Preservicing mission - On-orbit modifications to Hubble Space Telescope pointing control system

Because of unexpected thermally induced disturbances originating in the solar arrays (SAs), the Hubble Space Telescope was initially unable to meet its pointing error specifications of 0.007 arc-s (root mean square) for observations lasting from a few seconds to several hours. The effort to pinpoint the mechanism causing the problem and to redesign the onboard controller to accommodate the disturbance began shortly after deployment. After controller modifications, the pointing errors were well within the specification for all of the orbit except for short intervals during transitions in and out of the Earth's shadow. This paper presents a chronology of the redesign process and flight data showing the achieved pointing performance. During the December 1993 servicing mission the SAs were replaced with arrays of a modified mechanical design. A preliminary assessment of pointing performance since that time is presented. (Author)