Patent GB-1349861-A (GB)
Title: Antenna direction control system
IPC: G01S3/32
Filed: 1971-05-28
Abstract: 1349861 Aerials; radio direction-finding HUGHES AIRCRAFT CO 28 May 1971 [2 July 1970] 18170/71 Headings H4A and H4D [Also in Division G3] An arrangement for use in a communication system includes a directional aerial for receiving a FM signal radiated from a source and comprising feed means producing individual signals of different magnitudes whenever the aerial is not pointing at the source. Means responsive to said individual signals produces a sum signal and a difference signal, the magnitude of the latter being proportional to the angular deviation of the aerial from the source. A phase modulating means operating upon said difference signal produces periodic phase reversals therein at the frequency modulation rate of the radiating, and a combining means produces a signal exhibiting amplitude modulation from said phase modulated difference signal and from the sum signal, which is indicative of said angular deviation. As described, a synchronous satellite is stabilized by rotation about its axis, which is parallel to the axis of the earth. A de-spun section is provided in which is mounted a directional main aerial. An auxiliary aerial, conveniently polarized orthogonally with respect to the main aerial, is provided for use during the launch and transfer of orbit phases. A pilot station (8), Fig. 1 (not shown), transmits frequency modulated (FM) signals which are utilized for maintaining the correct pointing direction of the main aerial. This aerial comprises a parabolic reflector 10c, Fig. 2, and two offset feed horns 10a, 10b. Pilot signals received by the horns are coupled to the inputs of a hybrid junction 11 and sum and difference signals are derived at respective outputs 11a, 11b thereof. The difference signal will be zero when the aerial is pointing towards the station 8, and will increase in magnitude as the pointing direction diverges from the direction of said station, and will be in phase or in anti-phase with the sum signal according to whether the aerial is pointing to the right or the left of the station. The said difference signal is coupled to a directional filter 13 which is associated with a matching impedance 13a, and which is arranged to provide periodic phase reversals of the signal at the frequency modulation rate thereof. For example, if the peak-to-peak deviation of the pilot signal is 4 MHz, then the 180 degree peak-to-peak phase shift bandwidth of the filter 13 is also 4 MHz. The difference signal is thus phase modulated in passing through the filter 13 and is then combined with the sum signal from the output 11a of the hybrid junction so that a composite signal results which exhibits an amplitude modulation in synchronism with the frequency modulation of the pilot signals. The amplitude modulation index is proportional to the magnitude of the difference signal, and hence to the magnitude of the directional error, and the phase of the amplitude modulation signal relative to the phase of the frequency modulation signal is determined by whether the aerial is pointing to one side or the other of the pilot station (Figs. 3a, 3b, not shown). The composite signal is coupled through a rotary joint 7 to equipment in the rotating section of the satellite. Signals originating from the pilot station are separated by a directional filter 15 and are coupled by means of a hybrid junction 16 to duplicated pilot receivers 18, 20. The input to a receiver is coupled to an AM detector 26 after passing through a mixer 21 and an amplifier 23. The envelope of the amplitude modulated IF signal is detected by the AM detector 26 and is compared in a phase detector 28 with a reference signal supplied by a frequency discriminator 24. The output of the phase detector 28 is proportional in amplitude and related in sign to the directional error of the aerial 10. Said output is coupled through an amplifier 29 to the de-spin motor 30, which is thereby controlled to bring the aerial towards its correct pointing direction.