Online calibration for star trackers

Star trackers are perhaps the most accurate means of measuring a spacecraft's orientation in space and are becoming a popular sensing instrument for attitude determination systems amongst conventional larger satellites as well as micro satellites. In order to produce and maintain high fidelity measurements, the systematic effects of lens distortion and possible sensor alterations due to environmental changes and instrument aging must all be accounted for through calibration, both on the ground and on orbit. In this study, a calibration method is presented to account for errors in star camera parameters, namely the focal length, bore sight offset, higher order radial distortion terms and the tip and tilt of the detector array in relation to the lens arrangement. This method does not depend on a costly high-precision lab setup; instead it simply employs the star camera images and a star catalogue to calibrate the instrument given reasonable initial estimates. This allows for a reduction in pre-mission calibration requirements and is feasible for an online implementation, allowing the star tracker to calibrate itself through out its life-cycle.