It has been approximately 6.4 years since the Gyroscopes on HST have been replaced. During this time two Gyroscopes have failed and two have developed problems, but operational work-arounds are available. Further Gyroscope replacement will not occur until the anticipated Shuttle Servicing Mission scheduled for November, 2007. To extend the science mission life of HST up to an additional 15 months, the control system has been modified from a three/four-Gyro, to a two-Gyro control law. This paper describes the new two-Gyro Guide Star acquisition strategy to enable science observations, including the integration of Astrometry commanding with the Acquisition Logic.
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