# Centimeter-Level Precise Orbit Determination for the Luojia-1A Satellite Using BeiDou Observations
## 5 Luojia-1A Satellite POD Results and Discussion
### Precise Orbit Determination with BeiDou Observations  
The precise orbit determination of LEO requires precise BeiDou orbit, satellite clocks, and earth rotation parameters (ERP) products. One of the challenges for Beidou-based LEO POD is that the BeiDou orbit is generally not as good as the GPS orbit for many reasons. Presently, several analysis centers are providing multi-GNSS precise products with BeiDou constellation support. Four types of BeiDou precise products from different multi-GNSS experiment (MGEX) analysis centers are listed in Table 2. However, a few issues arise in applying these products at the current stage, such as that the Technische Universitat Munchen (TUM) products currently does not involve the ERP products. The precise products provided by CODE do not involve the BDS-2 GEO satellites. The German Research Centre for Geosciences (GFZ) and Wuhan University provide the full BDS-2 satellites orbit and clock, but different phase center variation (PCV) models are applied for different ACs, so the products are not directly compatible. In this study, the Wuhan University Multi-GNSS (WUM) products are adopted to determine the precise orbit of the Luojia-1A satellite. The BDS precise orbit and clocks products are critical factors for the Luojia-1A satellite POD. As reported, the precise orbit products of BDS GEO satellites are not as good as the IGSO and MEO satellites due to poor geometry relationship between the ground stations and the GEO satellites. Generally, the BeiDou GEO precise orbit only achieves a few decimeters accuracy, whereas the IGSO and MEO satellites achieve centimeter-level accuracy. Consequently, adopting the GEO satellites might decrease the LEO POD precision. WUM product employs a three-day arc for BDS orbit determination, and only the middle day is adopted as the final product. So the discontinuity is inevitably present at the day boundary (DBD) in orbit products [27], which can be used to roughly evaluate the orbit precision [28]. To analyze the orbit precise of BeiDou satellites, the average BDDs of 2018 DOY 153 to DOY 163 are calculated. Since the last epoch of WUM orbit products is 23h 45m, the orbit is extrapolated to the 24 h at midnight, then the difference is calculated at the day boundary with the next day orbit product in radial, along-track, and cross-track directions. After that, the average 3D BDDs of BDS satellites orbit products are shown in Figure 8. According to the figure, the GEO satellites present more significant BDDs than the IGSO and MEO satellites. The average BDDs of GEO satellites vary between 58 cm and 130 cm. As for IGSO satellites, the DBDs are less than 50 cm. The DBDs of MEO satellites are the smallest, which is generally less than 20 cm. The statistic result of different types of the BeiDou satellites are listed in Table 3, the average DBD of GEO satellites is 82.28 cm, which is more than three times of IGSO satellites. The precision of MEO satellites is best, which is 17 cm. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Institution** & **Indicate** & **Constellations** & **Orbit** & **Clocks** & **ERP** \\ \hline CODE & COM & GPS + GLO + GAL + BDS + QZS & 5 min & 30 s/5 min & 12 h \\ GFZ & GBM & GPS + GLO + GAL + BDS + QZS & 15 min & 30 s/5 min & 24 h \\ WHU & WUM & GPS + GLO + GAL + BDS + QZS & 15 min & 30 s/5 min & 24 h \\ TUM & TUM & GAL + BDS + QZS & 5 min & 5 min & / \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The information about BDS orbit and clock products. Figure 7: The concept of overlap comparison for the Luojia-1A precise orbit. According to the analysis of BeiDou satellites DBDs, the precision of GEO satellites orbit is not as good as that of IGSO and MEO satellites. Therefore, adopting the GEO satellite observations as same as IGSO and MEO satellite observations might decrease the LEO POD precision. [PERSON] et al. (2020) improve the BDS-based precise orbit determination result of FY-3C by excluding the GEO satellite observations.