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  • A passive gravitational attitude control system for communication satellites

    Paper ID



    • J.W. West


    Bell Telephone Laboratories






    An earth-pointing attitude control system is described which fulfils the basic requirements for orienting a commercial satellite repeater. The attitude control method discussed is particularly adaptable to a satellite communication system employing randomly spaced satellites at orbital heights in the neighborhood of 6,000 NM. In order to launch perhaps twenty satellites in a reasonable time and considering economic factors, it Is essential that more than one satellite be launched per vehicle. Since existing launch vehicles place restrictions upon the size and weight of satellites. It Is vital that the satellites be as small and as lightweight as possible. Another basic requirement Is long life in order that the attitude control system not be the life limiting factor of the satellite. The foregoing requirements for the attitude control system are met in the following ways. To minimize satellite size and weight, the satellite’s radiated power is directed to cover the earth. The design of the orientation hardware is compatible with multiple launch so that several satellites can be ejected from a single vehicle. Since the attitude control system is entirely passive, the satellite should remain earth-pointing indefinitely» No active controls or sensors are needed to accomplish the earth-pointing function. The particular passive gravity-gradient orientation system described consists of two bodies. The main body, which contains the satellite, is dumbbell-shaped and tends to align itself with the local vertical. A second body, which is connected to the main dumbbell through a damping unit, is employed to provide relative motion between the two bodies thereby affording damping. Damping of satellite librations or tumbling motions is accomplished by the hysteresis effect between small permanent magnets and permeable discs. Hysteresis damping is preferred over other methods because it is amplitude- dependent and is insensitive to temperature changes. A qualitative description of the basic principles of the attitude control system is presented. The type of hardware required for the system is described as well as the results on the testing of various components, such as? the ''' damping mechanism, etc c The various perturbing torques which act upon the satellite and their influence on the design of the attitude control system are reviewed. The performance of the system under various launching conditions has been simulated on a computer and typical results are presented graphically.