EVERYTHING IS RELATIVE IN SYSTEM ALIGNMENT CALIBRATION

Much of the literature on alignment calibration is based on the concepts of absolute and relative alignment calibration. Three degrees of freedom of attitude associated with absolute alignment calibration are unobservable unless payload data is processed. We show in this paper that an absolute alignment model is equivalent to a relative alignment model when the payload is regarded as an attitude sensor. We show that the extra degrees of freedom are eliminated by defining the gyro as the body reference frame, attributing only three non-orthogonal misalignment parameters to the gyro. The payload misalignment can then be parameterized and calibrated in exactly the same manner as any attitude sensor, or this can be left strictly to the payload data processing, thus creating a well-defined boundary between attitude control system calibration and payload calibration. The new parameterization introduced in this paper is illustrated via simulation results.