variations of the orbit accuracy were fitted by a trend line (dashed) in the results. In addition, the angles between orbital plane and Sun called \(\beta\) were also plotted in corresponding figures. Figure 8: Sketch of the orbit overlapping arcs. Figure 9: The 3D RMS of BDS-3 one-day overlapping arcs errors based on NES (**(a)** C31; **(b)** C32; **(c)** C33; and **(d)** C34). In Figures 9 and 10, the accuracy of BDS-3 was worse at the end of 2016, while it became better in 2017 due to the update of the iGMAS tracking network. Moreover, it was found that the \(\beta\) angles were highly-correlated with the accuracy of the BDS-3, especially for IGSO (C31 and C32). Because the solar radiation pressure model of GPS was taken into the orbit determination, this is not suitable for the new-generation BeiDou satellites and should be refined in later studies.