# Integrated Precise Orbit Determination of Multi-Gnss and Large LEO Constellations
## 4 Result Analysis and Discussion
### Integrated POD with Full LEO Constellation
#### 4.1.4 Integrated POD with Different Ground Network  
In order to evaluate the impact of station quantity and distribution on the LEO-augmented POD, fewer global stations are selected to perform the integrated POD with different LEO constellations. We firstly adopted about 22 global stations from iGMAS (see Figure 2). Typically, the three LEO schemes, i.e., 60 polar-orbiting LEO, 60 sun-synchronous-orbiting LEO, and 96 polar-orbiting LEO constellations at the altitude of 1000 km, are chosen. Figure 9 illustrates the integrated POD results for GPS, GLONASS, Galileo, and BDS using iGMAS ground observations. For comparison, we also plot the GNSS orbit results determined from the 65 MGEX stations and 60 sun-synchronous-orbiting LEO satellites. The corresponding statistical values are provided in Table 6. With regard to the numbers provided in Table 4, a clear degradation in the orbit accuracy can be recognized for all GNSS satellites compared with the POD result using MGEX network when only ground iGMAS observations are processed. This is reasonable, because fewer ground stations are employed to recover GNSS satellites orbit. Similar to the MGEX solution, the significant orbit accuracy improvement for all satellites can be recognized after the incorporation of LEO onboard observations into the GNSS orbit determination. Certainly, BDS GEO satellites exhibit the largest orbit accuracy improvement. The different LEO constellations present a similar performance to that in the MGEX solution. As shown in Table 6, the sun-synchronous-orbiting constellation yields slightly better performance in LEO-augmented POD compared to the two polar-orbiting constellations. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Altitude (km)** & **600** & **1000** & **1400** \\ \hline
**GPS** & 0.8 & 0.7 & 0.7 \\ \hline
**GLONASS** & 0.7 & 0.7 & 0.7 \\ \hline
**BDS GEO** & 3.6 & 4.7 & 4.3 \\ \hline
**BDS IGSO** & 1.8 & 1.8 & 1.7 \\ \hline
**BDS MEO** & 1.0 & 0.9 & 0.9 \\ \hline
**Galileo** & 0.8 & 0.8 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Average 3D RMS of orbit differences for the integrated POD with different altitude LEO constellations (Unit: cm). Meanwhile, it should be noted that although a sparse iGMAS network is employed to perform the integrated POD, the orbit accuracy of all three iGMAS integrated POD schemes is comparable to that of the corresponding MGEX solutions. The performance of the iGMAS and MGEX solutions mainly differs in their estimated BDS GEO orbits. Compared with the MGEX solution, the integrated POD with the iGMAS network provides a relatively worse orbit for the BDS GEO. The result demonstrates that the dependence of GNSS orbit determination on the ground stations can be largely reduced when introducing a large number of LEO satellites into the POD processing. Serving as moving stations, LEO satellites can not only provide a large amount of onboard observations, but also bring evident improvements to the tracking geometry for GNSS satellites. The above results indicate that the onboard observations play an important role in reducing the contribution of ground observations to GNSS orbit determination when introducing a large number of LEO satellites. To further investigate the dependency of the integrated POD on the ground stations, we design three integrated POD schemes with only 8 or 4 ground stations involved. The distribution of the small number of stations we selected is shown in Figure 10. The sun-synchronous-orbiting constellation with 60 LEO satellites is adopted due to its best performance in the previous study. It should be noted that the earth rotation parameters are fixed when we perform the integrated POD with a few stations.