Attitude determination and control of Clementine during lunar mapping

The Clementine spacecraft, launched in early 1994, recently completed a two-month lunar mapping mission in which it recorded high-resolution images over the entire surface of the moon. The attitude determination and control subsystems developed for the mapping phase of the mission are described. The vehicle's attitude determination subsystem consisted of a simple fixed-gain Kalman filter, uncoupled about each spacecraft axis, combining rate inputs from two solid-state optical gyros and attitude inputs from two wide field-of-view star trackers. The attitude control subsystem consisted of a simple proportional-integral-derivative algorithm operating on quaternion and rate feedback signals to provide pointing, tracking, and slewing functions. The primary actuating devices were three reaction wheels used for feedback error compensation and 12 thrusters used for occasional momentum dumping. During lunar mapping the attitude determination and control subsystems maintained vehicle pointing to within a three-axis rss value of 0.05 deg. On-orbit results are provided from outputs of the attitude determination filter, the lunar nadir-tracking controller, and the large-angle slew generator. Additionally, results are provided from a novel inertia estimator, which utilized sensor outputs during two slew maneuvers.