## 1 Introduction  
With the wide application of satellite navigation systems in many real-time fields such as aviation, high-speed railways and unmanned vehicles, users' demand for reliable GNSS has increased [1]. The integrity of GNSS is an important indicator used to measure the reliability of a system's service. It refers to the ability of the system to provide a timely alarm to users when any fault or error in the GNSS exceeds the allowable limit [2, 3]. With improvements in service accuracy, the \"Big 4\" GNSS (GPS/BDS/Galileo/GLONASS) are all paying more and more attention to improving their system integrity monitoring ability. In the Big 4 GNSS upgrade plans, improving the system's integrity service ability is one of the most pressing concerns [4; 5; 6]. At present, the GNSS integrity monitoring methods can mainly be divided into satellite autonomous integrity monitoring (SAIM), ground integrity monitoring and receiver autonomous integrity monitoring (RAIM) [7]. Among these, RAIM technology refers to the detection, analysis and processing of service performance anomalies by the user receiver of the received observations of multiple navigation satellites [8]. Generally, this technology can only monitor the obvious anomalies of a single satellite. Ground integrity monitoring technology refers to monitoring the accuracy of the spatial signal corresponding to the predicted ephemeris and clock error in the navigation message by using the observation data of the monitoring station established by the system and obtaining the corresponding integrity parameters and broadcasting them to the user together with the navigation message [5]. There is also an article examining the concept of relative receiver autonomous integrity monitoring (RRAIM) where using time differential carrier phase measurements is investigated [9]. SAIM technology involves the monitoring of the satellite's anomaly by the navigation satellite itself. Thus, SAIM could work independently, without being affected by ground equipment and propagation path errors, which has the advantage of highly efficient alarm times, and it is an important development direction of monitoring GNSS integrity [10; 11; 12; 13]. Study [14] mentions the use of inter-satellite link communication information to enhance SAIM monitoring capabilities. Study [15] describes an approach to SAIM and is prototyping it. The prototype has been tested against nominal satellite signals (to confirm that no fault alarms are rare enough to support civil aviation continuity requirements) and several classes of fault signals. Practical implementation issues such as satellite multipath and receiver clock calibration will also be addressed. This method can be applied to future GNSS satellites such as GPS III. Study [16] focuses on the technical solution of BDS-3's SAIM, analyzes the complete telemetry data of the new generation of Beidou satellites, and confirms that the alarm time through message is less than 6 s, and the alarm time through non-standard code (NSC) is less than 2 s.  
In the first GPS designs, SAIM was not fully considered, with most integrity indicators not meeting International Civil Aviation Organization (ICAO) requirements. In order to meet the needs of the civil aviation industry for integrity, many countries and regions have established satellite-based augmentation systems (SBAS) and ground-based augmentation systems (GBAS) to compensate for the lack of GPS integrity monitoring capability. GPS III also plans to enhance the risk controlling ability of ground control systems, to increase the ability of SAIM and to redesign the integrity parameters, thus ensuring that the probability of integrity risk in the system meets the index required by the ICAO, which is better than \(10^{-7}/\)h. The latest service performance specifications show that the global integrity alarm time of GPS is less than 10 s. The Galileo system is also carrying out SAIM research via the simulation analysis method [11; 12].