Patent GB-632357-A (GB)
Title: Improvements in and relating to radio systems and apparatus particularly for use in aircraft landing
IPC: G01S1/02
Filed: 1944-11-17
Abstract: 632,357. Radio navigation. SPERRY GYROSCOPE CO., Inc. Nov. 17, 1944, No. 22922. Convention date, Sept. 3, 1943. [Class 40 (vii)] A radio-controlled landing system for aircraft comprises an omnidirectional beacon which defines a circular course where the intensity of the signals received by the aircraft is constant, and overlapping beam equi-signal glide-path and localizer beacons which, when the aircraft enters the zones covered by these beacons, causes relay systems in the aircraft to operate to render the receiver unresponsive to the indications provided by the omnidirectional beacon and responsive to those provided by the localizer and glide-path beacons. The omnidirectional signal, which is given a frequency modulation to prevent the formation of a stable interference pattern with the localizer beam, is radiated with either vertically or horizontally polarized radiation on the same frequency as the localizer beam which is horizontally polarized. The glide-path beam which is also horizontally polarized is transmitted on an adjacent frequency. These signals are received on the aircraft by an aerial 29, which is adapted to receive both horizontally and vertically polarized signals if necessary, and are frequency changed by the mixer 37 and local oscillator 34 and applied to the I.F. amplifiers 38 and 46, 38 being tuned to receive the glidepath signals and 46 the omnidirectional and localizer signals. The signals detected at 39 and 47 are applied to A.V.C. detectors 42 and 49 which thus operate upon the intensity of the modulation. A.V.C. signals are fed back to each I.F. amplifier and also to the audiofrequency amplifiers. 41 and 48 to provide effective broadening of the beams as the beacons are approached. The outputs of the audiofrequency amplifiers are applied to the meter circuits 43 and 51 which develop voltages whose magnitude and sign depend upon the deviation of the aircraft from the course defined by the beams. These voltages are applied to a relay circuit 52 which is controlled by the output from the A.V.C. detector 49 and are passed, when the relay is suitably actuated, to a cross-pointer indicator and/or automatic pilot 31. The output voltage from the detector 47, which is proportional to the received carrier amplitude and is therefore a measure of the range from the omnidirectional beacon, and that from the unit 56 which generates a voltage proportional to height, are also applied to the relay 52. Normally, when the localizer and glide-path beams are not being received, the output from the A.V.C. detector 49 is insufficient to operate the relay 52 and the signals from 47 and 56, suitably backed-off by biassing voltages, are applied to the indicator 31 to give a central indication when the aircraft is flying at a predetermined height and range. When the aircraft enters the localizer beam the output from the A.V.C. detector 49 causes the relay 52 to operate and consequently apply the meter circuit outputs to the cross-pointer indicator to enable the aircraft to follow a landing-path. The relay 52 may alternatively be actuated by the output from the meter circuit 51, Fig. 9 (not shown). The omni-directional beacon may radiate an amplitude-modulated signal, the output from the detector 47 being amplified, filtered and rectified to provide a voltage indicating the range. To prevent the apparatus from coming into operation when the aircraft is far from the beacon and giving false indications, especially when the craft is being flown by the automatic pilot, means may be provided for rendering the indicator and/or automatic pilot inoperative when the signals received are below a certain strength. The omnidirectional beacon need not radiate on the same frequency as the localizer beam; if a different frequency is used no frequency modulation need be provided.