# Design and Performance Analysis of BDS-3 Integrity Concept
## 2 BDS Integrity Concept
### Alarming Approach
#### 2.2.1 Ground Monitoring and Alarming  
The ground monitoring and alarm approach using the BDS ground segment facilities the monitoring of the signal quality and the URE of the satellite SIS. When the failure is detected, the user will be notified of the satellite SIS health status through the integrity parameters in the broadcast message. The designed TTA of the ground monitoring and alarming approach is better than 60 s, and the delay mainly comes from processes such as data transmission, information processing, and message update, etc. The new BIC and B2a signals and the original B1I signal of BDS have different integrity parameter designs and alarm mechanisms. The B1I signal uses the \"autonomous satellite health flag (SatH1)\" parameter broadcast in the BDS D1 navigation message to indicate the satellite SIS health status, where \"SatH1 = 0\" indicates that the satellite SIS is available, and \"SatH1 = 1\" indicates that satellite SIS is not available (that is, URE > 4.17 \(\times\) URA), which are shown in Table 2[22; 23]. As a comparison, B1C and B2a signals use the \"satellite health status (HS)\" parameter to indicate the health status of the entire satellite, and the \"signal integrity flag (SIF)\" parameter to indicate the satellite SIS status. In addition, B1C and B2a signals use a \"data integrity flag (DIF)\" parameter to indicate the SIS accuracy (SISA) of satellites. This is mainly to take into account the differences in the sensitivity and tolerance of aviation and non-aviation users to satellite ranging errors. The integrity parameters of B1C and B2a signals are broadcast in sub-frame 3 of BDS B-CNAV1 and B-CNAV2 navigation messages, respectively [24; 25]. As the update frequency of B-CNAV2 is higher, it is recommended to use the integrity parameters broadcast in B-CNAV2, for B1C/B2a dual-frequency users. According to the above-mentioned integrity parameter design, BIC and B2a signals can take three different states as shown in Table 3, with the following meanings:  
* \"Healthy\": The SIS of the satellite meets the minimum service performance specified in the \"BeiDou Open Service Performance Specification\" [22];
* \"Unhealthy\": The SIS of the satellite is not providing services or is under test;
* \"Marginal\": The signal is neither of the two previous states. For some types of users, it is acceptable and tolerable, but for others, it is not. \begin{table}
\begin{tabular}{c c} \hline \hline
**B1 ISISHealthStatus** & **SatH1** \\ \hline Healthy & 0 \\ Unhealthy & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: B1I SIS health status indications. Further, the steps for the use of B1C and B2a signals can be described as following:  
* Step 1: Confirm whether the entire satellite is healthy according to the HS parameter in the message. If HS = 1, it indicates that the satellite is currently unhealthy, and the user should stop using the satellite. If HS = 0, it indicates that the satellite is currently healthy, and proceed to step 2. * Step 2: Confirm whether the satellite SIS is abnormal according to the SIF parameter in the message. If SIF = 1, it indicates that the satellite SIS has an anomaly affecting the pseudo-range, please stop using the satellite. If SIF = 0, it indicates that the satellite signal is normal, and proceed to step 3. * Step 3: Access the DIF parameter in the message. If DIF = 1, it indicates that the SISA of the satellite exceed the NTE (that is, URE > 4.42 \(\times\) SISA), and it is not recommended for users in the life safety field, such as aviation users. However, for the satellite, this is just that its SISA exceeds the limit at this time, not a failure. Other users who have less strict safety requirements can still choose to use it (for example, users in the mass consumer sector). If DIF = 0, it indicates that the SISA of the satellite does not exceed the NTE, and all users can use it with confidence. Currently, this alarm approach has been implemented in B1C and B2a signals. Here, we want to further highlight the design significance of DIF.