## 4 Conclusions  
Toward enhancing BeiDou integrity performance, this paper revisits the current status of the \"big 4\" GNSS constellations and provides an initial proposal for the development plans of the next-generation BeiDou integrity subsystem. The third-generation BeiDou Navigation Satellite System (BDS-3) utilizes both ground-based integrity monitoring and satellite autonomous integrity monitoring (SAIM) to address the issues caused by the limited coverage of ground monitoring stations. The BDS-3 fundamental integrity services and satellite-based augmentation system (SBAS) integrity services can meet the navigation performance requirements for the majority of aircraft operations, such as en-route and non-precision approaches (NPAs). However, the current-state BDS-3 integrity services cannot meet the stringent navigation performance requirements from the Category I precision approach (CAT-I) and the requirements from emerging applications such as automotive and urban air mobility.  
Therefore, this paper provides a preliminary proposal for the development plans of the next-generation BeiDou integrity subsystem by considering both advanced SAIM (A-SAIM) techniques and an SBAS with LEO satellites (L-SBAS). The A-SAIM technique will focus on efficient integrity monitoring against signal anomalies, clock anomalies, and ephemeris anomalies by utilizing both onboard hardware and inter-satellite links. The L-SBAS technique will employ the LEO satellites to augment GNSS integrity performance. Two different approaches to use LEO satellites are considered, i.e., LEO satellites acting as space monitoring stations and LEO satellites serving as navigation satellites. Simulation results suggest that if the LEO satellite and constellation fault probabilities are sufficiently low (e.g., \(10^{-5}\)), then employing LEO satellites as navigation satellites will provide more integrity performance improvement than using them as space monitoring stations. The results also indicate that the global coverage of CAT-I can be potentially improved from 67% to 99% after taking LEO satellites into account. The conclusion will be the opposite if the LEO constellation has high satellite and constellation fault probabilities. This paper is beneficial for realizing global and high-quality integrity services with the next-generation BeiDou system. Our future work will focus on experimental tests and will take carrier-phase positioning integrity into account.  
**Author Contributions:** [PERSON] proposed the idea of the article and determined the framework of the article; [PERSON]. ([PERSON]) designed the data analysis and wrote the manuscript; [PERSON] and [PERSON] analyzed the system architecture design of integrity services; [PERSON] also provided data analysis methods of the protection level; [PERSON]. ([PERSON]) and [PERSON] helped the analysis of A-SAIM and L-SBAS; [PERSON] conducted the analysis of A-SAIM; [PERSON] assisted in data collection. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research is sponsored by the National Natural Science Foundation of China (grant no. 42374044; grant no. 41974041).  
**Data Availability Statement:** The derived result data are available upon reasonable request. The GNSS ephemeris used for simulation in the section on illustrative examples and simulation results are available from [[https://celestrak.com/NORAD/ELEMENTS/](https://celestrak.com/NORAD/ELEMENTS/)]([https://celestrak.com/NORAD/ELEMENTS/](https://celestrak.com/NORAD/ELEMENTS/)), accessed on 6 August 2024.  
**Conflicts of Interest:** The authors declare no conflicts of interest.