## 3 Integrity Monitoring Method of Space-Ground Integration
### Architecture of Integrity Monitoring Method  
The overall framework of the integrated space-ground integrity monitoring method proposed in this paper is shown in Figure 3. It includes four components, namely medium- and high-orbit advanced SAIM (A-SAIM), LEO integrity enhancement, ground monitoring, and RAIM. Among them, the medium- and high-orbit A-SAIM serves as the core component, supported by ground monitoring. This system is enhanced by multi-source fusion for improving low-orbit integrity, with RAIM implemented as an additional method. Building on the integrity framework of the BDS-3, the integrated space-ground integrity monitoring method proposed in this paper includes four aspects. The first one is upgrading the integrity monitoring of the medium- and high-orbit constellation from autonomous integrity to A-SAIM. This upgrade not only enhances monitoring accuracy but also introduces new capabilities for monitoring ephemeris correctness and clock drift variations, utilizing intelligent prediction methods to proactively address potential faults. The second one is the new LEO integrity enhancement, leveraging the global multi-coverage characteristics of LEO satellites to achieve multiple redundancy monitoring of medium- and high-orbit satellites, effectively reducing the duration of faults in these satellites. For highly reliable low-orbit satellites, broadcasting navigation enhancement signals to users can increase the number of visible satellites and improve geometric configuration, thereby enhancing user positioning accuracy and lowering the integrity protection level. The third one is ground-based intelligent operation and maintenance, which will receive navigation signals from medium, high, and low orbits, providing auxiliary verification of integrity monitoring data and implementing anomaly correction and control when deviations occur in medium- and high-orbit autonomous integrity and LEO integrity enhancement. The last one is that user receivers will employ RAIM or A-RAIM algorithms, integrating integrity alert information from high, medium, and low orbits to deliver high-integrity services to users. In the integrated space-ground integrity monitoring method, LEO satellites play a crucial role as an important enhancement. First, LEO satellites, which function as space-based monitoring stations, can work in conjunction with ground monitoring stations to perform the integrity monitoring of medium- and high-orbit satellites. This will allow for quicker integrity assessments and a reduction in the mean time to failure of medium- and high-orbit GNSS satellites, thereby reducing the fault probability of GNSS satellites. Furthermore, a lower fault probability of GNSS satellites will result in a decrease in the protection levels. Second, LEO satellites can also serve as navigation satellites, broadcasting navigation signals to users. This increases the number of visible satellites for users, improving positioning accuracy and subsequently lowering the user protection level. Third, the high data transmission rates of LEO satellites can be utilized to broadcast space-based integrity enhancement information, providing globally covered space-based augmentation services. Based on the capabilities of LEO satellites, the integrated space-ground integrity monitoring method proposed in this paper can be divided into two technical approaches. The first involves LEO satellites acting as space-based monitoring stations to perform the integrity monitoring of medium- and high-orbit satellites. The second involves LEO satellites acting as navigation satellites, directly sending navigation signals to users and participating in user positioning. The second technical approach places higher reliability requirements on the LEO satellites themselves, necessitating low failure probabilities for both individual LEO satellites and the overall LEO constellation.