Determining high rate angular velocity from star tracker measurements

The paper focuses on the problem of determining high rate angular velocity based on star-tracker measurements only. A fully gyroless solution is highly interesting and appealing in order to reduce AOCS costs and complexity. The aim is to extend the maximum angular rate up to 20 deg/s or more, at which at least a first, coarse rate determination could still be performed. At high angular rate, the star-tracker acquires images with streaks related to the stars in the FOV of the sensor. This work aims to develop a High Rate Mode to be implemented in the APS based Star Trackers’ product line of Selex ES (AA-STR and SPACESTAR). As a result, the paper will focus on the characteristics of the APS-based star-trackers of Selex ES. The line read-out time of the APS sensor introduces a distortion in the image in which the streaks are stretched or shortened depending if the star moves, in the image plane, in the same direction or in the opposite direction of the line read-out. This effect is not negligible because the distortion induces an error of about 25 % in the estimation of the streak length. In the paper, an analysis of the APS read-out distortion is presented in order to reduce the error in the determination of the rate of the angular velocity. The angular velocity vector is estimation consists of three different information, the sign, the direction and the rate. The angular velocity direction is identified using two methods, a kinematical approach and a geometrical approach. A numerical simulator has been developed to simulate operative scenarios in order to test the algorithm. Finally, numerical results are shown assuming that only one star is detectable and considering angular rates from 10 deg/s to 25 deg/s.