It takes an image of a region of the sky using a CCD-like optical camera, and compares successive images to determine how much the orientation of the satellite has drifted. This information will be used by the spacecraft pointing system to determine the actual pointing direction of the satellite at any instant as it spins at 1 rotation every 2 minutes. In other satellites, this information is sent to an attitude control system which then corrects for the drift by using on-board thrusters or other motion-generating devices to maintain the correct satellite pointing.
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