Abstract: Apparatus and a related method for detecting failures of control elements, such as skewed-axis reaction wheels with closed-loop speed control, used in satellite attitude control systems, and for compensating for such failures and thereby maintaining continuity of operation. The apparatus includes a wheel loop model for simulating operation of each of the reaction wheels and speed control loops in the satellite, and for deriving an estimated wheel speed from a command speed for each wheel. By comparing the estimated speed of each wheel with the corresponding actual or observed speed, the apparatus determines whether a failure was occurred in the wheel, or in its control loop.

Abstract: An active attitude control apparatus for damping roll/yaw nutation motion within the control deadband of a satellite which is three-axis stabilized to local vertical by means of a pitch biased, body-fixed reaction wheel and offset roll-yaw thrusters. The apparatus is incorporated into a system in which large attitude motion outside the deadband is controlled by means of the offset roll/yaw control thrusters with an accompanying control mechanism for activating the thrusters in an appropriate direction when motion outside the deadband is sensed. The present apparatus includes logic circuitry apparatus to cause a second thruster activation, of the same sense, after each deadband actuated thruster firing, where such a firing in the same sense has not been triggered within a certain elapsed portion of the satellite nutation period, approximately 5/8ths of the period being preferred.