# Construction Progress and Aviation Flight Test of BDSBAS
## 4 BDSBAS Aviation Flight Test
### Flight Test-III  
The transition flight test was carried out on 25 September 2020. The plane took off from Caihu Airport and flew for about 180 km to the southwest before flying for about 180 km to the southeast to arrive at the second airport. It did not land at the second airport, but returned to Caihu Airport on the same route for great-circle flights before landing. The flight lasted 3 h and 55 min, and the trajectory is shown in Figure 19.  
Figure 18: Stanford triangle charts for flight test-II. The top panel and the bottom panel are the horizontal Stanford chart and the vertical Stanford chart, respectively. \begin{table}
\begin{tabular}{c c} \hline \hline
**Indicators** & **Statistical Results** \\ \hline Horizontal accuracy/m (95\%) & 1.27 \\ Vertical accuracy/m (95\%) & 4.30 \\ Minimum horizontal safety index & 8.860 \\ Minimum vertical safety index & 2.582 \\ NPA availability & 100.00\% \\ NPA continuity & 100.00\% \\ NPA HMI events & 0 \\ APV-I availability & 100.00\% \\ APV-I continuity & 100.00\% \\ APV-I HMI events & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: BDSBAS SF performance statistics of flight test-II. Figure 23: Stanford triangle charts for flight test-III. The top panel and the bottom panel are the horizontal Stanford chart and the vertical Stanford chart, respectively. Figure 22: The PE, PL, and AL sequences of flight test-III. In both the horizontal direction (see the top panel) and the vertical direction (see the bottom panel), the PL can completely cover the PE, which means that no false alarms occurred during the test. However, there is an epoch in the bottom panel where the VPL exceeds the VAL while the VPE does not, which means that a false alarm occurred during the test. Table 4 shows the statistics of the performance indicators. It can be seen from Figures 22 and 23 and Table 4 that for the flight test-III conducted on 18 September 2020, the BDSABS SF augmentation positioning accuracy meets the requirements of the civil aviation APV-I accuracy index, and the PL can completely cover the PE. No integrity risk events occurred during the test. However, it should be noted that during the test, a false alarm event occurred in which the PL exceeded the AL and caused the system's APV-I level service to be unavailable. This reduced the availability and continuity of the APV-I level service from 100% to 99.993% and 99.989%, respectively, and caused the continuity performance to fail to meet the APV-I level requirements under the ICAO standards. By analyzing the data, we found that this false alarm event was caused by the decrease in the number of the augmented GPS satellites available for the airborne receiver. As shown in Figure 24, in the 8834 th epoch, the number of the augmented GPS satellites available for the airborne receiver suddenly decreased from nine to five, which resulted in the increase in the dilution of precision (DOP) - especially the vertical dilution of precision (VDOP), which in turn caused the VPL (vertical protection level) to exceed the VAL (vertical alarm level). Comparing the observation data of the BDSBAS MSs in the same test period, we found that the number of the available augmented GPS satellites broadcast by BDSBAS has no abnormalities (as well as the DOP), as shown in Figure 25. Here, \"BJFS\" represents the MS located in Beijing, China, and \"CHAN\" represents the MS located in Changchun, \begin{table}
\begin{tabular}{c c} \hline \hline
**Indictors** & **Statistical Results** \\ \hline Horizontal accuracy/m (95\%) & 1.49 \\ Vertical accuracy/m (95\%) & 4.04 \\ Minimum horizontal safety index & 11.506 \\ Minimum vertical safety index & 3.099 \\ NPA availability & 100.000\% \\ NPA continuity & 100.000\% \\ NPA HMI events & 0 \\ APV-I availability & 99.993\% \\ APV-I continuity & 99.989\% \\ APV-I HMI events & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: BDSBAS SF performance statistics of flight test-III. Figure 24: Available augmented satellites number and DOP sequence of the airborne receiver. China.