Patent GB-1352085-A (GB)
Title: Satellite antenna autotrack system permitting error signals to appear at the earth station
IPC: G01S3/14
Filed: 1972-02-07
Abstract: 1352085 Radio directional control; aerials; radio signalling COMMUNICATIONS SATELLITE CORP 7 Feb 1972 [11 Feb 1971] 5561/72 Headings H4D H4A and H4L An aerial autotrack system for aligning the pointing direction of a transmitter aerial to correspond with the line of sight to a receiver aerial comprises means for generating four distinguishable electromagnetic beams, which are arranged in pairs. Said means includes first and second radiator means, each displaced in a respective plane on opposite sides of the axis of the transmitter aerial. At the receiver error signals are generated proportional to the differences in the received intensities of the beams of each pair, said error signals being proportional to the angular offsets from alignment of the transmitter aerial. Means are provided for transmitting said error signals from the receiver to the transmitter, and further means at the transmitter are responsive to said error signals to alter the alignment of its aerial in such sense as to reduce said error signals. As described, the aerial system of a communications satellite comprises a cluster of four horns A, B, C, D, Fig. 1, which feed a parabolic main reflector. The focal axis of the reflector is the pointing axis of the aerial and any directional offset is detected by the ground station with which the satellite is co-operating, whereupon said station transmits command signals to control the satellite aerial servomotors to reduce said offset. The horns are arranged in two planes, e.g. vertical and horizontal, in pairs A, C and B, D each of which is symmetrically disposed about the said axis. Each horn is excited by a pair of orthogonally disposed probes, e.g. the probes A V  and A H  in the horn A. Beacon signals of frequency f 1  are fed in phase quadrature to the probes of the horns A and C so that they radiate waves which are respectively of right and of left hand circular polarization. Similarly, beacon signals of frequency f 2  are fed to the probes of the horns B and D so that they radiate waves which are circularly polarized in opposite senses. The horizontal probes are directly coupled to the beacon signal generators, whilst the vertical probes are coupled through respective 90 degree delay lines 4, 8 and hybrid circuits 6, 10 (the latter producing a 180 degree phase difference between the vertical probes of a pair of horns). Down link communications signals of frequency f 3  are applied to the sum terminals of the hybrid circuits 6, 10 whereby the vertical probes are energized in phase, and radiate said signals as vertically polarized waves. The aerial system of the ground station, which is kept pointing at the satellite by a known autotrack system, comprises a single horn 14, Fig. 2, having probes E V  and E H  and a 90 degree phase shifter 16, said probes being respectively coupled to the input terminals of a hybrid circuit 18. When R.H. circularly polarized waves are received there is an output signal at the probe E H , whereas with L.H. circularly polarized waves there is an output signal at the probe E V . Equal amplitude output signals indicate that the satellite aerial is pointing at the ground station, whereas any difference in the amplitudes indicates an offset (Figs. 4A, 4B, not shown), and results in the appearance of a signal at the difference terminal of the hybrid circuit 18. Any such signal is separated by a diplexer 20 into components of frequencies f 1  and f 2 , which are applied to respective first input terminals of tracking receivers 26, 24. A coupler 19 applies part of the output from the sum terminal of the hybrid circuit 18 to a diplexer 22 where it is separated into components of frequencies f 1  and f 2 , which are applied to respective second input terminals of the receivers 26, 24. The outputs of said receivers respectively represent vertical and horizontal error signals. The vertically polarized communication signal waves of frequency f 3  result in equal output signals appearing at the probes E V  and E H , and these are combined at the sum terminal of the hybrid circuit 18. A command system is described in which ground station command tone carrier signals are deviated in frequency in accordance with said error signals. After modulation with the command tones, the deviated carriers are combined in a directional filter with communication signals, and are radiated to the satellite (Fig. 3, not shown). There, after separation of the communication signals by a directional filter, the command signals are coupled to discriminators which remove the carriers. The command tones and the error signals are separated by filters, and the latter are applied to suitable servo systems to drive the satellite aerial into alignment.