Everything is relative in spacecraft system alignment calibration

The concepts of absolute and relative alignment calibration of spacecraft attitude sensors are examined. It is known that three degrees of freedom of attitude associated with absolute alignment calibration are unobservable unless payload data are processed. It is shown that an absolute alignment model is equivalent to a relative alignment model when the payload is regarded as an attitude sensor. Then it is shown that the three unobservable degrees of freedom are eliminated by defining the gyro as the body reference frame, attributing only three nonorthogonal misalignment parameters to the gyro. The payload misalignment can then be parameterized and calibrated in 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 is illustrated via simulation results.