# Orbit Determination Method for BDS-3 MEO Satellites Based on Multi-Source Observation Links
## 3 Test Analysis
### Test of Orbit Determination for BDS-3 Satellites and LEO Satellites
#### 3.2.2 Test Scenarios for Orbit Determination with Simulated Data  
By constructing scenarios with different observation data, the orbit determination accuracy could be determined in different scenarios. The designed test scenarios are listed in Table 3. \begin{table}
\begin{tabular}{c c c} \hline
**Type** & **Model** & **Strategies** \\ \hline \hline \multicolumn{2}{c}{Arc length} & 24 h \\ \hline \hline \multicolumn{2}{c}{Data sampling} & 30 s \\ \hline \hline \multicolumn{2}{c}{Cutoff elevation} & 5\({}^{\circ}\) \\ \hline \hline \multicolumn{2}{c}{Relativity} & IERS Conventions 2010 \\ \hline \hline \multicolumn{2}{c}{Earth tide} & IERS Conventions 2010 \\ \hline \hline \multicolumn{2}{c}{Troposphere} & Saastamoninen \\ \hline \hline \multicolumn{2}{c}{Antenna phase center bias} & BDS: igs14.atx \\ \hline \hline \multirow{4}{*}{Observation equation} & \multirow{4}{*}{Atmospheric drag} & LEO: the Box-wing model is used to calculate the atmospheric drag perturbation acceleration, with the DTM94 model employed to compute atmospheric density.BDS: not considered \\ \hline \hline \multicolumn{2}{c}{Solar radiation pressure} & LEO: Box-wing modelBDS: 5-parameter \\ \multicolumn{2}{c}{ECOM model} \\ \hline \hline \multicolumn{2}{c}{Weight} & Ground, LEO and ISL are equally weighted \\ \hline \hline \multirow{4}{*}{Kinematics equations} & Simulated pseudo ranging and phase data obtained by the gts, CHAN, HKSL, LHAZ, URUM and WUHZ; simulated pseudo ranging and phase between the satellites; real data \\ \hline \hline \multicolumn{2}{c}{Station coordinates} & Fixed to IGS weekly solutions \\ \hline \hline \multirow{4}{*}{Kinematics equations} & Earth gravity field & EGM2008(BDS: 12 \(\times\) 12; LEO: 140 \(\times\) 140) \\ \cline{2-2} \cline{3-3}  & N-body perturbation & DE430 \\ \cline{2-3} \cline{2-3}  & Empirical force & Segmented estimation \\ \cline{2-3} \cline{2-3}  & Ocean tide & FES 2004 (50 \(\times\) 50) \\ \hline \hline \multirow{4}{*}{Estimated parameters} & Station coordinates & Tightly constrained \\ \cline{2-3}  & BDS orbit & 6 orbital elements5 ECOM parameters: \\ \cline{2-3}  & LEO orbit & 6 orbital elements based on the single-point positioning (SPP); piecewise empirical parameters: 3 h drag scale coefficient: 3.0 hsolar radiation pressure coefficient: 3 h \\ \hline \hline \multirow{4}{*}{Estimated parameters} & \multirow{4}{*}{Tropospheric delay} & For each ground station; piecewise constant zenith delays for 1 h intervals; piecewise constant horizontal gradients for 4 h intervals \\ \cline{1-1} \cline{2-3}  & Phase ambiguities & Float \\ \cline{1-1} \cline{2-3}  & Clock offsets & Satellites and receivers; epoch wise; pre-eliminated \\ \hline \hline \multicolumn{3}{c}{Farch rotation parameters} & Fixed as known values with a priori value: IERS Bulletin A product; Rotation pole coordinates and UT1 for 24 h intervals; piece-wise linear modeling \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Strategies for orbit determination. The evaluation results of orbit determination over one day showed that:  
1. As shown in Figure 11, the accuracies of orbit determination in the radial (R), tangential (T), normal (N) and three-dimensional (3D) directions were, respectively, 0.050 m, 0.128 m, 0.133 m and 0.191 m in Scenario 1. Accuracy would gradually increase when the satellite left the ground monitoring area and would become more than 0.4 m. Compared with Scenario 1, the accuracy could be improved as satellite-based observation links were added. Accuracy in the 3D direction was 0.067 m in Scenario 2. When the satellite left the ground monitoring area, the accuracy was still within 0.1 m.
2. As shown in Figure 12, the three-dimensional accuracy of orbit determination in Scenario  
The results for Scenario 1, Scenario 2 and Scenario 5 were, respectively, 0.194 m, 0.067 m and 0.077 m, verifying that LEO satellites and inter-satellite links are both important augmentation means of improving the visual arc of MEO satellites, and that their augmentation effects are basically the same.