# Research on Satellite Selection Strategy for Receiver Autonomous Integrity Monitoring Applications
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A counterexample is that when several satellites are near to each other, the removal of either one among them shows little impact on the protection level as they are redundancies to each other, but abandoning all of them may cause significant performance deterioration. A heuristic satellite selection strategy is proposed in [31] to improve the horizontal navigation accuracy and integrity of ARAIM. It uses a similar measurement ranking selection strategy for the initial subset then iteratively optimizes them in a \"greedy exchange\" way thus improves the selection quality but accompanies with an increased computational cost. Later, this heuristic satellite selection method is applied to the GBAS applications and augmented against timescale constraints, including limitations in satellite visibility, loss of satellites during approach, and convergence times in the airborne processing until satellites are usable [33]. Unfortunately, all the methods above require integrity aid information from the ground station or geostationary satellite, and thus are not suitable for the traditional RAIM process [25; 34; 35]. However, the principles of these satellite selection strategies are also instructive for our study. In the aerial navigation or air transport area, navigation devices are always required to satisfy certain criteria that are dependent upon the operational need such as traffic density and airspace complexity. These criteria include accuracy, integrity and other performance requirements and are of different acceptable ranges of values during en route, terminal, approach, and departure operations, as shown in Table 1. In these applications, the primary purpose of satellite selection methods is to fit the requirement of the flight phase, then it is to reduce the tracking channels so that computation load is mitigated. This paper tries to develop a satellite selection strategy for the RAIM process from a practical aspect, that is how to achieve the required accuracy and integrity performances, such as the protection level, with a relatively small subset, so it can adapt to applications that need to fit the criterion of the flight phase by low-cost navigation devices. Several issues are investigated under this motivation, such as the impacts of subset size on the accuracy and integrity performances, and the impacts of a single satellite on both accuracy and integrity of the current subset. Then a performance-requirement-driven fast satellite selection algorithm is proposed based on the former analysis to construct an appropriate subset. The main contributions of this paper can be summarized as follows:  
1. The impacts of subset size on the accuracy, integrity and computation load are analyzed in the ideal and practical satellite distribution scenes to illustrate the inefficiency of satellite increase in performance improvement when sufficient satellites have been in use. Consequently, the importance of the satellite selection strategy in RAIM process is confirmed;
2. The impacts of a single satellite on the accuracy and integrity of the subset are theoretically evaluated and a cost function is presented according to the performance requirement to choose the valuable satellite for the current subset. This valuable satellite is then used to construct the final subset so that the accuracy and integrity requirements can be both efficiently satisfied;
3. A performance-requirement-driven fast satellite selection algorithm is raised according to the above investigation. It constructs the initial subset through geometric selection strategy to keep a tolerable accuracy and integrity, then improves the performance by adding a most valuable satellite to the subset until the required performance is met, thus a small size feasible subset is obtained.