Modified Horizontal Protection Levels for ARAIM

Abstract The baseline algorithm for horizontal advanced receiver autonomous integrity monitoring (HARAIM) provides a reference method for computing a horizontal protection level (HPL) for aircraft en-route navigation safety requirements. The baseline HPL bounds the radial positioning error in three steps by iteratively solving for an east protection level (PL), solving for a north PL, and then combining the two PLs. Each iterative process optimally allocates an integrity risk requirement across fault-free and fault hypotheses. We develop two new HPLs that require the iterative process to be performed only once. One approach is more compact, whereas the other provides a tighter bound than the baseline. Additionally, we derive a theoretical generalized chi-square horizontal reference boundary for analytical purposes. Using simplified single- and two-hypothesis examples, we assess how satellite geometry and measurement error model parameters impact the HPL bounds. Finally, we conduct a worldwide analysis comparing the proposed HPL approaches against the baseline for an example HARAIM-based aircraft navigation application.