## 1 Introduction
Due to some objective factors, such as geographical politics, the monitoring coverage of the BDS-3 ground monitoring network for on-orbit satellites is limited. Ground integrity monitoring alone cannot meet the needs of global monitoring. Faced with this problem, BDS-3 has engineered the first realization of SAIM for all constellation satellites all around the world, and the alarm time performance is theoretically better than 6 s. Fusion of the information of SAIM and ground integrity monitoring can achieve global coverage and a timely response for integrity monitoring, thus ensuring the integrity of the service ability of the system [17; 18]. BDS-3 SAIM already has the ability to monitor the satellites' atomic clock frequency, the satellites' atomic clock phase, the downlink navigation signal power and the pseudo-range measurement, but it does not have the capacity for autonomous monitoring of satellite ephemeris. At present, the monitoring of ephemeris anomalies relies only on ground monitoring stations, which also means that when the satellite runs in the arc section outside the area covered by the ground monitoring stations, every time an ephemeris anomaly occurs, the alarm will be completely missed; that is, the user will receive the wrong navigation information but BDS-3 will not send an effective and timely warning to the user. As BDS-3 only has a few monitoring resources outside of China, a satellite ephemeris anomaly occurring outside China is likely to be one of the most important risks to the integrity of BDS-3's service [19]. Therefore, supplementing and enhancing the autonomous integrity monitoring ability for satellite ephemeris anomalies has become an important part of upgrading BDS-3. According to studies [18; 19], during the two-year period from 27 December 2018 to 27 December 2020, six SAIM misses occurred in BDS-3 (the main reason was that SAIM did not have the ability to monitor the ephemeris anomalies), and the probability of an integrity risk obtained by the final evaluation was about 0.9 \(\times\) 10\({}^{-7}\), which only just meets the index requirement of 1.0 \(\times\) 10\({}^{-7}\). Therefore, in order to ensure that BDS-3 can still meet the requirements of the integrity indicators required by the ICAO for the future period of a more severe space environment, it is very necessary to carry out research on how to enhance SAIM's capability to monitor ephemerides. To sum up, this paper firstly describes the BDS-3 SAIM design scheme, including current SAIM monitoring methods of signal power, pseudo-range, clock frequency and phase. In view of the lack of autonomous ephemeris monitoring capability of BDS-3 SAIM, an enhancement design is proposed, which consists of three relatively independent methods. Then, using the actual telemetry data of BDS-3 satellite on-orbit, the actual performances of current SAIM in navigation signal power, pseudo-range measurement, satellite atomic clock frequency jump monitoring and phase jump monitoring are quantitatively analyzed. According to the analysis results, we obtained the actual characteristics of SAIM monitoring data. Meanwhile, the validity and feasibility of the ephemeris monitoring methods are verified by simulation data, which ensures that the method can provide direct and effective support for the realization and application of SAIM in the near future.