Fault Free Integrity of Mid-Level Voting for Triplex Differential GPS Solutions
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Landing systems for large unmanned air vehicles have stringent integrity requirements as well as demanding system continuity requirements that often lead to triplex avionics architectures. Triplex avionics architectures are designs that have triple redundancy for key functions. Mid-level voting (MLV) algorithms that select the median value from among the three solutions are commonly used to select among available sensors and navigation solutions. When each solution is computed using a single suite of avionics, such median values are robust to single airborne sensor failures and provide improved unfaulted accuracy as well. Robustness to single faults results because a single faulted sensor will not impact the solutions computed by the other two sets of avionics. System accuracy is improved for zero mean error solutions because the median value is more concentrated about the truth than any of the single solutions. When performing fault tree analysis for integrity risk in the unfaulted case, it is common to treat sensors’ errors as being mutually independent. In the case of multiple carrier phase differential GPS (CDGPS) solutions, this assumption is invalid due to common atmospheric errors and common reference receiver errors. This paper aims to quantify the unfaulted integrity risk from triplex correlated CDGPS solutions for float, fixed, and almost fixed baselines that use a MLV algorithm. The bound on the integrity risk is compared with that of independent solutions to show the impact of incorrectly assuming independence of CDGPS solutions. Triplex performance is compared to simplex to show improvement or degradation in unfaulted availability of integrity.