Patent GB-628956-A (GB)
Title: Improvements in or relating to devices for landing aircraft by means of ultra-high frequency signals
IPC: G01S1/02
Filed: 1945-12-07
Abstract: 628,956. Radio navigation; waveguides; pulsing carrier frequencies. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. Dec. 7, 1945, No. 33172. Convention date, Oct. 26, 1944. [Classes 40 (v) and 40 (vii)] Relates to overlapping-beam equi-signal beacon systems for defining a landing path for aircraft. According to the invention the azimuthal component of the landing path is defined by two aerials having radiation patterns overlapping in azimuth which transmit pulses of ultra-high frequency energy modulated with different low frequencies and the vertical component of the landing path is defined by the locus of points at which the amplitude of the field from a third directive aerial is equal to the sum of the amplitudes of the fields of the first two aerials. Means are also described for, radiating a fourth beam to define the boundary of the landing ground. In the embodiment described the pulses of R.F. energy are produced by coupling a R.F. generator to an aerial through a rectangular waveguide dimensioned to sustain waves in the H01 mode, the waveguide containing a variable attenuator similar to that disclosed in Specification 594,368. As shown in Figs. 1 and 2, the attenuator comprises a rectangular frame positioned in the guide g and rotating about an axis y, y<1> of the frame perpendicular to the direction of the electric vector E at a speed of rotation equal to half the required pulse recurrence frequency. The length of the frame h parallel to the axis of rotation is made equal to #/2 so that the frame acts as a #/2 resonant parallel-wire line and prevents the transmission of energy through the guide in all positions except when the angle between the plane of the frame and the longitudinal axis of the guide is less than # degrees where 6 is of the order of 5 to 10 degrees, thus resulting in the formation of pulses of angular width 2 #, Fig. 3 (not shown). In the embodiment of the invention shown in Figs. 4 and 7, three sets of intercalated wave pulses, Fig. 8 (not shown), are formed in the guides g1, g2 and.g3 by means of the rotating frames m1, m2 and m3 which are mounted on a common shaft and are angularly displaced with respect to each other by angles #. The guides are fed in parallel by an ultra-short wave generator Em which is sequentially modulated by frequencies F1, F2 and F3 applied from the modulator MOD through the commutators B1, B2, B3 so that the three pulsed wave trains are distinctively modulated by the frequencies Fl-F3. The guides g1 and g2 are terminated by directional horns c1, c2, the polar diagrams of which overlap in azimuth to define an equi-signal plane P2, the longitudinal axes 01, Z1 and 02, Z2 of the horns being elevated at an angle equal to half the angular width # of the vertical polar diagram to reduce ground reflection effects, Fig. 5 (not shown). The guide g3 feeds a third directional horn c3 mounted symmetrically above the other two horns so that the glide path is defined by the surface MOM, Fig. 6 (not shown), at which the intensity of the field radiated by the horn c3 is equal to the sum of the fields radiated by the horns c1 and c2 and the landing path 0, ZA is defined by the intersection of this plane and the vertical plane P2. At the aircraft receiver, Figs. 9 and 10 (not shown), the signals received from the three horns are separated after the detector stage by means of circuits tuned to the frequencies F1-F3 and each separated pulse train is applied to a rectifier with a long time-constant output circuit giving a positive D.C. voltage proportional to the amplitude of the received pulses. The D.C. voltages corresponding to the radiations from the horns c1 and c2 are applied respectively to the left and right X-plates of a cathode-ray tube and the sum of these voltages is applied to the lower Y-plate, the D.C. voltage corresponding to the radiation from the horn c3 being applied to the upper Y-plate so that the cathode-ray spot is deflected from its central position in accordance with departures of the aircraft from its landing path, Fig. 12 (not shown). In order to prevent false indications when the navigational signals fade, the bias of the cathode-ray tube is controlled by a voltage proportional to the strongest received signal so that the spot is blacked out when this voltage falls below a minimum value. In order to indicate the boundary of the landing ground, a fourth directional horn fed with similarly produced pulses modulated at a frequency F4 is located to produce a beam transverse to the landing path which is elevated so as to intersect the landing path above the boundary, Figs. 13 and 14 (not shown). At the aircraft receiver this additional signal is separated by a circuit tuned to the frequency F4, the rectified signal being then applied to a relay operating an indicator lamp. The Specification as open to inspection under Sect. 91 describes a simpler system providing azimuthal guidance only, Figs. 7 to 12 (Cancelled) (not shown), in which the horn c2 is mounted above the horn c1. When a long route is to be marked out, a number of equi-signal beacons are spaced along the route such that a good signal to noise ratio is always obtained, Fig. 10 (not shown). This subject-matter does not appear in the Specification as accepted.