## 4 Precise Orbit Determination (POD)  
### POD Strategy  
In order to analyze the POD performance for BeiDou-3e satellites, only stations with the ability to capture BeiDou-3e and BeiDou-2 signals were used. Position And Navigation Data Analyst software (PANDA), which was developed by the GNSS Research Center at Wuhan University [24], was used for POD in this study. In the combined POD of BeiDou-3e and BeiDou-2 satellites, the prior position and velocity of the satellites were imported from the broadcast ephemeris provided by iGMAS stations. The orbit parameters included the initial position and velocity, five parameters of solar radiation pressure (SRP) in the Empirical CODE Orbit Model (ECOM) [25], with a constant acceleration bias in along-track direction [26]. The entire data processing workflow for the combined POD of BeiDou-2 and BeiDou-3e satellites can be separated into two steps. Firstly, GPS precise orbit and 30 s interval clock products provided by IGS were used to estimate the coordinates, zenith troposphere delays (ZTDs), and receiver clock offsets of the stations. Secondly, the parameters estimated above were fixed and the BeiDou orbit parameters, together with the clock offsets of the satellites, were estimated in batch mode. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**BeiDou-2 IGSO**} & \multicolumn{2}{c}{**BeiDou-2 MEO**} & \multicolumn{2}{c}{**BeiDou-3e IGSO**} & \multicolumn{2}{c}{**BeiDou-3e MEO**} & \multicolumn{2}{c}{**GPS**} \\ \cline{2-10}  & **B1** & **B3** & **B1** & **B3** & **B1** & **B3** & **B1** & **B3** & **L1** & **L2** \\ \hline
**Mean** & 1.8 & 1.5 & 1.2 & 1.1 & 0.2 & 0.1 & \(-\)0.3 & 0.3 & \(-\)0.1 & \(-\)0.1 \\
**RMS** & 40.7 & 23.8 & 52.5 & 30.3 & 33.1 & 21.9 & 29.9 & 26.0 & 34.1 & 34.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean and root mean square (RMS) values (cm) of MP combinations for BeiDou-2, BeiDou-3e, and GPS satellites. Figure 3: MP combinations for BeiDou-2 and BeiDou-3e satellites as a function of azimuth and elevation. The first and second columns represent BeiDou-2 inclined geosynchronous orbits (IGSOs) and medium Earth orbits (MEOs), respectively, the third column represents BeiDou-3e satellites, and the last column represents GPS satellites. The MP for the B1I and L1 signals is shown at the top and the MP for the B3I and L2 signals is shown at the bottom. Satellite attitude is essential for solar radiation calculations as well as for phase center offset (PCO) corrections. In general, in order to transmit navigational signals to the ground and receive energy from the Sun, satellites must be ensured that (1) their transmitting antenna points to the Earth and (2) their solar panels are perpendicular to incoming sunlight [27]. Two control modes, namely, yaw-steering (YS) and orbit-normal (ON), are used in BeiDou satellites system to simplify attitude control during eclipse periods [28; 29; 30]. Generally, in the satellite-fixed reference frame, center of satellite mass is set as the coordinate origin, the z axis is along the antenna transmitting direction and points to the geocenter, the y axis is along the solar panels and is perpendicular to either the satellite-Sun direction (YS mode) or the satellite orbital plane (ON mode), and the x axis completes the right-handed coordinate system. As shown in Figure 4, in YS mode, a satellite always conforms to two laws of satellites attitude control (mentioned above) and the satellite bus constantly yaws, whereas in ON mode the GNSS antenna points to the Earth but the satellite bus does not yaw and the solar panels are perpendicular to the orbital plane. Of the BeiDou-2 satellites, GEOs are constantly operated in ON mode, while IGSOs and MEOs are operated in both YS and ON modes. YS mode is used when the absolute value of the solar elevation angle (beta angle) above the orbital plane is greater than 4\({}^{\circ}\); otherwise, ON mode is utilized [29]. According to an analysis by [PERSON] et al.