###### Abstract
Equivalently, the receiver can determine whether the integrity performance of the onboard navigation system meets the requirements by comparing the integrity risk with the target integrity risk. The integrity risk is defined as the joint probability that the positioning error exceeds the AL without issuing an alarm within the TTA [14; 15]. To improve the integrity performance of global navigation satellite systems (GNSSs), GNSS service providers have also been developing satellite-based augmentation systems (SBASs) [16; 17; 18] independently. The SBAS uses geostationary orbit (GEO) satellites to broadcast SBAS messages, providing high-accuracy and high-integrity correction information on satellite orbits, clock offsets, and ionospheric delays [16; 19; 20; 21]. The SBAS also provides timely alerts to users with GEO satellites if the navigation information contains faults. Currently, the BeiDou Satellite-Based Augmentation System (BDSBAS), the Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), and the Russian System for Differential Corrections and Monitoring (SDCM) have completed their system construction. Among them, the WAAS has received single-frequency LPV-200 certification from the ICAO, and the EGNOS has received single-frequency NPA certification. Meanwhile, the BDSBAS is working toward the certification of single-frequency APV-I, which is expected to be completed around 2025 [22]. Although the SBAS can effectively enhance the integrity performance of GNSSs, there are still certain limitations regarding their performance and applicability. Firstly, the current SBAS primarily relies on GEO satellites, like the WASS and BDSBAS, making it impossible to provide global high-level integrity services (GEO satellites maintain a \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameters** & **En-Route** & **Terminal** & **NPA** & **APV-I** & **LPV-200** & **CAT-I** \\ \hline Time To Alert (TTA) & 5 min & 15 s & 10 s & 10 s & 6 s & 6 s \\ \hline Horizontal AL (HAL) & 7.4 km & 1.85 km & 556 m & 40 m & 40 m & 40 m \\ \hline Vertical AL (VAL) & N/A & N/A & N/A & 50 m & 35 m & 15 m \\ \hline Target Integrity Risk (PHMI) & \(10^{-7}\)/h & \(10^{-7}\)/h & \(10^{-7}\)/h & \(2\times\) & \(2\times\) & \(2\times\) \\  & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Integrity performance requirements for different operations in civil aviation. rotational period that coincides with the Earth's rotation, appearing stationary relative to the ground. Consequently, their signal coverage is regional). Secondly, the current SBAS cannot meet the performance requirements from CAT-I operations. Thirdly, the compatibility of the SBAS in applications beyond civil aviation has not been validated. In recent years, the United States has proposed the concept of satellite autonomous integrity monitoring (SAIM). Meanwhile, the BDS-3 has achieved the implementation of SAIM, significantly enhancing the system integrity performance. SAIM monitors anomalies in navigation signals, messages, and satellite clocks [23; 24; 25; 26]. Among them, the monitoring of signal and message anomalies relies on GNSS receivers installed on GNSS satellites. These receivers receive the GNSS signals from the satellite itself to determine whether there are any anomalies in the radio frequency signals and messages. Satellite clock anomaly monitoring refers to the GNSS satellites autonomously assessing whether their onboard clocks have experienced abnormal jumps or other behaviors. SAIM has the advantages of not relying on the satellite-ground data link and offering rapid integrity responses. Practical experience has shown that SAIM can effectively reduce the fault probability of GNSS satellites [27; 28; 29; 25]. In addition to SAIM, it has also been a hot topic to enhance the integrity performance of GNSSs using LEO satellites [30]. On one hand, LEO satellites can act as space monitoring stations, providing additional integrity monitoring capabilities aside from ground monitoring stations.