Hubble Space Telescope precision pointing control system

The Hubble Space Telescope has the most stringent pointing requirements imposed on any spacecraft to date. The overall HST pointing stability shall be 0.007 arcsec rms or less. The Pointing Control System uses fineguidance sensors and rate gyros for attitude reference and rate information. Control torques are provided by reaction wheels. A digital computer collects the sensor data, performs the control law computations, and sends torque commands to the reaction wheels. To attain this precision pointing, improvements were made to the rate gyros to lower their noise characteristics and to the reaction wheels to reduce their induced vibration levels. The control system design was validated in a test sequence that progressed from model verification tests on an airbearing to operations-or iented, closed-loop testing on the assembled vehicle. A test system is described that allowed the simultaneous production of test-case command loads for the flight computer and plots of predicted profiles to assist in test data analysis. Testing and analysis indicated that the HST will be capable of meeting the requirements for precision pointing.