Initial analysis of the BDS satellite autonomous integrity monitoring capability

The integrity of Global Navigation Satellite System (GNSS) has the capability of sending a timely alarm to users when the GNSS is out of use, and it is one of the significant parameters of GNSS. Compared with ground-based integrity monitoring, integrity monitoring within the satellite constellation itself could lessen the effects of possible failures in the signal propagation path and ground systems and shorten the alarm time, which would quickly alert users of the system failure. Developing satellite autonomous integrity monitoring (SAIM) has been one of the main objectives of the upgrade plan for future GNSS such as GPS III, Galileo and the third generation of Beidou satellite navigation system (BDS3). To test the new technology, BDS3 experimental satellites are equipped with SAIM payload which is independently developed by China and it is the first satellite navigation system that applies SAIM method to monitor the navigation signals integrity. SAIM monitors signal quality and clock and frequency stability. We briefly introduce the payload design of SAIM on BDS3 experimental satellites, and evaluate in detail the monitoring measurements stability for two new civil signals of BDS3 open service, B1C_pilot signal, and B2a_pilot signal. By artificially setting signal faults, we analyze the time-to-alert performance of SAIM under its two kinds of alarming modes. The results show that the onboard observation stabilities of signal power, pseudorange, code carrier bias, and code carrier divergence are better than 0.12 dB, 0.1 ns, 0.14 ns, and 3.5 mm/s, respectively, which were reliable at 1-year interval. The onboard satellite clock step monitoring precision is approximately 0.5 ns. Once the abnormal signal is detected, the BDS SAIM may provide a warning within 6 s. With the characteristics of overcoming the ground monitoring network limitation and fast alarming ability, SAIM is expected to be an effective method to enhance the BDS3 system integrity monitoring capability.