Attitude Control of Orbiting Satellites at High Eccentricity

Abstract

Stringent requirements of attitude control of a satellite in an eccentric orbit cannot be satisfied with readily available techniques. The basic limitations are raised by the kinematics of the vehicle itself, rather than by imperfections in instrumentation. It is shown that the gravity gradient field referred to an orbiting satellite is dissymmetrical around the local instantaneous vertical. This is because of centrifugal accelerations created by the rotation of the vehicle at the Schuler period within the orbital plane in following the local vertical. In an eccentric orbit, a periodic forcing function at the Schuler frequency is applied to the vehicle. The latter is supposed to have at least two unequal axes of inertia. Accordingly, the vehicle will oscillate at three different frequencies : 1) Schuler frequency of the forcing function; 2) natural frequency within the orbital plane; and 3) natural frequency perpendicularly to the orbital plane. It is impractical to damp out the imposed oscillations. An attitude prediction computer is suggested which should provide information on future attitude deviation during the flight history of the vehicle.