Spacecraft attitude sensor calibration from on-orbit experience

Soon after launch, most spacecraft go through an in-orbit checkout phase. Activities during this phase often include calibration of the attitude sensors. Occasionally, recalibration is required during the life of the mission. This paper contains a description of the types of calibration that are performed, the reasons for performing them, factors that must be considered when planning calibration, and the limitations that attitude sensor calibration requirements place on autonomous operation. Illustrations are given from experience at NASA's Goddard Space Flight Center. The types of calibration described include alignment calibration, determination of parameters in transfer functions, and calibration of attitude rate sensors (gyroscopes). As a rule, alignments do not significantly change after the first few days of a mission. Biases and scale factors, on the other hand, have been known to drift and in some cases even alignments have shifted significantly. Any plan for autonomous operations may be limited by the need for monitoring, and perhaps recalculating, these parameters. Onboard, autonomous calibration appears to be a much more distant goal than are other autonomous attitude functions.