Simplified processing of star tracker commands for satellite attitude control

A method for using gimbaled star trackers to obtain useful attitude control signals is presented. Gimbaled trackers are required if the spacecraft is to be pointed in an arbitrary direction and if any suitable star is to be chosen as a guide star for the star tracker. With the trackers locked onto their guide stars, deviation of the gimbals from their commanded angles provides a measure of the error in vehicle attitude. Since the errors are generated about axes which are not necessarily mutually orthogonal nor aligned with the spacecraft's control axes, they must be processed somehow to make them useful as control signals. The study considered here develops a realizable approximation of the nonlinear processor required to generate vehicle attitude errors from the gimbal angle error measurements. The approximate processor is simple enough for use on board current spacecraft and allows satisfactory performance over as wide a range of star tracker gimbal angles and satellite attitudes as desired.