ANTENNA DIRECTION CONTROL SYSTEM

A satellite communications system comprises signal tracking for a directional antenna by detecting any directional error of the antenna relative to a pilot station, or predetermined offset angle therefrom, to provide phase modulated error signals which are combined with frequency modulated pilot signals radiated by the pilot station in order to pass composite error and pilot signals through a single receive channel including a rotary joint coupler connecting a spinning section and a despun antenna section of a spin-stabilized satellite. Any directional error of the despun antenna about the spin axis of a spinning satellite results in sum and difference signals being produced in the receive feeds of the antenna wherein the magnitude of the difference signal is proportional to the angle of deviation of the antenna pointing direction and the pilot station. The difference signal is phase modulated in an over-coupled, double-tuned, directional filter and combined with the sum signal to produce a composite signal which exhibits amplitude modulation in synchronism with the frequency modulation of the sum signal. The amplitude modulation index is proportional to the difference signal and hence the directional error; and the phase of the amplitude modulation relative to the phase of the frequency modulation indicates the sign of the error. After passing through the rotary coupler, the envelope of the AM signal is detected and compared in a phase detector, with a reference signal derived from the FM of the composite signal. The output of the phase detector is the directional error signal which provides a tracking signal for controlling a despin motor to correct the error in pointing of the antenna.