Star Tracker Alignment Determination For Resourcesat-I

Two identical star trackers, flown on Resourcesat-I spacecraft, provide star vector measurements, attitude quaternion to Attitude and Orbit Control System (AOCS) for spacecraft pointing and control. The star vector measurements of brightest five stars, and attitude quaternion are telemetered to ground. Initially the pre-launch measured alignment angles were commanded. The internal misalignments of the trackers (between optical axis the optical cube) were not considered. This resulted in small difference between attitudes measured by two trackers. Since only one of two trackers was used in closed-loop attitude control, the misalignment reflected as pointing error in imageries. Star tracker alignment angles were determined iteratively, using data from both star trackers, and commanded in subsequent two phases. A program to process on-orbit star vector measurements was developed. Using this program the absolute alignment angles are arrived at iteratively with a goal of reducing the attitude difference between the two trackers. Each of the iterations resulted in reducing pointing errors of imageries and final pointing error improved to less than 200 meters. This paper presents these tracker alignments and results obtained.