## 4 Results
### BDS-2 Anomaly Detection Results  
In this section, six-year BDS-2 anomalies are detected. Two satellite indicators contained in the broadcast navigation message, i.e., the URAI and SV health, were used to detect satellite anomalies to see if they can correctly reflect the satellite status. After presenting the URAI and SV health detection results, BDS-2 anomalies detected by comparing broadcast navigation messages with the precise products are presented. Firstly, a one-week C07 anomaly detection result from 15 May to 22 in 2013 is depicted in Figure 4. An apparent anomaly occurred from May/17 0:0:0 to May/20 5:0:0. We further separate this period into three parts:  
1. May/17 1:0:0-May/18 0:0:0. The URAI in this period retained a value of 2 (3.40-4.85 m URA), whereas SV health changed between 0 (healthy) and 1 (unhealthy). Since no precise product was available for this period, broadcast navigation messages were unevaluable, as illustrated in the Section 3.1. 2. May/18 0:0:0-May/18 23:0:0. The URAI and SV health patterns were similar to that during the previous period, except that SV health was marked as healthy during most time. However, the precise product was available in this period. The orbit error series showed a poor accuracy worse than 20 m.
3. May/18 23:0:0-May/20 5:0:0. The URAI in this period still held a value of 2 (3.40-4.85 m URA), while SV health held a value of 0, indicating the satellite was in healthy condition. The orbit error series in this period, however, still showed a worse than 20 m orbit error, which indicates the satellite was not in an applicable condition. We can conclude from the weekly anomaly detection result that the URAI could not detect any satellite anomalies. SV health can only partially identify the satellite abnormal status. However, the comparison, with respect to the precise product, was able to detect significantly more satellite anomalies than the other two. Figure 3: Availability of BDS broadcast navigation message in Multi-GNSS Experiment (MGEX)-merged navigation files and the precise product generated by Wuhan University. (**a**) Six-year series of available BDS-2 broadcast navigation message and the precise product; (**b**) Availability of BDS-2 broadcast navigation message and the precise product. Table 1 shows six-year anomaly detection results using the URAI and SV health. The URAI in broadcast navigation messages could not tell any anomalies throughout the six years. While SV health identified 5261 unhealthy messages, occupying 0.81% of all tested data. The C13 (MEO) was verified to be problematic again with 31.78% broadcast navigation messages marked as unhealthy. The SV health detected unhealthy ratios were 0.09%/0.31%/1.94% for GEO/IGSO/MEO (except for C13), respectively. Table 2 shows six-year anomaly detection result with reference to the precise products. We can see that a total of 20,241 broadcast navigation messages were detected as anomalies, which occupy 3.11% of all tested data. Among these anomalies, 18,432 messages were detected as orbit anomalies, while only 2011 were detected as clock anomalies. The anomaly ratios are 2.81%/2.92%/4.01% for GEO/IGSO/MEO (without C13), respectively.