4. Attitude and Orbit Control The control system must compensate for all forces acting on the SPS, both orbit perturbations and attitude disturbances. Orbit perturbations are discussed in section IV-A-3. The predominant attitude disturbances are gravity gradient torques of two kinds: short-period (12 hour) torque about an axis perpendicular to the orbit plane, and long-period torque about an axis in the orbit plane (see fig. IV-10). Other attitude disturbances include solar radiation pressure and microwave recoil (if not acting through the center of mass) and antenna angular accelerations (if eccentricity or inclination is not zero). Figure IV-10.- Gravity gradient torques.
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