# An Integrity Monitoring Method for Navigation Satellites Based on Multi-Source Observation Links
## 1 Introduction  
The BeiDou-3 navigation satellite system (BDS-3) has been officially providing high-accuracy positioning, navigation, and timing services to global users since 31 July 2020. The positioning accuracy has been improved from the submeter level to the centimeter level, which has basically solved the problem of high-accuracy positioning. With the expansion of satellite navigation applications to emerging fields, such as unmanned autonomous operating systems, intelligent driving, and automated precision operations, demand has gradually changed from requiring high accuracy to high integrity.  
Integrity parameters are significant and fundamental performance indexes to predict the application security of navigation satellite systems. The definition of integrity has been constantly improved by updating the navigation system [1]. The existing technology for integrity monitoring is mainly designed with alarm limits at the meter level based on the positioning accuracy at the meter or submeter level and is not fully adapted to high-precision positioning technology.  
BDS-3 has adopted a new design concept for integrity parameters, which takes full consideration of compatibility and interoperation with BDS-2 and references the modern design of CNAV-2 and F/NAV messages [1; 2; 3]. Based on the integrated design of the basic navigation service and satellite-based augmentation service in BDS-3, integrity parameters can be divided into two types: basic integrity parameters and augmentation integrity parameters. Diversified design can not only improve the reliability of system integrity monitoring but also reduce the cost of different users.  
Basic integrity parameters are solved with satellite-to-ground and inter-satellite observation data to indicate the state of navigation satellites in real time and in the short term and long term. These messages can be modulated and broadcast in new signals (B1C and B2a) and transitional signals (B1I and B3I) [4; 5; 6; 7] to global users with the support of satellite-to-ground and inter-satellite observation links. As deployment limitations of monitoring stations and constraints on the communication capacity of inter-satellite links, real-time integrity monitoring within a global area is rather difficult to realize.  
Augmentation integrity parameters are generated by the BDS-3 satellite-based augmentation system (BDSBAS), which can realize real-time monitoring at the second level with the support of satellite-to-ground observation links. Only regional users in China and surrounding areas can obtain augmentation monitoring service.  
With the comprehensive positioning, navigation, and timing (PNT) concept proposed [8], research on low earth orbit (LEO) constellations as a backup and augmentation system for the global navigation satellite system (GNSS) has garnered increasing attention both domestically and internationally. An increasing number of LEO satellites are being equipped with high-precision satellite-based GNSS monitoring receivers [9; 10; 11]. The proliferation of LEO satellites will facilitate the acquisition of full-arc navigation satellite observation data, thereby significantly enhancing real-time monitoring capabilities. Nevertheless, it also poses challenges related to excessive observational data. Consequently, combined utilization of satellite-to-ground, inter-satellite, and satellite-based observation links will be crucial for achieving real-time monitoring data to enhance the systematic capability of integrity monitoring.  
This study first constructs multi-source observation links with satellite-to-ground, inter-satellite, and satellite-based observation data. Then, an optimization method of observation links is proposed to realize the constrained return of navigation messages and rapid calculation of satellite orbit. Finally, integrity monitoring capacity is evaluated with different observation links to verify the feasibility of improving the integrity monitoring capability of BDS-3.