###### Abstract  
It is essential to provide high-integrity navigation information for safety-critical applications. Global navigation satellite systems (GNSSs) play an important role in these applications because they can provide global, high-accuracy, all-weather navigation services. Therefore, it has been a hot topic to improve GNSS integrity performance. This paper focuses on an initial proposal of the next-generation BeiDou Navigation Satellite System (BDS) integrity subsystem, with the aim of providing high-quality and global integrity services for the BDS. This paper first reviews the current status of the third-generation BDS integrity service. Following this, this paper proposes a space-ground integrated integrity monitoring design for the BDS that integrates the traditional ground-based integrity monitoring method, the advanced satellite autonomous integrity monitoring (A-SAIM) method, and the augmentation from low-earth-orbit (LEO) satellites. Specifically, this work offers an initial design of the A-SAIM method, which considers both single-satellite autonomous integrity monitoring and multi-satellite joint integrity monitoring. In addition, this work describes two different ways to augment BDS integrity with LEO satellites, i.e., (a) LEO satellites act as space monitoring stations and (b) LEO satellites act as navigation satellites. Simulations are carried out to validate the proposed design using CAT-I operation in civil aviation as an example. Simulation results indicate the effectiveness of the proposed design. In addition, simulation results suggest that if the fault probability of LEO satellites is worse than \(1\times 10^{-4}\), LEO satellites can contribute more to BDS integrity performance improvement by acting as space monitoring stations; otherwise, it would be better to employ LEO satellites to broadcast navigation signals. The results also suggest that after taking LEO satellites into account, the global coverage of CAT-I can be potentially improved from 67% to 99%. This work is beneficial to the design of the next-generation BDS integrity subsystem. BeiDou; integrity; SBAS; SAIM; LEO 2024 16 2024 1  
Integrity is the measure of trust that can be placed in the correctness of the information supplied by a navigation system [11]. Integrity also includes the ability of the system to provide timely warnings to users when the system should not be used for navigation. This concept was first introduced in the aviation domain and has recently attracted increasing interest from other safety-critical navigation applications such as intelligent transportation systems, railways, autonomous driving, and low-altitude economies [12]. It is worth noting that we will focus on aviation applications in this work. In our future work, we will extend this work to support urban air mobility and autonomous driving through combing high-precision positioning techniques and integrity monitoring techniques. Table 1 presents the integrity performance requirements from the International Civil Aviation Organization (ICAO) considering the following flight phases: en-route, terminal, non-precision approach (NPA), approach with vertical guidance I (APV-I), localizer performance with vertical guidance 200 (LPV-200), and Category I (CAT-I). As shown in Table 1, the integrity performance requirements include metrics such as time to alert (TTA), target integrity risk (also known as probability of hazardously misleading information (PHMI)), and alert limit (AL). Table 1 gives the integrity performance requirements for different operations from the ICAO [13]. To determine whether the integrity performance of the onboard navigation system meets the corresponding performance requirements, the navigation receiver needs to calculate the protection level (PL) in real-time and compare it with the associated AL. The PL is defined as the probabilistic upper bound of the positioning error under a target integrity risk (i.e., PHMI).