A sequential on-orbit calibration method based on Extended Kalman Filter was proposed in this paper. Considering the distortion feature of most star camera optics lens, focal length of star sensor and first order radial distortion coefficient were calibrated separately with radial aberration be considered only. Results from Extended Kalman Filter were optimum estimation for calibration parameters founded on principle that Inter-Star Angles are invariants, whether measured in image frame or inertial frame. The simulation results showed that this method could respond to changes of internal parameters very soon, which would do perfectly in working situation where real time was needed. When the standard deviation of star positional noise is set to 0. 05 pixels, accuracy of 1.2 arc-second could be obtained by this scheme, and it can meet the accuracy demand given to on-orbit calibration of star sensor.
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