## 3 Integrity Monitoring Method of Space-Ground Integration
### LEO Satellites as Space Monitoring Stations  
LEO satellites acting as space-based monitoring stations transmit integrity observation data to the medium- and high-orbit satellite navigation system, which then makes comprehensive integrity assessments. This technical route relies on the medium- and high-orbit navigation system to make integrity decisions and disseminate integrity information without adversely affecting the integrity risk of the medium- and high-orbit navigation satellites themselves. Compared to the technical route, where LEO satellites serve as navigation satellites, this approach has lower reliability requirements for the LEO satellites. The alarm process for space-based monitoring station-type integrated integrity enhancement is as follows:  
**(1)**: **BeiDou Single-Satellite Autonomous Integrity Monitoring**  
This module uses three redundant onboard integrity monitoring receivers to fulfill integrity monitoring against all-channel signal anomalies, clock anomalies, and ephemeris anomalies, as shown in Figure 4. This aims to provide comprehensive monitoring of a single satellite's operational status. The following describes the monitoring methods:  
First, signal monitoring aims to achieve real-time monitoring for all-channel signal power, pseudorange, carrier phase, and signal quality. The satellite's autonomous integrity receiver tracks navigation signals from the power amplifier, obtaining information such as signal power, pseudorange, carrier phase, and signal quality. It then compares these data against preset thresholds, which are derived from historical data gathered during prior stable operations. If these measurements exceed the thresholds, signal anomalies are considered to have occurred. Then, the satellite will inform the users with a broadcast message or non-standard codes (NSCs). In addition, an intelligent prediction of signal anomalies will also be considered by monitoring the change rate of all-channel signal  
Figure 4: Single-satellite autonomous integrity monitoring.  
power, pseudorange, carrier phase, and clock offset. If there is an abnormal change rate detected, then the satellite will inform the ground integrity processing center to verify whether there is a risk of signal anomalies in the following hours.  
Second, single-satellite clock monitoring aims to fulfill real-time integrity monitoring against abnormal clock jumps. The satellite checks the phase jump of the satellite clock (current value minus the value at the previous moment) and compares it against preset thresholds. If there is an abnormal clock jump detected (i.e., exceeding the predefined threshold), then the satellite will inform the users with the broadcast message or the non-standard codes.  
Third, single-satellite ephemeris monitoring aims to detect the anomalies in the ephemeris. The satellite's autonomous integrity receiver tracks navigation signals and obtains ephemeris information. Then, this is achieved by a three-step procedure. The first step is to validate the correctness of the ephemeris modulation process, i.e., check the consistency between the ephemeris that is uploaded from the ground and the ephemeris that is broadcast to users. The second step is to check the consistency among the B-CNAV1, B-CNAV2, D1, and D2 ephemeris. This benefits from the fact that the ephemeris for B1C/B2a (i.e., B-CNAV1 and B-CNAV2) and the ephemeris for B1I/B3I (i.e., D1 and D2) are generated independently. Finally, the third step is to verify the consistency between the previous ephemeris (i.e., the message that is valid in the last hour) and the current one. If there is any inconsistency detected in the three steps above, then the satellite will determine whether the ephemeris is faulted by jointly considering the single-satellite monitoring result, the inter-satellite consistency check result, and the ground verification result.