High angular rate determination algorithm based on star sensing

The paper focuses on the problem of determining high rate angular velocity directly from star-tracker measurements. A fully gyroless solution would result in reduced AOCS costs and complexity. The aim is to extend the maximum angular rate magnitude up to 20 deg/s or more, at which at least a first, coarse rate determination could still be performed. At a 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 on Selex ES’ APS based Star Tracker line of products (AA-STR and SPACESTAR). The APS sensor line read-out time introduces an image distortion 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 length of the streak. In the paper, an analysis of the APS read-out distortion is presented in order to reduce the error in the determination of the magnitude of the angular velocity. The angular velocity direction is identified using 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. At lower rates, different algorithms (simpler) will be used for the rate determination. These are not discussed in this paper. The Rate Mode under development will allow to achieve reliable and accurate rate measurements necessary for de-tumbling satellites and for star trackers to switch back to tracking mode.