A Simple Satellite Exclusion Algorithm for Advanced RAIM

Abstract

Advanced Receiver Autonomous Integrity Monitoring is a concept that extends RAIM to multi-constellation and dual frequency that could provide worldwide coverage of vertical guidance [1], [2]. A baseline algorithm for the user receiver has been described in [3]. This algorithm includes the computation of the Protection Levels, the Effective Monitor Threshold, the accuracy, and the description of a preliminary exclusion algorithm, which was refined in [4]. The exclusion function helps maintain availability through the duration of a fault or to meet the continuity requirements. In our previous work [4], we had proposed an exclusion algorithm for ARAIM whose goal was to provide a set of conditions under which a fault (satellite or group of satellites, for example) can be excluded and treated as if it was flagged by the ground. This property allows the receiver to treat the satellites remaining after exclusion as an all-in-view situation, which greatly simplifies the processing. To achieve this goal, that algorithm used exclusion confirmation tests. The thresholds of the exclusion confirmation tests were tied to the Protection Level equation. There were two minor drawbacks in this algorithm. First, the exclusion confirmation tests, although not computationally expensive, require a complex description and a somewhat complicated logic. Second, when the exclusion confirmation tests do not pass, (that is, in practice when there are other consistent sets,) there can be a momentary loss of performance due to the ambiguity in which is the correct exclusion. In this paper we present an evolution of the exclusion scheme presented in [4] that, while retaining its qualities, solves the first issue and mitigates the second one. To address the first one, the logic of the algorithm has been simplified: it now uses the same function to compute the Protection Levels whether exclusion has been performed or not. The second issue, that is, the loss of performance, is mitigated by exploiting the ambiguity as to which satellite must be excluded. In this new algorithm, the exclusion confirmation tests are now only an output of the all-in-view Protection Level calculation. We will present a step by step description of the algorithm, and the associated analytical proof of integrity. To show the performance of the algorithm in a realistic setting, we will test the algorithm using GPS and GLONASS measurements (both with real faults and injected faults).