# An Integrity Monitoring Method for Navigation Satellites Based on Multi-Source Observation Links
## 4 Analysis of Optimization Strategies
### Performance Analysis of Integrity Parameter  
UDRE can effectively indicate service area and service accuracy, representing the service performance of the system. As to the service area, the calculation of integrity parameters related to real-time service performance is restricted by the layout of ground monitoring stations. With the support of LEO satellites, variation in the service area can be evaluated by the effective duration of integrity parameters. Regarding service accuracy, UDRE is a pseudo-distance residual of the spatial signal, which can be determined using the original observed data and theoretical distances between the monitoring receiver and the target satellite. Then, service accuracy could be estimated by the product of UDRE and the position dilution of precision (PDOP). This study constructs scenarios with the support of different LEO constellations to verify the effect on service area and accuracy, as shown in Table 4. \begin{table}
\begin{tabular}{c c} \hline \hline
**Scenario** & **Monitoring Points** \\ \hline
1 & Six ground monitoring stations \\ \hline
2 & Six ground monitoring stations + 6 LEO satellite (Walker 60/6/1, \\  & each orbital plane chooses one satellite) \\ \hline
3 & Six ground monitoring stations + 12 LEO satellite (Walker 60/6/1, \\  & each orbital plane chooses two satellites) \\ \hline
4 & Six ground monitoring stations + 60 LEO satellite (Walker 60/6/1) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Designed scenarios of UDRE duration. Figure 14: Number of available links and SPDOP values in scenario 4. Figure 13: Number of available links and SPDOP values in scenario 3. The following can be seen:  
1. The effective duration of UDRE is about 8.00 h, which is only 33.31% of the length of a single day because of the limitation of ground monitoring stations. The average simulation value of UDRE is 0.427 m, which is basically the same as the real performance of BDS-3. 2. With the support of 6, 12, or 60 LEO satellites, the effective duration of UDRE is respectively 16.11 h, 20.93 h, and 24 h, as shown in Table 5, meaning that the single-day effective rate is 67.11%, 87.19%, and 100%, respectively. As the number of LEO satellites increases, the area where UDRE parameters can be solved in real time can be gradually expanded to the global area, verifying that LEO satellites could be used as satellite-based monitoring stations to obtain more observation data. 3. The average value of UDRE is respectively 0.397 m, 0.388 m, and 0.351 m in scenarios 2 to 4, which have 7%, 9%, and 18% improvement compared with the results in scenario 1, verifying that the satellite-based observation data of LEO satellites could support the solution of UDRE. As shown in Table 6 and Figure 15, PDOP could be improved by the addition of satellite-based observation links. There is no significant difference in PDOP values among different regions, meaning that the deployment of the constellation is relatively uniform. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**PRN**} & \multicolumn{3}{c}{**Scenario 1**} & \multicolumn{3}{c}{**Scenario 2**} & \multicolumn{3}{c}{**Scenario 3**} & \multicolumn{3}{c}{**Scenario 4**} \\ \cline{2-13}  & \begin{tabular}{c} **Duration** \\ **(h)** \\ \end{tabular}  & \begin{tabular}{c} **Rate** \\ **Rate** \\ \end{tabular}  & \begin{tabular}{c} **UDRE** \\ **(m)** \\ \end{tabular}  & \begin{tabular}{c} **Duration** \\ **Rate** \\ \end{tabular}  & \begin{tabular}{c} **Duration** \\ **(m)** \\ \end{tabular}  & \begin{tabular}{c} **Rate** \\ **Rate** \\ \end{tabular}  & \begin{tabular}{c} **UDRE** \\ **(m)** \\ \end{tabular}  & \begin{tabular}{c} **Duration** \\ **Rate** \\ \end{tabular}  & \begin{tabular}{c} **Duration** \\ **(m)** \\ \end{tabular}  & \ \begin{tabular}{c} **Rate** \\ **Rate** \\ \end{tabular}  & \begin{tabular}{c} **UDRE** \\ **(m)** \\ \end{tabular}  \\ \hline C19 & 8.09 & 33.71\% & 0.476 & 13.44 & 56.00\% & 0.435 & 21.54 & 89.75\% & 0.347 & 24 & 100\% & 0.355 \\ \hline C20 & 7.47 & 31.13\% & 0.474 & 14.76 &  
5.