# Research on Satellite Selection Strategy for Receiver Autonomous Integrity Monitoring Applications
## 1 Instruction  
The global navigation satellite system (GNSS) plays a critical role in nowadays civil aviation system. It not only provides a navigation solution for the crew, but also monitors the reliability of the navigation output through a series of augmentation techniques [1, 2, 3, 4, 5, 6], such as receiver autonomous integrity monitoring (RAIM), which is achieved by a consistency check among redundant pseudo-range measurements [7, 8, 9]. The RAIM monitoring technique has been commonly implemented especially in aerial navigation and air transport area for keeping the safety of the flight [10, 11, 12, 13, 14, 15], while also being investigated in the maritime navigation field [16, 17, 18]. The snapshot RAIM process only needs the satellite signals in current epoch for operating and requires no additional aid information beyond the receiver, thus has been widely investigated and employed for the last decades [19, 20, 21]. Along with the modernization of GNSS and the combination of different constellation navigation systems, the significant increase in operating satellites in view greatly improves the accuracy of navigation solution and the availability of the RAIM process. If we take the worldwide constellation navigation systems include global positioning system (GPS), Galileo, global navigation satellite system (GLONASS), and BeiDou into consideration, the total operating satellites can achieve 98 and more [22], and the operating satellites in view can reach an average number of about 28 [23]. Sufficient satellites in view surely improve the accuracy and availability of the navigation system but also turn outa challenge to the processing capability of the navigation device, as well as the channel limitation of the GNSS receiver processor [24]. Besides, adding new satellite signals into the navigation processor not always effectively improves the performance of the equipment, especially when sufficient measurements have been considered [23; 25], as will also be illustrated in this study. Consequently, satellite selection could be an effective way to address these drawbacks. It retains the satellite signals which are most critical in the visible satellite subset while abandoning the ones which are relatively not so critical for positioning and fault detection at a cost of acceptable accuracy or integrity degradation so that the real-time requirement and low-cost demand can be satisfied. Various satellite selection methods have been developed since as early as the 1980s [26; 27; 28; 29; 30]. Most of them are carried out in the motivation of optimizing the accuracy of a fixed-size subset. However, these methods did not consider the integrity performance of the extracted satellite subset in safety-critical applications. Additionally, the subset chosen for positioning not always performs well in integrity performance such as protection level (PL) [23; 31], thus the satellite selection strategy for RAIM applications still needs further investigation. Researches have also been carried out in recent years to improve the integrity performance of chosen subset in augmentation systems such as the satellite-based augmentation system (SBAS) [25], advanced RAIM (ARAIM) [23; 31], and ground-based augmentation systems (GBAS) [32; 33]. A \"measurement downdate\" method is carried out for SBAS applications in [25], which once determines a set of impact values termed \"measurement\" for the all-in-view solution and retains the several satellites with the largest impact values to construct a superior subset for both vertical and horizontal protection levels. Later it is developed by [23] for the vertical protection level (VPL)-favorable subset in ARAIM, while a parameter optimization process is introduced to calculate a more suitable \"measurement\" for each satellite. This \"measurement downdate\" method is quite computationally efficient but lacks of evidence to show the consistency between the impact of a satellite in all-in-view solution and that in its selected subset.