# An Integrity Monitoring Method for Navigation Satellites Based on Multi-Source Observation Links
## 5 Conclusions  
In the face of deployment limitations of monitoring stations and constraints on the communication capacity of inter-satellite links, this study proposes an integrity monitoring system with the support of multi-source observation links, which consist of satellite-to-ground, inter-satellite, and satellite-based observation links. Moreover, optimizing strategies are proposed to improve monitoring efficiency for navigation satellites. The main conclusions from this study are summarized as follows:  
1. According to the observation conditions of the target satellite, the designed integrity monitoring system can conduct multi-dimensional monitoring in signal and message fields. In the signal field, the state of the satellite clock, hopping indicators of frequency and phase in signal, and signal delay should be monitored. In the message field, basic navigation messages, satellite-based augmentation messages, and autonomous messages should be considered comprehensively to evaluate the state of the target satellite. Indicator parameters, residuals of observation data, orbit determination errors, and clock errors can all indicate the state of the navigation satellite. 2. LEO satellites can effectively make up for the shortage of ground monitoring stations and realize real-time return of navigation messages for navigation satellites. With the support of 30 LEO satellites, the coverage rate of four-multiple numbers can \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Scenario**} & \multicolumn{2}{c}{**BIFS**} & \multicolumn{2}{c}{**CHAN**} & \multicolumn{2}{c}{**HKSL**} & \multicolumn{2}{c}{**LHAZ**} & \multicolumn{2}{c}{**URUM**} & \multicolumn{2}{c}{**WUH2**} & \multicolumn{2}{c}{**Average**} \\ \cline{2-13}  & \multicolumn{2}{c}{\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **PDOP** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **PDOP** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **PDOP** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{\
\begin{tabular}{c} **PDOP** \\ **(m)** \\ \end{tabular} } & \multicolumn{2}{c}{
\begin{tabular}{c} **Service** \\ **Accuracy** \\ **(m)** \\ \end{tabular} } \\ \hline
1 & 2.32 & 0.99 & 2.36 & 1.01 & 2.36 & 1.01 & 2.39 & 1.02 & 2.41 & 1.03 & 2.40 & 1.02 & 2.37 & 1.02 \\ \hline
2 & 2.26 & 0.90 & 2.29 & 0.91 & 2.31 & 0.92 & 2.32 & 0.92 & 2.34 & 0.93 & 2.33 & 0.93 & 2.31 & 0.93 \\ \hline
3 & 2.19 & 0.85 & 2.22 & 0.86 & 2.27 & 0.88 & 2.26 & 0.88 & 2.20 & 0.85 & 2.28 & 0.88 & 2.24 & 0.88 \\ \hline
4 & 1.76 & 0.62 & 1.77 & 0.62 & 1.96 & 0.69 & 1.86 & 0.65 & 1.77 & 0.62 & 1.86 & 0.65 & 1.83 & 0.65 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Service accuracy and PDOP. Figure 15: Number of available links and PDOP values of BIFS in scenario 4.  
reach 100%. This means that it does not matter where the target satellite is; it can be monitored by no less than four LEO satellites. A small amount of LEO satellites could realize full-arc monitoring of navigation satellites. For a large LEO constellation, it is necessary to construct a minimum observation unit for the effective return of navigation messages and to avoid data redundancy. 3. Realization of the orbit determination needs not only effective observation links but also reasonable observation geometry conditions. As the number of LEO satellites increases, the augmentation effect of LEO satellites to SPDOP would gradually weaken.