# An Integrity Monitoring Method for Navigation Satellites Based on Multi-Source Observation Links
## 4 Analysis of Optimization Strategies
### SPDOP Analysis of Orbit Determination Unit  
Considering that the calculation of SPDOP needs at least four observation links, more than 30 LEO satellites should be chosen to carry out the analysis of SPDOP. With the choice of six IGS stations mentioned above, the designed scenarios with support of different orbit determination units and test results are shown in Table 3.  
The following can be seen from the results in Table 3:  
Figure 10: Coverage rate of more than four-multiple numbers in scenario 3. (The grids in the blue area can be monitored by no less than four LEO satellites; the grids in the yellow area can be monitored by less than four LEO satellites).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Scenario**} & \multirow{2}{*}{**Monitoring Points**} & \multirow{2}{*}{**SPDOP**} & \multicolumn{2}{c}{**Number of**} & \multicolumn{2}{c}{**Duration of**} \\  & & & & **Observation Links** & & **Observation Links** \\ \cline{3-8}  & & **Min** & **Ave** & **Min** & **Ave** & **Min** & **Ave** \\ \hline
1 & Six ground monitoring stations & 428.02 & 545.00 & 0 & 1.60 & 10,250.36 & 19,800.80 \\ \hline
2 & Six ground monitoring stations + & \multirow{2}{*}{21.09} & \multirow{2}{*}{38.37} & \multirow{2}{*}{4} & \multirow{2}{*}{9.94} & \multirow{2}{*}{91.92} & \multirow{2}{*}{2000.98} \\  & 30 LEO satellite (Walker30/6/1) & & & & & & \\ \hline
3 & Six ground monitoring stations + & \multirow{2}{*}{17.14} & \multirow{2}{*}{24.60} & \multirow{2}{*}{12} & \multirow{2}{*}{18.26} & \multirow{2}{*}{46.75} & \multirow{2}{*}{1849.72} \\  & 60 LEO satellite (Walker60/6/1) & & & & & & \\ \hline
4 & Six ground monitoring stations + & \multirow{2}{*}{13.04} & \multirow{2}{*}{15.71} & \multirow{2}{*}{33} & \multirow{2}{*}{41.29} & \multirow{2}{*}{10.45} & \multirow{2}{*}{1878.55 s} \\  & 144 LEO satellite (Walker144/12/1) & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: Designed scenarios of orbit determination units and test results.  
Figure 9: Coverage rate of four-multiple numbers in scenario 3.  
In scenario 1, the valid number of observation links is only 1.60. The minimum and average values of SPDOP are 428.02 and 545.00, respectively, which is quite large even with the support of six stations synchronously, as shown in Figure 11.
2. As shown in Figures 12-14, the value of SPDOP is respectively 38.37, 24.60, and 15.71, which could be improved by 92.95%, 95.49%, and 97.12% compared with the results in scenario 1. The value of SPDOP in scenario 2 changes greatly when the observation links are less than five. With the increasing number of LEO satellites, the augmentation effect of LEO satellites on SPDOP will gradually weaken. It means that a small number of LEO satellites can effectively improve the observation geometry the target satellite.
3. The minimum duration of observation links is respectively 10,250.36 s, 91.92 s, 46.75 s, and 10.45 s. Although the addition of LEO can effectively increase the number of observation links, it cannot guarantee continuous observation, which brings about the problem of frequent switching for observation links.  
Figure 11: Number of available links and SPDOP values in scenario 1.  
Figure 12: Number of available links and SPDOP values in scenario 2.