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

This paper describes a preliminary analysis of the potential for an integrated GPS IIIC/inertial navigation system to provide Category IIIB (CAT IIIB) precision approach and landing services. The CAT IIIB landing requirements are expressed as restrictions 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 range fault. The integrated system comprises a navigation grade inertial sensor tightly coupled with a GPS receiver, using pseudorange and delta range measurements to update the inertial 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 missed detection performance of the integrated system. Both step faults and ramp faults are considered. Results indicate that the integrated GPS IIIC/inertial system as modeled could likely meet the CAT IIIB fault detection requirements for any size step or ramp without even using delta range measurements.