# Integrated Precise Orbit Determination of Multi-Gnss and Large LEO Constellations
## 6 Conclusions  
This paper is devoted to investigating the integrated precise orbit determination with large LEO constellations. We performed LEO-augmented multi-GNSS precise orbit determination in this study. Based on simulated GNSS observations, several integrated POD schemes are implemented to investigate the performance of integrated POD under different tracking conditions. The potential influence factors of the integrated POD, including LEO satellites number, altitude, and orbit type as well as ground station number and distribution, are analyzed and discussed in detail. The result shows that joint processing ground observations from the global network and the large LEO constellation onboard observations can significantly improve the orbit accuracy for GNSS satellites, especially for BDS GEO satellites. The accuracy improvement percentage with respect to the ground-based POD results can reach over 70% for all the integrated POD schemes with 60-, 66-, 96-LEO satellites. The largest orbit accuracy improvement of over 98% can be recognized for BDS GEO, since the fast motion of LEO satellites brings tremendous variations to the tracking geometry of GEO satellites. By incorporating a large number of LEO satellites, BDS GEO satellites can obtain a centimeter-level orbit. Compared with the 60- and 66-LEO schemes, a slightly better orbit quality is observed in the 96 LEO scheme due to the introduction of more LEO satellites. However, the increase of the involved LEO satellites results in a sharp rise in the computational time of the integrated POD because of more unknown parameters to be solved. We also present the impact of the LEO orbit type on the integrated POD. With the same number of LEO satellites, the sun-synchronous-orbiting constellation presents a stronger enhancement to GNSS orbits than the polar-orbiting constellation. The improvement can be attributed to the more rapid variation of GNSS tracking geometry provided by a sun-synchronous-orbiting constellation. In terms of the LEO altitude, the orbit altitude of LEO satellites is found to have little influence on the enhancement to GNSS orbits except for with BDS GEOsatellites. Benefiting from the faster motion of lower LEO satellites, the 600 km scheme achieves a better orbit accuracy for BDS GEO than the 1000 km and 1400 km schemes. In order to assess the impact of station number and distribution, global and regional networks with different numbers of stations are employed. With the inclusion of large LEO constellation, the integrated POD appears to be more sensitive to the distribution of ground stations, rather than the station number. This is because in the integrated POD, the dependence of GNSS orbit determination on ground stations is largely reduced by the onboard GNSS observations when a large LEO constellation is considered. Based on 8 regional stations, the orbit accuracy of the integrated POD with a sun-synchronous-orbiting constellation is at the centimeter level. However, a subcentimeter accuracy can be recognized for the orbits of GNSS satellites when 8 globally distributed stations are adopted, which is comparable to the result of the integrated POD with a denser global network of 65 stations. Although GNSS orbit estimations can greatly benefit from the incorporation of a full LEO constellation, the integrated POD suffers from long computation time due to the addition of a large number of LEO satellites. To balance the conflict between orbit accuracy and computational efficiency, the integrated POD with a partial LEO constellation is investigated. The orbit accuracy of GNSS satellites improves gradually as the LEO satellites increase. The more LEO satellites, the better the orbit accuracy. However, with an increasing number of LEOs, the accuracy improvement of GNSS orbits becomes smaller. Meanwhile, the increase of LEO satellites results in a steep rise in computational time. Considering both orbit accuracy and computational efficiency, we prefer 40 LEO satellites in a sun-synchronous orbit.