# Integrated Precise Orbit Determination of Multi-Gnss and Large LEO Constellations
## 1 Introduction
The results show that the integrated solution achieves better accuracy for both orbits and Earth parameters. [PERSON] et al. [17] presented an initial comparison between the performances of integrated POD with different ground networks. [PERSON] et al. [18] reported an increase of about 0.7% in the ambiguities fixed rate and better GPS orbits when Jason-2 data were introduced into the orbit determination. [PERSON] and [PERSON] [19] jointly processed a network of 53 global ground stations and four LEO satellites to assess the improvement on geocenter coordinates. [PERSON] et al. [20] used a few LEO satellites to discuss the impact of LEO with different orbital altitudes and inclinations, as well as ground station networks, on the GPS orbit determination. In terms of the GPS and BDS dual-system integrated POD, [PERSON] et al. [21] performed an integrated POD of FengYun-3C (FY-3C), GPS, and BDS satellites using FY-3C onboard GPS and BDS observations as well as global and regional ground measurements. The results evidence the contribution of FY-3C onboard observations for BDS orbits, especially for BDS GEO. Based on the Iridium satellite constellation and global network, [PERSON] et al. [10] tested the performance of integrated POD with the GPS, BDS, and LEO constellationsusing simulated data. Their initial results demonstrated the significant accuracy improvements of GPS and BDS orbits as a result of the LEO constellation. The aforementioned works evidence the contribution of onboard GNSS observations to the GNSS POD, and shed light on the performance of the integrated POD. However, these studies mainly focus on the integrated POD with a few LEO satellites, since no onboard GNSS observations from large LEO constellations are available at present. Additionally, the impact of different LEO configurations, such as LEO number, orbital altitude, and orbit type, on integrated POD has not yet been well studied. Motivated by the aforementioned studies, we propose a LEO constellation-augmented multi-GNSS precise orbit determination method and assess the LEO-augmented POD performance with different configurations of ground stations and LEO constellations. Within this paper, we mainly focus on the geometric factors that limit the accuracy of GNSS orbit determination. Six LEO constellations with different orbital altitudes, satellite numbers, and orbit types are designed to evaluate the influence of LEO constellations on the GNSS POD. Several station networks with different station number and distributions are employed to discuss the dependency of the integrated POD on ground stations. The paper is structured as follows. Section 2 starts by introducing GNSS and LEO constellations we used, and gives a detailed description of the sophisticated simulation processing methods used for GNSS observations. Then, the LEO constellation-augmented four-system precise orbit determination algorithm and strategy are introduced in Section 3. Section 4 analyzes LEO-augmented GNSS POD results under different tracking conditions in detail. A discussion of our result is given in Section 5. Finally, conclusions are provided in Section 6.