ESA GNC Conference Papers Repository
In Flight Star Sensors Self Calibration Using Inter-stars Angular Separation Measurements
For some future missions the need has arisen for highly accurate star sensors, e.g. formation flying and Earth observation satellites. Optical distortion error is the main contributor of STR mean frequencies errors or FOV errors that have an important impact in a STR-based attitude restitution system. A way to improve the performance of star sensors is to perform on-orbit automatic calibrations that are more accurate than difficult and expensive on ground calibrations. Star sensors optical errors are usually calibrated on-ground using accurate measurements on optical benches. These calibration processes are quite long and difficult - especially when the desired accuracy is high. Furthermore, the environmental conditions of the calibration - especially vacuum and temperature effects - can be different from the on-orbit conditions and degrade the accuracy of the calibration. The dynamics disturbances due to vibrations during the launch can also have a non-negligible effect on optical parameters. The advantages of on-orbit calibration are obvious: accuracy, the calibration is done in exact flight conditions; cost, no need to develop very accurate optical test facilities controlled wrt temperature and pressure. Moreover, the in-flight calibration allows a long term intelligent monitoring of the optical characteristics of the star sensors. The information contained in the angular separation of two stars does not depend on the attitude and gives an information of the quality and coherence of the measurement of both stars positions.