Alignment Estimation for Terrestrial Multihead Star Trackers Based on Starlight Direction Stability

Under terrestrial observation conditions, the alignment estimation for multihead star trackers (MHSTs) is significantly influenced by atmospheric refraction, which is challenging to fully compensate for. However, we find that both determining factors of atmospheric refraction, meteorologic conditions and zenith direction, change slowly, resulting in the refracted starlight remaining high directional stability. This stability implies that atmospheric refraction can be eliminated by certain relative measurement approaches as a common bias. A feasible relative measurement is the relative rotation of an optical head (OH) estimated by the shared stars observed across consecutive exposures. Due to misalignments, there are discrepancies in the rotations of the MHST main body that are calculated individually from the rotations of different OHs. We derive a linear equation to correlate the discrepancies with the misalignments, upon which an alignment estimator is established. The estimator is independent of specific atmospheric models or assumptions, allowing alignment estimation without the need to correct for atmospheric refraction. In addition, we analyze the effects of motion parameters of calibration maneuvers on the estimator sensitivity, through which some design guidelines for calibration maneuvers are presented. Finally, experiments and simulations were conducted. The results indicate that the accuracy of the proposed estimator was superior to 2.0 arcsecs.