# Orbit Determination Method for BDS-3 MEO Satellites Based on Multi-Source Observation Links
## 4 Conclusions  
With the construction of multi-source observation links, an orbit determination method is proposed to simultaneously determine the orbit of navigation satellites and LEO satellites. Through the designed tests with different observation links, some conclusions could be gained from this study, as follows:  
1. With the support of 6, 12 and 60 LEO satellites, the accuracy of orbit determination for BDS-3 MEO satellites under conditions of simulation tests could be, respectively, improved by 65.5%, 73.7% and 79.4%. Nevertheless, the scope for further improvements in accuracy is limited when the observation geometry meets the requirements of basic orbit determination. 2. With the addition of five LEO satellites, the accuracy of orbit determination based on real data could be, respectively, improved by 72%, 78%, 71% and 76% in the R, T, N and 3D directions. The augmentation effects of satellite-based observation data are basically the same in different directions. 3. The accuracy of orbit determination could be augmented significantly with the addition of LEO satellites. When the navigation satellite leaves the monitoring area of the ground monitoring stations, the reduction in the orbit accuracy can also be slowed down with the support of the satellite-based observation links. 4. With the support of either inter-satellite or LEO satellites, the accuracy of orbit determination was at the centimeter level. Inter-satellite links and satellite-based links could be used as each other's backups for navigation satellites. 5. The augmentation results of the multi-source observation links were not better than the results obtained with the support of the satellite-to-ground and inter-satellite links or the satellite-to-ground and satellite-based links. This might be caused by some unconsidered systematic errors. For the observation equations, the weights of the satellite-to-ground, inter-satellite and satellite-based observation links were set to be the same, something which should be further improved. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**PRN**} & \multicolumn{4}{c}{**Scenario 1**} & \multicolumn{4}{c}{**Scenario 2**} \\ \cline{2-9}  & **R** & **T** & **N** & **3D** & **R** & **T** & **N** & **3D** \\ \hline C19 & 0.217 & 0.929 & 0.355 & 1.018 & 0.035 & 0.111 & 0.142 & 0.184 \\ \hline C20 & 0.184 & 0.596 & 0.351 & 0.716 & 0.040 & 0.097 & 0.115 & 0.156 \\ \hline C21 & 0.101 & 0.358 & 0.525 & 0.643 & 0.059 & 0.097 & 0.063 & 0.129 \\ \hline C22 & 0.122 & 0.429 & 0.180 & 0.481 & 0.069 & 0.111 & 0.082 & 0.154 \\ \hline C23 & 0.123 & 0.590 & 0.372 & 0.709 & 0.043 & 0.142 & 0.213 & 0.259 \\ \hline C24 & 0.271 & 1.345 & 0.598 & 1.497 & 0.137 & 0.315 & 0.135 & 0.369 \\ \hline C25 & 0.530 & 3.031 & 1.178 & 3.295 & 0.050 & 0.127 & 0.110 & 0.175 \\ \hline C26 & 0.404 & 1.213 & 0.535 & 1.386 & 0.076 & 0.176 & 0.166 & 0.253 \\ \hline C27 & 0.594 & 2.354 & 0.875 & 2.581 & 0.036 & 0.120 & 0.094 & 0.157 \\ \hline C28 & 0.196 & 0.507 & 0.358 & 0.651 & 0.050 & 0.157 & 0.083 & 0.184 \\ \hline C29 & 0.159 & 0.458 & 0.352 & 0.599 & 0.081 & 0.278 & 0.110 & 0.310 \\ \hline C30 & 0.200 & 1.207 & 1.074 & 1.628 & 0.028 & 0.185 & 0.116 & 0.220 \\ \hline C32 & 0.620 & 1.842 & 1.081 & 2.224 & 0.033 & 0.070 & 0.095 & 0.123 \\ \hline C33 & 0.303 & 1.818 & 0.726 & 1.980 & 0.064 & 0.136 & 0.134 & 0.201 \\ \hline C34 & 0.174 & 0.615 & 0.350 & 0.729 & 0.044 & 0.203 & 0.164 & 0.264 \\ \hline C35 & 0.395 & 0.805 & 0.806 & 1.206 & 0.149 & 0.445 & 0.480 & 0.671 \\ \hline Average & 0.287 & 1.131 & 0.607 & 1.334 & 0.062 & 0.173 & 0.144 & 0.238 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Accuracy of orbit determination with real data (unit: m). Author Contributions:Conceptualization, formal analysis, investigation, data curation and writing--original draft preparation, J.X. ; software and validation, K.L. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (grant nos. 12103077).