With constant body spin-rate control, any secular disturbance torques cause a systematic increase or decrease in wheel momentum. When die wheel momentum approaches the maximum wheel capacity or minimum desired momentum, momentum dumping or desaturation must be performed using gas jets or spin-plane magnetic coils. (See Sections 19.2 and 19.3.)

An alternative design for a momentum bias control system is illustrated by the SEASAT system which uses a pair of canted scanwheels (see Section 6.2) in the pitch-yaw plane, as shown in Fig. 18-9. The scanwheels use the pitch and roll attitude error signals to maintain closed-loop three-axis attitude control. The pitch and yaw momentum components are given by

hp = {hx^h2)cosa hy^i^-h2)sma where a is the cant angle between the pitch axis and the momentum wheels. The

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