Abstract: Attitude determination of star tracker is executed by observing star light, which is rather weak, the optics and the imaging array are not applicative to the sun and the moon, the two strong light sources in the space, so a geometrical analytic method is put forward for star tracker to evade strong light sources. First, the sun and moon direction vectors at the observing time are calculated according to the astronomical equations and converted to the local geographic coordinate system. Then, the geometrical relationships among the direction vectors of the sun, the moon the geocenter and the conical FOV of star tracker are divided into 3 classes totally, combined with the exclusive angle parameters of the sunlight and of the moonlight designed for the baffle, the vectorial equations are built for each case to solve the command boresight orientation to direct the following attitude maneuver. Finally, the algorithm verification is carried out using STK (Satellite Tool Kit). The simulation environment is set up by selecting a spell of time and settling the boresight orientation of the star tracker, to conduct a scenario that has more chances for the Sun and the Moon lights interfering into the star tracker FOV or the baffle. After many times of tests, the commanded orientation of the boresight by the algorithm always succeeds in evading the sun, the moon and the horizon plane. This method can conduct protecting of the optics of the star tracker autonomously, selection of star observation window, and can carry out the valid starlight observation by the evasion of the Sun and the moon through attitude maneuver.

Key words: star tracker, optics, baffle, exclusive angle, attitude maneuver