# Design and Performance Analysis of BDS-3 Integrity Concept
## 1 Introduction  
Accuracy, integrity, continuity, and availability are the four core performance indicators of satellite navigation systems. Among them, integrity refers to the ability of the system to alert users in time when the service is abnormal or experiences failure, and it characterizes the security and reliability of system services [1, 2]. If there is an abnormality or failure in the service, but the system fails to detect it or fails to alarm in time, an \"integrity event\" has occurred. Once an integrity event occurs, it will have a security impact on the user, especially for civil aviation, maritime, railway, and other users related to life safety. System reliability is even more strategically important with the widespread use of low-cost sensors for various applications including personal positioning and autonomous navigation [3, 4]. In addition, the integrity of the core constellation is also an important foundation for the construction of the satellite-based augmentation systems (SBAS) such as WASS (wide area augmentation system) [5, 6], ENGOS (European geostationary navigation overlay system) [7, 8], MSAS (multi-functional satellite augmentation system) [9, 10], BDSBAS (BeiDou satellite-based augmentation system) [11], and airport ground-based augmentation systems (GBAS). These augmentation systems are constructed to further augment the global navigation satellite system (GNSS) constellation and provide higher integrity navigation services. Satellite navigation systems in the world are paying more and more attention to the construction and improvement of their integrity capabilities. GPS and Galileo considered their integrity function at the beginning of the system design, defined the integrity parameters and algorithms, and planned its application scenarios [2; 12; 13; 14]. China's BeiDou also attaches importance to the construction of integrity. China's BeiDou global navigation satellite system (BDS-3) carried out a brand new integrity concept design work, which defines and achieves the integrity functions for major civil open service (OS) signals such as B1C, B2a, and B1I. In addition to GNSSs, regional satellite navigation systems have also begun to upgrade their integrity capabilities. For example, Japan's QZSS not only defines and provides the integrity function for its basic positioning, navigation, and timing (PNT) service, but also for its sub-meter level augmentation service (SLAS) and centimeter-level augmentation service (CLAS) [15; 16]. Due to the numerous components, functions, and processing operations of the system, it is not easy to ensure the integrity of satellite navigation systems, especially for global ones. On 13 July 2019, a mass outage occurred in Galileo--all 24 networking satellites in orbit entered an \"unavailable\" or \"test\" state, and the system was immediately paralyzed. After 48 h of full maintenance, the system did not return to normal until 8 o'clock on 18 July. During this period, the service was knocked offline for up to 117 h (about 5 days) [17]. Although the European GNSS Agency (GSA) issued a warning of service outage on the Galileo official website at 20:00 on 13 July [18], it still had a serious impact on users around the world. This incident also prompted countries to take measures to ensure the safe operations of satellite navigation systems. After this incident, Russia stated that it has set up a multi-level software detection system in the ground segment of GLONASS, with a redundant design and an autonomous control function to prevent such failures from occurring. On 27 December 2018, BDS-3 completed the construction of the basic constellation (consisting of 18 MEOs) and began to provide the initial global service [19]. On 31 July 2020, BDS-3 completed all construction (consisting of 24 MEOs, 3 IGSOs, and 3 GEOs) tasks and officially opened the global service [20]. Since the initial service was provided, BDS-3 has been in a stable operation state, and there have been no integrity events affecting the system and user services.