## 5 Conclusions  
Through analyses of MGEX archived BDS-2 broadcast navigation messages from 2013 to 2018, this study reports the six-year BDS-2 broadcast navigation message performance from the perspectives of message availability, satellite anomalies, and SIS UREs. Precise products generated by Wuhan University were collected during the same period (DOY 42, 2013 to DOY 365, 2018) for reference purpose. Firstly, all BDS-2 satellites provide consecutive availability for six years except for C13, which was launched as an MEO satellite and then switched to an IGSO satellite. The BDS-2 service availability assessment using MGEX navigation files shows a significant difference between GEO/IGSO and MEO. Better than 98.51% navigation message availability is reported for GEO/IGSO, while -90.03% availability is reported for MEO. Among all available BDS-2 broadcast navigation messages, 651,038 were retained after excluding invalid messages in the preprocessing stage. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Orbit** \\ **Type** \\ \end{tabular} } & \multirow{2}{*}{**PRN \#**} & \multicolumn{6}{c}{**RMS (m)**} \\ \cline{3-10}  & & & **R** & **A** & **C** & **T** & \begin{tabular}{c} **Orbit-Only** \\ **URE** \\ \end{tabular} & \
\begin{tabular}{c} **Global-Average** \\ **URE** \\ \end{tabular} &
\begin{tabular}{c} **Wost-Case** \\ **URE** \\ \end{tabular} \\ \hline \multirow{4}{*}{**GEO**} & C01 & 1.040 & 15.933 & 1.364 & - & 2.122 & - & - \\  & C02 & 1.006 & 8.135 & 1.390 & 3.493 & 1.579 & 4.152 & 5.311 \\  & C03 & 1.099 & 4.427 & 1.441 & 2.091 & 1.323 & 2.612 & 2.962 \\  & C04 & 1.113 & 9.200 & 1.577 & 1.307 & 1.676 & 2.191 & 2.335 \\  & C05 & 0.889 & 4.663 & 1.549 & 1.713 & 1.173 & 2.282 & 2.635 \\ \hline \multirow{4}{*}{**IGSO**} & C06 & 0.951 & 1.636 & 1.281 & 2.262 & 0.951 & 2.872 & 2.907 \\  & C07 & 0.666 & 1.570 & 1.309 & 1.782 & 0.767 & 2.248 & 2.334 \\  & C08 & 0.927 & 1.783 & 1.610 & 2.034 & 1.058 & 2.774 & 2.850 \\  & C09 & 1.004 & 1.529 & 1.267 & 2.117 & 1.098 & 3.041 & 3.072 \\  & C10 & 1.028 & 1.618 & 1.292 & 2.266 & 1.134 & 3.279 & 3.334 \\  & C13(IGSO) & 1.559 & 1.574 & 1.242 & 1.720 & 1.939 & 3.061 & 3.152 \\ \hline \multirow{4}{*}{**MEO**} & C11 & 0.838 & 1.945 & 0.830 & 1.921 & 1.004 & 2.447 & 2.725 \\  & C12 & 0.802 & 1.935 & 0.849 & 2.102 & 0.973 & 2.794 & 3.093 \\ \cline{1-1}  & C13(MEO) & 0.309 & 1.487 & 0.659 & 2.357 & 0.440 & 2.299 & 2.558 \\ \cline{1-1}  & C14 & 0.732 & 1.867 & 0.798 & 2.053 & 0.879 & 2.627 & 2.916 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Statistics of six-year BDS broadcast navigation message orbit/clock errors and SIS UREs RMS. Figure 8: Six-year BDS broadcast navigation message orbit/clock errors and signal-in-space (SIS) UREs RMS. Secondly, satellite anomalies were detected based on broadcast navigation messages and precise products. Two indicators in the broadcast navigation messages, the URAI and SV health, were used to detect satellite anomalies. However, our results indicate that the URAI could not reflect any satellite anomalies, while SV health can only partially report satellite anomalies, with 5261 unhealthy messages detected. With a precise orbit/clock as the reference, a total of 20,241 abnormal messages were detected. In these abnormal navigation messages, 18,432 were recognized as orbit anomalies, which is much more frequent than the 2011 clock anomalies. Finally, 630,797 messages were retained for the six-year BDS-2 nominal SIS UREs assessment. The statistics show that the radial component has the best accuracy for all BDS-2 satellites (except for C13 (IGSO)), with an RMS better than 1.559 m. A significant difference is reported in the along-track component between GEO and IGSO/MEO. A better than 1.945 m RMS is obtained for the IGSO/MEO along-track component, while a >4.427 m RMS is achieved for GEO. However, the difference in the along-track component is not absorbed by the orbit-only URE, since the criterion is not dominated by along-track error.