Investigation and Comparison of Horizontal Protection Level and Horizontal Uncertainty Level in FDE Algorithms.

The fault detection and exclusion (FDE) function in a GPS receiver ensures the integrity of the navigation solution and prevents the display and use of hazardous and misleading information (HMI). The FDE function is required to provide a figure of merit of its current protection capability which is known as the horizontal protection level (HPL). For primary-means GPS navigation, the FDE function is also required to provide an estimate of current horizontal position uncertainty, based on measurement inconsistency, which is the horizontal uncertainty level (HUL). There are subtle, but important, differences between HPL and HUL that have not been fully explained in the current literature. This paper compares design philosophies of HPL and HUL and examines differences between them for stand-alone GPS navigation. HPL and HSJL algorithms for snapsot parity space FDE approaches are proposed which meet requirements for all phases of non-precision flight. The performance of various HPL algorithms discussed in the literature are evaluated through detailed Monte Carlo simulation studies.