# An Integrity Monitoring Method for Navigation Satellites Based on Multi-Source Observation Links
## 2 Systematic Integrity Design for BDS-3 Satellite  
### Integrity Monitoring System of BDS-3  
Regarding integrity monitoring of the target satellite, the main control station of BDS-3 should collect satellite-to-ground and inter-satellite observation data to obtain the pseudorange and satellite autonomous integrity monitoring (SAIM) information, evaluate the state of the satellite, and generate various integrity indicator messages. These messages can be broadcast to the users through basic navigation messages and satellite-based augmentation messages. Different types of users in different regions can receive different integrity monitoring services. The process of integrity monitoring for BDS-3 is shown in Figure 1.  
Basic integrity parameters broadcast in basic navigation messages and SAIM messages mainly include the healthy indicator (SatH1), healthy status (HS), healthy indicator (Hea or Health), regional user range accuracy indicator (RURAI), signal-in-space accuracy indicator (SISAI), signal-in-space monitoring accuracy indicator (SISMAI), integrity flag of data (DIF), integrity flag of signal (SIF), and integrity flag of alarm (AIF).  
Augmentation integrity parameters broadcast in satellite-based augmentation messages mainly included user differential range error indicator (UDREI), grid-point ionospheric vertical delay error index (GIVEI), dual frequency range error indicator (DFREI), and dual frequency range error change indicator (DFRECI).  
Parameters have different update cycles and broadcast strategies, as shown in Table 1. HS and SIF can be generated not only by the main control station but also by the satellite itself with the application of SAIM technology.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & \begin{tabular}{c} **Integrity** \\ **Parameter** \\ \end{tabular} & **Message Type** & **Signal** & **Update Cycle** & **Data Source** & **Service Object** \\ \hline \multirow{8}{*}{\
\begin{tabular}{c} Basic integrity \\ parameters \\ \end{tabular} } & \multirow{2}{*}{SatH1} & D1 & \multirow{2}{*}{B1I/B3I} & 30 s & Main & \\ \cline{3-5}  & & D2 & & 3 s & control station & \\ \cline{3-5}  & & B-CNAV1 & B1C & 18 s & Main & \\ \cline{3-5}  & & B-CNAV2 & B2a & 3 s & control station/ & \\ \cline{3-5}  & & B-CNAV2 & B2a & 3 s & satellite & \\ \cline{3-5}  & & & D1 & 30 s & & Global \\ \cline{3-5}  & & D2 & & 3 s & Main & user \\ \cline{3-5}  & & & B-CNAV1 & B1C & 18 s & control station & \\ \cline{3-5}  & & B-CNAV2 & B2a & 3 s & & \\ \cline{3-5}  & & RURA & D2 & B1I/B3I & 18 s & \\ \cline{3-5}  & & & B-CNAV1 & B1C & 30 s/360 s/changing & Main & \\ \cline{3-5}  & & DIF, SIF, AIF & B-CNAV2 & B2a & time & control station/ & \\ \hline \multirow{8}{*}{
\begin{tabular}{c} Augmentation \\ integrity parameters \\ \end{tabular} } & \multirow{2}{*}{UDREI} & D2 & B1I/B3I & 3 s & \\ \cline{3-5}  & & B-CNAV & BDSBAS-B1C & 6 s & \\ \cline{3-5}  & & D2 & B1I/B3I & 180 s/360 s & Main & Regional \\ \cline{3-5}  & & B-CNAV & BDSBAS-B1C & 240 s & control station & user \\ \cline{3-5}  & & DFREI & B-CNAV & BDSBAS-B2a & 6 s/alarm time & \\ \cline{3-5}  & & DFRECI & B-CNAV & BDSBAS-B2a & 6 s/alarm time & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Update cycles and broadcast strategies of integrity parameters.  
Figure 1: Design of integrity monitoring system for the BDS-3 satellite.