# BDS-3 Integrity Risk Modeling and Probability Evaluation
## 2 BDS-3 Integrity Monitoring Architecture  
### Integrity Concepts and Indicators  
Due to the high requirements for the safety and reliability of position information services, the concept of integrity first appeared in the civil aviation field. To be applied in the civil aviation field, GNSS around the world needs to carry out and complete the research and formulation of relevant standards under the framework of ICAO and promise to meet the service performance indicators required by the corresponding standards.  
For a more detailed and accurate analysis, the integrity risk probability is further divided into \"Psat, which is the single-satellite integrity risk probability\" and \"Pconst, which is the constellation integrity risk probability\".  
Psat refers to the probability that the SISRE of any single satellite exceeds the alarm limit and that the system fails to issue warning information within the promised alarm time. Pconst refers to the probability that the SISRE of more than two satellites exceeds the alarm limit, and the system fails to issue warning information within the promised alarm time. Anomalies that result in a single-satellite integrity risk usually affect only the satellite with no effect on the service performance of other satellites and systems. Abnormalities that lead to constellation integrity risks generally affect system service performance.  
At present, the Big 4 GNSSs around the world are attempting to develop and formulate their own integrity-related technical indicators. From ICAO SARPs, the Big 4 GNSS integrity indicators are shown in Table 1.  
Table 1 shows that the Big 4 GNSS integrity indicators are quite different. GPS has global ground monitoring and injection abilities, so once the service is abnormal, the system can send alarms in real time. Specifically, there are many GPS warning methods, such as using health status identification in navigation messages and setting the satellite Pseudorandom Number (PRN) code to \"No. 37\" and broadcasting nonstandard PRN code. GLONASS passes the system's running status to users through health status identification in navigation messages. Galileo provides users with integrity information through HS, DVS, SISA and other parameters in navigation messages. BDS-3 adopts two methods, namely, ground integrity monitoring, which is similar to other GNSS, and Satellite Autonomous Integrity Monitoring (SAIM), which is the first applied to the actual GNSS, effectively making up for the deficiency that the ground monitoring network can only achieve satellite coverage in China due to geopolitical influence. With the application of SAIM, BDS-3 updates the promised integrity alarm time from 300 s to 10 s in the upcoming ICAO SARPs to better meet the application requirements of the ICAO ARAIM. BDS-3 provides users the system's alarm information with AIF, DIF and SIF parameters in navigation messages. If  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Index Item** & **GPS** & **GLONASS** & **BDS-3** & **Galileo** \\ \hline Alarm limit & SISRE \textgreater{} 4.42 \textasci{} IAURA & SISRE \textgreater{} 70 m & SISRE \textgreater{} 4.42 \textasci{} SISA & SISRE \textgreater{} 4.17 \textasci{} URA \\ Alarm time & 10 s & 10 s & 300 s (10 s) & — \\ Integrity risk & Psat & 1E-5 & 1E-4 & 1E-5 & 3E-5 \\ probability & Pconst & 1E-8 & 1E-4 & 6E-5 & 2E-5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: GNSS integrity indicators in ICAO SARPs.  
the status of the signal is healthy, these parameters are assigned values of \"0\"; otherwise, the parameters are assigned values of \"1\".