High integrity GPS/INS filter for precise relative navigation

A GPS/INS filter design for absolute and relative state estimation is discussed. The GPS code and Doppler measurements are used for absolute state estimation while the GPS carrier phase and Doppler measurements are used for relative state estimation. Real-time results obtained from a Hardware-in-the-Loop Simulation and actual F-18 flight tests are presented. To ensure accurate state estimation, the observability of the GPS/INS system is analyzed analytically when the system is at rest and during maneuvers. The system observability enhancement during two different maneuvers is discussed. Simulation results illustrating the observability of the system during the different stages of the GPS/INS system operation are also presented. To allow successful use of the high-accuracy carrier phase measurements, two methods for resolving GPS integer ambiguity are introduced. The first combines an efficient method for obtaining the admissible integer ambiguity hypotheses within a probabilistic volume with an integer hypothesis testing method to reduce the convergence time with high probability to the GPS carrier phase integers. The second GPS integer ambiguity resolution method is based on a log-formulation of the Multiple Hypothesis Wald Sequential Probability Test (MHWSPT). The computational time requirement of the latter method is shown to be smaller than the former method. Real-time results are presented to show the performance of the two methods introduced.