## 3 Results
### BDS3 Joint POD Results Based on Ground Monitoring Stations and ISL  
In this section, we explore the use of ISL data and ground station data to validate the POD and time synchronization capabilities of the BDS-3 constellation. BDS-3 employs ISL technology, whereby each satellite carries ISL equipment that allows two-way communication and measurements in Ka-band between satellites, or between satellites and ground stations (anchor stations) equipped with the same equipment [38]. Broadcast ephemerides can be updated using ISL technology, allowing the autonomous navigation of the BDS-3 constellation to be completely independent of ground-monitoring stations. Ka-band pseudorange measurements between the BDS-3 satellites, and between the satellites and the anchor station, provide the independent orbit and time synchronization capabilities of the ground monitoring stations. We divided the complete POD simulation process into four parts. Firstly, we converted the Kepler orbit parameter to the position and velocity in J2000 inertial system (namely the initial conditions) at the beginning epoch. Secondly, the initial conditions were extended to three days by orbit integration, in this process, we set the force model to the parameters in Table 2. Next, BDS's ground monitoring stations and ISL observation data were simulated based on the satellite orbit inherited by step two. Finally, in the POD process, the estimated parameters fitted by POD included BDS-3 satellites position and velocity, solar radiation pressure parameters, satellite clock bias, zenith tropospheric delay (for anchor stations), and receiver clock bias. Firstly, we performed a joint POD experiment using the seven sta (seven stations) and seven sta and ISL schemes with an orbital arc of 3 days. The seven sta scheme uses only ground stations and no ISL data, while the seven sta and ISL scheme uses ground stations and BDS-3 ISL data for a joint POD to analyze BDS-3 POD and time synchronization. The observation data of the BDS-3 ground station are simulated, the elevation angle threshold is set to 5 degrees. The pseudorange measurement is added with a random noise of 1.000 m and a systematic error of 0.030 m, while the carrier-phase measurement is added with random noise of 0.002 m and a systematic error of 0.030 m [29; 39]. We simulate BDS-3 ISL observations between GEO and MEO, IGSO and MEO, MEO and MEO, and IGSO and IGSO. The satellite geometry visibility condition is judged as the ISL height above 1000 km \begin{table}
\begin{tabular}{c c} \hline \hline
**Project** & **Parameters and Models** \\ \hline Earth Gravity Field & EIGEN6C (12 \(\times\) 12) for BDS-3 \\ N-body Perturbation & JPL DE405 \\ SRP & ECOM 5 model for BDS-3 \\ BDS-3 Phase Center Offset (PCO) and & igs14.atx \\ Station PCO and PCV & igs14.atx \\ Solid tide and Pole tide & IERS 2010 \\ Relativity & IERS 2010 \\ Ocean tide & FES 2004 \\ Earth rotation parameters & One set per arc \\ Orbit parameters to be estimated & Satellite initial position, velocity and solar pressure parameters \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detailed processing strategy for the ISL POD. on the Earth's surface. We added 0.100 m random noise and 0.100 m systematic error to the ISL observation values [11]. We simulated seven ground-monitoring stations, located in Harbin, Beijing, Xi'an, Urumqi, Lhasa, Shanghai, and Sanya, as shown in Figure 1.  
The BDS-3 satellites adopt the 3 GEO + 3 IGSO + 24 MEO constellation configuration. The three GEO satellites are positioned over the equator at 80\({}^{\circ}\)E, 110.5\({}^{\circ}\)E, and 140\({}^{\circ}\)E, respectively. The three IGSO satellites have a figure-of-eight ground-track, with their intersection point over the equator at 120\({}^{\circ}\)E, and their orbital planes differ by 120\({}^{\circ}\)E from each other in space.