Patent GB-1003157-A (GB)
Title: Improvements in or relating to radio guidance systems for aircraft
Inventor: WALKER PHILIP HULME
IPC: G01S1/54
Filed: 1962-03-29
Abstract: 1,003,157. Aircraft radio guidance systems. DECCA Ltd. June 28, 1963 [March 29, 1962], No. 12178/62. Heading H4D. A radio system for the guidance of an aircraft along a required track, comprises a radio transmitter located at or beyond one end of said track and having a directional aerial system to radiate two narrow overlapping directional beams, and to oscillate said beams about the track line, in the plane in which guidance is required, and, on the aircraft, means for receiving signals radiated from said aerial and means responsive to the differences in time periods between successive simultaneous reception of signals of equal intensity from both beams, as the overlapping region of the beams scans across the aircraft, to determine thereby the deviation of the aircraft from the required track line. The transmitter 10, Fig. 1, comprises a trigger generator 11 pulse modulating a magnetron 15 directly, to cause a train of pulses to be radiated by aerial 17, and also pulse modulating a magnetron 16 via a delay 14, to cause a train of pulses to be radiated by aerial 18, lagging that radiated by aerial 17. Aerial 17 radiates a beam 19, Fig. 2, whilst aerial 18 radiates a beam 20, overlapping beam 19. The two beams are oscillated in azimuth, about the required track between angular limits defined by lines 21 and 22. The aerial arrangement may be as described in Specification 816,244. The two trains of pulses are received at the aircraft by aerial 31 and are detected and amplified by diode 32 and video amplifier 33 respectively. The pulses produced at the output of amplifier 33 is passed to the inputs of a coincidence gate 34, directly to one input, and via a delay 35, having the same delay time as delay 14, to the second input. The arrangement is such that, at the inputs to the coincidence gate, the pulses in the output of amplifier 33, due to aerial 17, are brought into phase with those due to aerial 18, the gate 34 therefore producing a train of pulses in phase with those in the amplifier output due to aerial 18. The output of amplifier 33 is also passed to gates 36 and 37, which are controlled by the output of gate 34, such that gate 36 is opened on receipt of a control pulse and gate 37 is closed on receipt of a control pulse. The output of gate 36 therefore consists solely of the pulses due to aerial 18 and output of gate 37 therefore consists solely of the pulses due to aerial 17. The amplitudes of the pulses in the outputs of the two gates vary in accordance with state of the respective beam as it sweeps past the aircraft. The pulses in the two outputs will therefore be of equal amplitude when the plane of overlap of the two beams passes through the aircraft. To detect these times of equality, the outputs of the gates are passed to two peak detectors 40 and 41 to produce outputs such as shown at 56 and 57. The outputs are fed in turn to a comparator 42 which produces a wide pulse output shown at 43, a leading or trailing edge of these pulses occurring when the inputs to the comparator are equal. The mark/space ratio of output 43 will depend on the position of the aircraft in the angular region defined by the limits 21 and 22. Thus an aircraft B, Fig. 2, will have a one to one mark/space ratio, since, as shown in Fig. 4, which illustrates the time variation of the intensity of the transmitter radiation received by the aircraft, the times T 1  and T 2 , between successive passages through the aircraft of the plane of overlap of the beams, indicated by the dotted lines, are equal. For an aircraft A, T1 < T2, whilst for an aircraft C, on the opposite side of the required track to aircraft A, T1 > T2. To produce a signal proportional to the mark/space ratio, output 43 is passed to a differentiator 44 which produces at one output a positive pulse (waveform 45) for every leading edge of a pulse in waveform 43, and at the other output, a negative pulse (waveform 46) for every trailing edge. The differentiator outputs are sent to control a flip/flop 47, which produces an output waveform similar to waveform 43. The differentiator outputs are also inverted in inverters 50 and are then used to control a flip/flop 51 which produces a waveform wherein pulses replace the spaces of waveform 43. Amplifiers 48 and 52 amplify the outputs of the flip/flop and extract the average values therefrom. The difference in the two average values represents in magnitude and sign the value of T2-T1 and thus the deviation of the aircraft from the required track. This difference or error signal is applied directly to a course reading instrument 53, and, via an amplifier, to a fine reading instrument 55. Automatic gain control is preferably provided to ensure that the two gate/peak detector channels have the same gain and amplitude control is preferably used at the transmitter to ensure equality of the radiated powers of magnetrons 15 and 16. The same transmitting valve may be switched alternately to the two aerials 17, 18. For guidance in elevation an asymmetrical scan is preferably used. Instead of using pulse modulation, continuous wave signals may be radiated modulated sinusoidally at different frequencies. A monitoring and connection system may be employed in order to ensure that the amplitudes of the two beams are correctly matched. As a check the feeds to the two magnetrons may be reversed for every one scan in ten and any change in the magnitude of the indicated track deviation, other than the change of sign, noted. For checking operation in an approved or landing aid for an aicraft, an alignment checking ground monitor may be provided on the required track close to the touchdown point, similar to the airborne receiver. The said monitor may be coupled with a radar aligned along the glide path. The system may be used in an automatic landing system.