Patent GB-750436-A (GB)
Title: Radio navigation system
Inventor: SHEARER DOUGLAS GRAHAM
IPC: G01S1/02
Filed: 1953-08-18
Abstract: 750,436. Radio navigation. SHEARER, D. G. Aug. 18, 1953, No. 22755/53. Class 40 (7). In a radio navigation system a mobile beacon station, e.g. an aircraft, transmits a radio frequency F1, e.g. less than 600 Kc/s, which is amplitude modulated with frequencies of 3,000 c/s and 30 c/s such that the phases of the modulation signals as detected at a distant beacon receiver are dependent on the direction of the beacon receiver from the aircraft in two perpendicular planes respectively, e.g. azimuth and elevation; a 1,500 c/s reference modulation signal synchronized with the direction-dependent signals may also be transmitted on the same carrier, the phase of the signal being independent of direction. The modulation signals received at a plurality of spaced beacon receivers are correlated at a control station to give a video signal which is modulated on to a V.H.F. carrier F2 and transmitted back to the aircraft to produce a visual representation of the locations of the beacon receivers as seen from the aircraft. Such a representation may also be produced at a ground monitor station by transmitting the display signals from the aircraft back to the monitor station. The invention is described as applied to an aircraft landing system in which the beacon receivers are spaced round the landing area and the visual representation in the aircraft displays images of the beacon receivers or a televised image of the airfield or a model thereof, the position and orientation of the display being dependent on the position and altitude of the aircraft relative to the airfield. The invention is also applicable to (1) facilitate navigation of boats in restricted waters and of aircraft along a route defined by spaced beacon receivers, and (2) an aircraft-to-aircraft manning system. Aircraft transmitter. In one embodiment, Figs. 4 and 6 (not shown), one output from the R.F. oscillator (frequency F1) is modulated in phase quadrature without suppression of the carrier by an output from a 3,000 c/s oscillator, the two outputs being applied respectively to crossed vertical loops 30, 32 which intersect on a vertical axis 39. Another output from the R.F. oscillator is modulated in a similar manner by a 30 c/s signal obtained by frequency dividing an output from the 3,000 c/s oscillator, the two modulated outputs being applied respectively to two crossed loops 37, 38 which are each inclined at 45 degrees to the horizontal and which intersect on a horizontal axis 40 perpendicular to axis 39. Both outputs from the R.F. oscillator are modulated cophasally by a 1,500 c/s reference signal obtained by frequency dividing a third output from the 3,000 c/s oscillator. For test purposes it is possible to apply the 3,000 c/s modulated signal to loop 32 only and the 1,500 c/s modulated signal to loop 30 only. In a modification, Fig. 5 (not shown), the 3,000 c/s and 30 c/s signals are generated by two-phase alternators and the 1,500 c/s signal is generated by an angle phase alternator, the three alternators being driven by a single motor. Beacon receivers. As shown, a plurality of beacon receivers 45 ... 45M are spaced around the landing area and the detected modulation signals are transmitted by line to a control station 331, Figs. 7, 11 and 12 (not shown). At the control station the 3,000 and 30 c/s modulation signals from each beacon receiver are separated by filters, the 3,000 c/s signal being converted to a 6,000 c/s pulse train (pulse duration 0.8 Ás, pulse period 167 Ás) which is. gated by a 30 c/s pulse train (pulse duration 167 Ás) produced from the 30 c/s modulation signal to give a 30 c/s train of position pulses (pulse duration 0.8 Ás), the position pulses derived from all the beacon receivers being applied to amplitude modulate (75 per cent modulation) a transmitter of frequency F2. The 1500 c/s reference signal from a selected single beacon receiver is converted to a 6,000 c/s sync. pulse train (pulse duration 0.8 Ás) which also amplitude modulates the ground transmitter (100 per cent modulation). A beacon receiver linked to the control station 331 by line or radio may be mounted on objects moving on the airfield and similar receivers linked to the control station 331 by radio may be installed on other aircraft. Phase shifters are inserted in each of the 3,000 and 30 c/s signal channels to compensate for the different distances of the beacon receivers from the control station 331 and the phase shifters may be adjusted so that the transmitted position pulses correspond to a selected approach path, i.e. the effective location of the beacon receivers is thus adjustable. Aircraft receiver, Figs. 13 and 14 (not shown). The sync. and position pulses received from the ground transmitter are separated by an amplitude discriminator, the sync. pulses being applied to synchronize the 6,000 c/s horizontal time base of a cathode-ray tube whose 30 c/s vertical time base is synchronized by a signal from the 30 c/s modulating signal at the aircraft transmitter and the position pulses being applied to intensity modulate the cathode-ray tube to produce images 651, Fig. 3, representing the location of the beacon receivers as seen from the aircraft. The relative amplitude of the position pulses may be controlled at the ground control station to create an illusion of depth and the absorbent amplitudes may be controlled so that the displayed images are brighter if the aircraft deviates from some predetermined approach path. In order to simulate infinity receiving conditions . the cathode-ray screen may be received through two eye-pieces and a prismatic image splitter, Fig. 1b (not shown); alternatively two identical cathode-ray displays 55 and 56 may be viewed through two eye-pieces, Fig. 15 (not shown). The duration and timing of the horizontal and vertical time bases are controlled so that the display corresponds to a 90 or 60 degree seg. ment of space centred about the longitudinal axis of the aircraft. Alternatively a televised image of the airfield may be transmitted from the ground control station and displayed on the cathode-ray tube, the position and orientation of the display being controlled by the position pulses in accordance with the position and altitude of the aircraft relative to the airport or the position pulses may be utilized at the ground control station to control the position and orientation of a television camora viewing a three-dimensional model of the airport. Modification. Instead of transmitting the 1,500 c/s reference signal from the aircraft, it may be applied to synchronize the horizontal timebase of the aircraft display via a phase shifter adjusted in accordance with the distance of the beacon receivers from the aircraft or a 3,000 c/s reference signal may be transmitted from the aircraft on a different camera. Communication. Speech signals may be transmitted to the aircraft from the ground control station by time modulation of a train of 0.8 Ás pulses intercalated with the position pulse signals. When a plurality of aircraft are approaching the airport, the transmitters on these aircraft may be operated sequentially under the control of additional pulse signals transmitted from the ground control station such that the display on the aircraft about to land has the greatest definition.