## 4 Results
### Nominal SIS URE Assessment  
After the anomaly detection in Section 3.2, 630,797 (= 651,038-20,241) BDS-2 broadcast navigation messages are retained for nominal SIS URE assessment in this section. First, three satellites C02/C06/C11 are selected as representatives of GEO/IGSO/MEO, respectively. Figure 6 shows a six-year orbit/clock error series of the three selected satellites. The Root Mean Square (RMS) of the radial errors are 1.006/0.951/0.838 m, respectively, which are the best out of the three directions. The RMSs of 8.135/1.636/1.945 m are obtained for along-track component, respectively. C02 had a worse along-track accuracy than C06/C11. As for the cross-track direction, the six-year time series were all stable with RMSs of 1.390/1.281/0.830 m, respectively. The clock precision of C02/C06/C11 was 3.493/2.262/1.921 m, respectively.  
Figure 7 shows the six-year time series of three SIS URE criteria for C02/C06/C11. The orbit-only URE RMS of the three satellites is 1.597/0.951/1.004 m, respectively. It is clear that C02 has the worst accuracy. The global-average URE, considering both the orbit and clock errors, owns a larger value than the orbit-only URE. The global-average URE RMSs of C02/C06/C11 are 4.152/2.872/2.447 m, respectively, which is about twice as the orbit-only URE. The instantaneous maximum URE indicator (worst-case  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **URAI Detected** & **SV Health Detected** & **Precise Product Detected** \\ \hline Counts & 0 & 5261 & 20,241 \\ Ratio & 0.00\% & 0.81\% & 3.11\% \\ \hline \hline \end{tabular}
\end{table}
Table 3: Six-year BDS anomaly detection result using the URAI, SV health and the precise product.  
Figure 6: Six-year C02/C03/C11 broadcast navigation message orbit and clock error time series.  
URE) owned relatively larger value than the global-average URE, with an RMS of 5.311/2.907/2.725 m for C02/C06/C11, respectively.  
Figure 8 and Table 4 show the six-year orbit/clock error and SIS URE RMSs for all BDS-2 satellites. Since C01 was selected as the reference satellite for clock assessment, no clock precision statistics were available for C01. Several patterns can be summarized:  
1. Except for C13 (IGSO), radial accuracies for all satellites are the best among three orbit components--better than 1.113 m.
2. A significant difference between the GEO and IGSO/MEO satellites is reported in the along-track component. A better than 1.945 m RMS is attained for the IGSO/MEO along-track error, whereas a worse than 4.427 m RMS is attained for GEO. C01 is the worst, with a 15.933 m along-track RMS.
3. With an RMS better than 2.357 m, the clock performance is similar for all satellites except for C02.
4. Though a large orbit accuracy difference is found in the along-track component among three orbit types, no significant difference is reported for the orbit-only URE. This is because the along-track error is scaled by a factor of 1/54 and 1/127 for MEO and IGSO/GEO, respectively. The orbit-only URE RMS for all satellites is better than 2.122 m.
5. Better than 3.279 and 3.334 m RMSs are obtained for the BDS-2 global-average URE and worst-case URE, respectively, except for C02. Since the clock error dominates the global-average URE and worst-case URE, C02 owns the worst accuracy on these two criteria, with an RMS of 4.152/5.311 m, respectively, due to the worst clock precision.  
All in all, although significantly orbit difference exists between the GEO and IGSO/MEO satellites, the satellite orbit type (GEO/IGSO/MEO) does not impact the orbit-only URE, global-average URE, and worst-case URE.  
Figure 7: Six-year C02/C03/C11 orbit-only user range errors (UREs), global-average UREs, and worst-case UREs.