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A General Observation Matrix for Attitude Error Estimation with an Offset GPS Antenna

Abstract : In conventional integrated GPS/INS navigation systems, attitude errors are estimated through correlation to, primarily, velocity errors. This correlation develops due to the coupling of the attitude errors to the velocity errors via the presence of specific forces. Other GPS-based attitude measurement techniques typically employ multiple antennas and special receiver hardware. An alternate approach to attitude estimation is proposed. This report exploits the attitude (and attitude rate) information inherently present in pseudo-range and, particularly, delta-range measurements obtained when a GPS antenna is offset from the GPS/INS reference point of a platform undergoing attitude changes, independent of the value of specific forces. A more general observation matrix that captures this information is derived. The attitude information is recovered with standard recursive estimation techniques such as the Kalman Filter algorithms typically used in these systems. The proposed approach is ideally suited for applications where the antenna is mounted on a spinning platform, such as a spinning vehicle in exoatmospheric, free-fall conditions. Simulation results are provided to illustrate the attitude estimation capability in a free- falling vehicle application. The technique developed here has the advantage of being readily implementable through minor software modifications (i.e., there are no special hardware requirements).