  Star sensor's image plane can have three kinds of displacement after a long
time working in space, and the displacements are the principal point drift,
incline displacement and rotation displacement. These displacements can
severely decrease star sensor's measuring accuracy, therefore it's necessary to
analyze and calibrate them. The previous researches have only considered the
principal point drift of image plane, which is three-degree-of-freedom. In
contrast, the image plane displacements under the rest three degrees of
freedom, that are the incline displacement and the rotation displacement, have
been modeled in this paper. These two kinds of displacement's influences on
star sensor's accuracy have been analyzed. And the necessity to calibrate them
has been pointed out. At last, the Extended Kalman Filter has been used to
on-orbit calibrate the six-degree-of-freedom image plane displacement. And the
simulation results reveal that the on-orbit calibration algorithm can
effectively calibrate the image plane displacement of star sensor. The
measuring accuracy of star sensor has been increased to 0.23'' after the
calibration. Therefore the new six-degree-of-freedom image plane displacement
model has made up the deficiency of the conventional displacement model and
enhance the performance of star sensor greatly.
