A CCD tracker for closed-loop instrument pointing at Halley's comet

An optical tracker based on CCD technology has been developed for the closed-loop pointing of the narrow-angle imaging system of the proposed Halley's comet intercept mission at the comet nucleus. The target tracker, needed to assure the accurate pointing of science instruments as the nucleus rapidly passes the spacecraft, is a component of the pointing control system, which also includes a two-axis articulated instrument platform and high-resolution optical encoders to measure platform orientation relative to spacecraft structure. The device was designed on the basis of a CCD imaging array to calculate target centroid position based on the optical intensity of all pixels containing the image, and a processor to perform spacecraft attitude control, pointing control and target tracker control and data processing functions. The tracker has been configured to operate in both target acquisition and tracking modes. Preliminary simulation results and past experiments on similar trackers indicate that excellent tracking performance can be achieved, and a breadboard version of the tracker is currently being fabricated.