The Challenge of Implementing Reaction Wheel Biases on the Cassini Spacecraft During Extended Mission Sequence Development

Abstract

During its orbital tour of the Saturnian system, the Cassini spacecraft must point its instruments at a variety of targets while under reaction wheel assembly (RWA) control. To preserve the long-term health of the reaction wheels, they must frequently be biased—that is, the wheel speeds are altered to redefine the spacecraft's angular momentum vector while thrusters maintain the spacecraft attitude. The biases (either standalone or part of other activities on thruster control) are defined early in sequence development to divide the science sequence into multiple segments, which are carefully examined for the presence of undesirable reaction wheel rate conditions. In many cases, instead of redesigning science activities, the most efficient solution is to introduce additional biases. Moreover, as the Cassini mission has transitioned from Prime to Extended Mission, the number of biases required per sequence has increased. To ease the challenge of managing these RWA biases, several tools were developed to automate the arduous steps in this RWA bias implementation and simulation process. This paper describes the RWA biasing strategy and bias automation tools, and demonstrates how the integrated tools ensure the successful execution of RWA biases in flight.