## 4 Results
### BDS-2 Anomaly Detection Results
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Orbit Type**} & \multirow{2}{*}{**PRN \#**} & **\# of Messages** & **\# of URAI Detected** & **\# of SV Health** & **Anomaly** \\  & & **Messages** & **Anomalies** & **Detected Anomalies** & **Ratio (\%)** \\ \hline \multirow{6}{*}{**GEO**} & C01 & 48,226 & 0 & 17 & 0.04 \\  & C02 & 48,234 & 0 & 46 & 0.10 \\  & C03 & 47,240 & 0 & 50 & 0.11 \\  & C04 & 49,143 & 0 & 48 & 0.10 \\  & C05 & 48,686 & 0 & 58 & 0.12 \\ \cline{2-6}  & **Total** & **241,529** & **0** & **219** & **0.09** \\ \hline \multirow{6}{*}{**IGSO**} & C06 & 49,599 & 0 & 181 & 0.36 \\  & C07 & 49,654 & 0 & 110 & 0.22 \\  & C08 & 49,878 & 0 & 365 & 0.73 \\  & C09 & 49,744 & 0 & 71 & 0.14 \\  & C10 & 49,836 & 0 & 85 & 0.17 \\  & C13 (IGSO) & 19,573 & 0 & 12 & 0.06 \\ \cline{2-6}  & **Total** & **268,284** & **0** & **824** & **0.31** \\ \hline \multirow{6}{*}{**MEO**} & C11 & 45,344 & 0 & 1305 & 2.88 \\  & C12 & 45,610 & 0 & 517 & 1.13 \\ \cline{1-1}  & C13 (MEO) & 4949 & 0 & 1573 & 31.78 \\ \cline{1-1}  & C14 & 45,322 & 0 & 823 & 1.82 \\ \cline{1-1} \cline{2-6}  & **Total** & **136,276** & **0** & **2645** & **1.94** \\ \cline{1-1} \cline{2-6}  & **(without C13)** & **136,276** & **0** & **2645** & **1.94** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: BDS-2 anomalies detected by the URAI and SV health. Figure 4: The User Range Accuracy Index (URAI), space vehicle (SV) health (Top) and orbit error (Bottom) time series of C07 broadcast navigation messages from May/15 to May/22 in 2016. To further reflect BDS anomalies, Figure 5 illustrates the comparison of anomalies detected by the URAI, SV health, and the precise product. For most satellites, anomalies detected by precise products were more than that by SV health except for C13 (MEO). Though 31.78% messages of C13 (MEO) were marked as unhealthy, only 3.11% were detected as anomalies. The significant difference indicates that, although marked as unhealthy, C13 (MEO) can still broadcast valid orbit/clock messages. To sum up, three conclusions are summarized in Table 3: (1) The URAI is incapable of detecting any anomalies. (2) The SV health flag can only reflect satellite anomalies partially (5261). (3) With precise products as the reference, 20,241 out of 651,038 broadcast navigation messages were detected as anomalies. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Orbit Type** & **PRN \#** & **\# of Messages** & **\# of Orbit Anomalies** & **\# of Models** & **\# of Orbit/Clock Anomalies** & **Anomaly Ratio (\%)** \\ \hline \multirow{4}{*}{**GEO**} & C01 & 48,226 & 1532 & 65 & 1575 & 3.27 \\  & C02 & 48,234 & 989 & 88 & 1055 & 2.19 \\  & C03 & 47,240 & 1135 & 112 & 1208 & 2.56 \\  & C04 & 49,143 & 1490 & 65 & 1554 & 3.16 \\  & C05 & 48,686 & 1319 & 81 & 1399 & 2.87 \\ \cline{2-7}  & **Total** & **241,529** & **6465** & **411** & **6791** & **2.81** \\ \hline \multirow{4}{*}{**IGSO**} & C06 & 49,599 & 1697 & 74 & 1769 & 3.57 \\  & C07 & 49,654 & 1376 & 63 & 1429 & 2.88 \\  & C08 & 49,878 & 1461 & 166 & 1625 & 3.26 \\  & C09 & 49,744 & 1135 & 31 & 1166 & 2.34 \\  & C10 & 49,836 & 1291 & 41 & 1332 & 2.67 \\  & C13 (ICSO) & 19,573 & 483 & 24 & 506 & 2.58 \\ \cline{2-7}  & **Total** & **268,284** & **7443** & **399** & **7827** & **2.92** \\ \hline \multirow{4}{*}{**MEO**} & C11 & 45,344 & 1607 & 639 & 2215 & 4.88 \\  & C12 & 45,610 & 1401 & 232 & 1621 & 3.55 \\ \cline{1-1}  & C13 (MEO) & 4949 & 78 & 102 & 154 & 3.11 \\ \cline{1-1}  & C14 & 45,322 & 1438 & 228 & 1633 & 3.60 \\ \cline{1-1} \cline{2-7}  & **Total** & **136,276** & **4446** & **1099** & **5469** & **4.01** \\ \cline{1-1} \cline{2-7}  & **(without C13)** & **136,276** & **4446** & **1099** & **5469** & **4.01** \\ \hline \hline \end{tabular}
\end{table} 
Table 2: BDS-2 anomalies detected by precise orbit/clock products. Figure 5: Comparison of the URAI, SV health, and the precise product anomaly detection results.