Feasibility Analysis of GPS III Integrated with an Inertial System to Provide CAT IIIB Services

This paper describes a follow-on analysis of the potential for an integrated GPS III/inertial reference system (IRS) to provide Category IIIB (CAT IIIB) precision approach and landing services. Three different levels of GPS III capability are considered: 1) one without cross-link (GPS IIIA), 2) one with cross-link but without improved integrity assurance (GPS IIIB), and 3) one with cross-link also providing improved integrity assurance (GPS IIIC). The CAT IIIB landing requirements are expressed as constraints on the vertical navigation sensor error (NSE) to ensure a high probability of safe landing under both fault-free and faulted conditions. In particular the most restrictive requirement dictates the probability of missed detection of a satellite fault resulting in erroneous range measurements. GPS fault detection is performed by applying a threshold to the innovation residual (difference between predicted and measured ranges). A Monte Carlo simulation is used to estimate the missed detection performance of the integrated system. Both step faults and ramp faults are considered. The integrated system comprises an IRS with navigation grade sensors tightly coupled with a GPS receiver using pseudorange and possibly also delta range measurements to continuously update corrections to the inertial measurements. Results indicate that an inertial system integrated with a GPS receiver using delta range measurements could likely meet the CAT IIIB fault detection requirements for any step or ramp faults during the timeframe of any of the three phases of GPS III implementations. However, if the delta range measurements are not used, the CAT IIIB requirements could only be met with GPS IIIC.