## 4 Conclusions  
The BDS-3 has realized satellite autonomous integrity monitoring in an actual satellite navigation system for the first time, making up for the lack of monitoring stations abroad. This study introduced the SAIM design of the BDS-3, analyzed the long-term stability of various satellite monitoring data and studied the distribution characteristics of various monitoring data. The results show that the standard deviation of the power monitoring value of the satellites' BIC signal is better than 0.1 dBm, and the standard deviation of the power monitoring value of the B2a signal is better than 0.2 dBm. The standard deviation of pseudo-range measurement of the BIC signal is better than 0.1 m, and that of the B2a signal is better than 0.3 m. The standard deviation of the frequency hopping monitoring data of the satellites is better than 0.1 mHz, and the standard deviation of the phase hopping monitoring data is better than 0.1 ns. These results prove that the BDS-3 SAIM monitoring data have good long-term stability and a basically normal distribution.  
Moreover, to solve the problem that BDS-3 SAIM does not have the capacity for autonomous ephemeris monitoring, a SAIM enhancement design is proposed, which consists of three relatively independent methods. In addition, the three methods are verified and analyzed with on-orbit data. The method which used the ephemeris extrapolated from the previous cycle, is highly dependent on the ground station, although it is the simplest and most reliable method, with the monitoring accuracy at the decimeter level. The method which used the ephemeris generated by autonomous orbit determination can realize autonomous integrity monitoring in the autonomous operation mode of the satellite, with a monitoring accuracy in the order of 10 m. The method which used inter-satellite link distance measurement data depends on the working state of the inter-satellite link, meaning that real-time performance of this method would not be good, with the monitoring accuracy also at the decimeter level. In engineering practice, these three methods can be integrated and applied according to the advantages and disadvantages, providing a reference for the upgrade of the SAIM monitoring function of BDS-3.  
Data curation, L.W.; Formal analysis, Y.C. and L.P.; Investigation, Q.R.; Methodology, Y.D. and Z.H.; Software, J.Z.; Supervision, W.G.; Validation, X.N. and R.S.; Writing original draft, L.C. and Y.D. All authors have read and agreed to the published version of the manuscript.  
This paper is supported by the National Natural Science Foundation of China (No. 41974041).  
Data Availability StatementPart of the historical ephemeris data can be downloaded from the following website: [[http://www.igs.gnsswhu.cn/index.php/home/data_product/igs.html](http://www.igs.gnsswhu.cn/index.php/home/data_product/igs.html)]([http://www.igs.gnsswhu.cn/index.php/home/data_product/igs.html](http://www.igs.gnsswhu.cn/index.php/home/data_product/igs.html)). The rest of the integrity data is internal data of the Beidou system and cannot be downloaded from the Internet.  
The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Statistical Results (m)**} \\ \cline{2-3}  & Variance & 0.14 \\ \cline{2-3}  & Skewness & \(-\)0.14 \\ \cline{2-3}  & Kurtosis & 3.76 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Statistical results of the distance between the C25 satellite and the C26 satellite calculated by inter-satellite ranging and the ephemeris.