Satellite Selection for Aviation Users of Multi-Constellation SBAS

N O V E M B E R / D E C E M B E R 2 0 1 6 www.insidegnss.com The advent of multiple constellations provides the opportunity to eliminate geometry weakness as a source of satellite-based augmentation system (SBAS) unavailability. GPS users occasionally encounter areas where an insufficient density of satellites exists to support all desired operations. This most often occurs when a primary slot satellite is out of service. However, adding one or more constellations easily compensates for this geometric shortcoming. In fact, we may now experience the opposite problem of having more satellites that can be tracked by a receiver. There are many possible methods for selecting a set of satellites to use for the GPS position solution. Very often, elevation angle is used to rank satellites. A receiver may sort the satellites by their elevation angle and keep k (number of receiver hardware channels) highest ones. While this choice is good from a tracking robustness point of view, it does not lead to the best availability. Ideally, when choosing from n total satellites in view, the user will be able to find k that produce protection level values that are below the required integrity alert limits. In general, for aviation SBAS users it is desirable to find an algorithm that minimizes the vertical protection level (VPL) and the horizontal protection level (HPL). A brute force search, through all combinations, yields the optimal set for a given k, but may be costly and impractical when there are many possible satellite subsets. In this article, we examine and compare several methods that are more practical than the “optimal” brute force search. One such method is a “greedy” algorithm that iteratively The incorporation of multiple constellations into satellite-based augmentation systems may lead to cases where more satellites are in view than an aviation receiver has tracking channels. This article examines methods for selecting the optimal set of satellites and further describes a method to specify performance instead of a minimum hardware channel capacity. This form of specification allows for greater flexibility in receiver design. Manufacturers would be allowed to choose between more channels combined with a simpler algorithm versus fewer channels and a more sophisticated algorithm. Satellite Selection for Aviation Users of Multi-Constellation SBAS