###### Abstract
The user algorithm used in ARAIM with two constellations is explicit and clear as the airborne receiver can monitor two kinds of fault modes only in most cases: the single satellite fault and single constellation fault. On the other hand, once three or four constellations are included in ARAIM, although the worldwide availability can reach nearly 100%, apparently the computational complexity will explode as hundreds of fault modes need to be monitored simultaneously in the airborne receiver. Not only the two fault modes mentioned above, but also the dual satellite fault, dual constellation fault and single satellite and single constellation fault will be monitored as more satellites and more constellations result in more possible fault modes. Once one fault has been detected, the fault exclusion is also difficult as there are so many potential fault modes and the computational burden will increase further as the exclusion function will result in a second layer detection test [20]. The recent research works showed that even with four constellations, the protection levels of ARAIM still cannot meet the requirements of Category I, which is the first step for precision approach [21]. The performance upper bound of ARAIM still stays at localizer precision with vertical guidance down to 200 feet altitude (LPV-200) disappointedly without any external data assistance. What's worse, the fault detection and exclusion (FDE) threshold and probability of hazardous misleading information will enlarge due to more fault-tolerant subsets. In summary, more constellations will increase computational complexity dramatically and there has no significant performance improvements but potential hazards. Regarding the explosive growth of monitored fault subset, many studies have recognized this problem and proposed many methods to reduce the number of monitored fault subset [22; 23]. However, there is a risk that the values of protection levels and Effective Monitor Threshold (EMT) will enlarge by increasing the assigned a priori probability of the subsets that remove many satellites. Furthermore, the excessive subsets consolidation is extremely negative for the fault detection and exclusion, as too many fault modes are combined to the constellation or orbit simultaneously and it is hard to locate the fault immediately. What cannot be ignored is that the robustness of ARAIM with two constellations is not enough. The hidden trouble is that the single constellation fault will result in a similar performance of conventional RAIM (single constellation). Then, the fault-tolerant positioning result of the single constellation subset will depend on the performance of the corresponding constellation only [24]. The worldwide availability will be badly damaged under depleted scenarios, where satellites is removed due to fault, update or in maintenance. The removed satellites will affect the geometry of the corresponding constellation. Then the impact will pass to the single constellation subset. The recent studies have verified that the depleted scenarios would hurt the ARAIM availability seriously [25; 26]. Although some response solutions are proposed, these methods mostly sacrifice the other performance like vertical positioning accuracy and cannot make up the loss completely [27; 28]. To evade the performance uncertainty and the computational complexity of ARAIM with three or four constellations and improve the robustness of ARAIM with two constellations, the concept of constellation dynamic selection is proposed and implemented for ARAIM user algorithm. Only the two constellations with the best geometry will be used for ARAIM estimation. The proposed method can get an encouraging high worldwide availability with a low computation burden. Facing satellite outage or even constellation outage, the method also shows superiority in robustness. Most importantly, the flourishing development of multi-constellation provides solid foundation for constellation dynamic selection.