## 4 Experiment and Discussion
### Worldwide Availability Coverage Test
#### 4.1.3 Results under Depleted Scenario  
As mentioned in the section of introduction, the robustness of ARAIM with two constellations is not enough as the depleted scenarios like satellite outages would hurt the ARAIM availability seriously. To evaluate the robustness of the proposed constellation selection method. The availability under depleted scenario is tested in this section further. A GPS satellite is removed from the constellation to simulate the scenario that one GPS satellite is in maintenance or excluded due to fault. The availability under three above ARAIM modes is tested. The results are shown in Table 4.  
Figure 5: Performance of ARAIM with constellation dynamic selection. From top to bottom, from left to right is VPL, HPL, EMT, and \(\sigma_{acc}\), respectively. Figure 6: Number of fault modes in ARAIM. (**a**) ARAIM with constellation dynamic selection. (**b**) ARAIM with four constellations. signals are got from the open access high-quality GNSS data with the support of International GNSS Service (IGS) network, including observation files from different stations around the world and the combined multi-GNSS broadcast ephemeris file. Considering the latitudes and continents, observations from 12 stations are chosen for statistical analysis and every station can track GPS, GLONASS, Galileo and BDS. Distribution of the stations chosen are show in Figure 9. The observation data between 2019-July-09 00:00 and 2019-July-09 23:59 are collected for statistics. The data interval is 30 s and the number of valid observations in every station is 2880. Four ARAIM groups are considered in this test: 'GPS + Galileo', 'GPS + BDS', four constellations, and constellation dynamic selection. The statistical results are shown in Table 5. The values in one row represent the availability results of different ARAIM modes at one station and the values in one column represent the availability results of one ARAIM mode at different stations. Particularly, the mean availability time is given in the last row. Bold numbers are values beyond 99.5%. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number** & **Station Name** & \begin{tabular}{c} **GPS + Galileo** \\ **Constellations** \\ \end{tabular} & \
\begin{tabular}{c} **GPS + BDS** \\ **Constellations** \\ \end{tabular} & **Four Constellations** &
\begin{tabular}{c} **Constellation Dynamic** \\ **Selection** \\ \end{tabular} \\ \hline
1 & HAL1 & 80.90\% & 87.88\% & 100\% & 99.34\% \\ \hline
2 & JPLM & 86.25\% & 88.65\% & 99.34\% & 99.31\% \\ \hline
3 & GODS & 77.08\% & 92.36\% & 99.34\% & 99.13\% \\ \hline
4 & UFPR & 97.05\% & 68.75\% & 100\% & 99.86\% \\ \hline
5 & HERS & 90.76\% & 97.19\% & 99.24\% & 99.24\% \\ \hline
6 & METG & 92.71\% & 98.54\% & 100\% & 100\% \\ \hline
7 & BSHM & 88.75\% & 97.05\% & 100\% & 100\% \\ \hline
8 & SUTM & 96.60\% & 97.95\% & 100\% & 99.31\% \\ \hline
9 & SIN1 & 89.30\% & 99.06\% & 100\% & 99.79\% \\ \hline
10 & XMIS & 91.84\% & 97.85\% & 100\% & 100\% \\ \hline
11 & JFNG & 61.46\% & 98.89\% & 100\% & 99.65\% \\ \hline
12 & SYDN & 76.60\% & 91.81\% & 99.79\% & 99.76\% \\ \hline \multicolumn{3}{c}{Mean value} & 86.06\% & 92.97\% & 99.99\% & 99.62\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: ARAIM availability statistical results of observation stations. Figure 9: Distribution of observation stations. The black points are station locations, and the red words are station names. It should be noted that the current GNSS constellations are not all in their nominal configurations. The number of Galileo and BDS MEO satellites have not reach those of their nominal configurations. On the other side, GPS deploys more spare satellites than the need of nominal configuration. It resulted in the availability time varied from about 60% to nearly 100% when fixed dual-constellation is adopted. The ARAIM availability is not evenly spread and rare to reach 99% under the current GNSS constellations. The mean availability results of 'GPS + Galileo' and 'GPS + BDS' are only 85.76% and 93.00%.