Spacecraft actuator alignment estimation

Spacecraft attitude control has been well studied for several decades. Dozens of algorithms using a variety of techniques have been developed. One of the factors that these algorithms have in common is that they assume accurate knowledge of the actuator alignments. As with other spacecraft parameters, this knowledge will never be perfect. Whether due to finite manufacturing tolerances or warping of the spacecraft structure during launch, some alignment error will exist. Additionally, there is a possibility of launching a satellite with a sign error in at least one actuator. Whether due to a software or hardware error, this problem in and of itself can cause the spacecraft to fail. This paper develops methods for on-orbit estimation of actuator alignments. Sub-degree accuracy is demonstrated for several different scenarios. Simulation results show that alignment errors using filtered data primarily come from the finite difference approximations used to estimate the angular acceleration. Inertia knowledge errors yield alignment errors between seven and twelve degrees. While this error is significant, the rough alignment estimates provide an option for fault checking of ground-determined alignment calibrations. Overall, these approaches provide an effective means for on-orbit estimation of spacecraft actuator alignments.