Synchronous Satellite Station-Keeping

Abstract

The orbit control process required to maintain a stationary orbit is called station-keeping. Station-keeping is necessary to offset the effect of perturbations, principally due to the Earth's triaxiality and lunar and solar attractions, which tend to precess the orbit normal and alter the orbital energy. These variations are both secular and periodic in nature. They are separable due to a method of orbit determination which frees the orbital elements from periodic terms. The periodic motions, whose period is large compared to the period between orbit corrections, may be effectively removed by the cyclic control necessitated by the secular effects with little additional fuel expenditure. The peak deviation of the satellite from the beam center may be reduced below a milliradian with corrections every two weeks. This can be accomplished using spin stabilization with pulsed jet control as exists on the Syncom and Early Bird satellites. Cross coupling of orbit plane correction into orbital energy may be removed by cancellation to preclude inclusion of spin axis reorientation in the control cycle. This method is suggested to reduce operational complexity. The control velocity required is less than 200 ft/sec/yr, most of which is expended to produce orbital plane changes. A water electrolysis propulsion system is described as a promising candidate for precise long life fine station-keeping since it possesses the characteristics of high specific impulse, small and variable impulse-bit, and time and usage independent performance parameters. The relatively simple spin-stabilized control system is favored over gravity-gradient and three-axis stabilized configurations from the standpoint of long term reliable station-keeping.