Patent GB-633755-A (GB)
Title: Improvements in or relating to radio navigation systems for landing aircraft
IPC: H01Q25/00
Filed: 1945-02-21
Abstract: 633,755. Radio navigation. SPERRY GYROSCOPE CO., Inc.- Feb. 21, 1945, Nos. 4419/45, 4420/45 and 4421/45. Convention dates, Nov. 22, 1943, Jan. 29, 1944, and Jan. 29, 1944. [Class 40 (vii)] A radio navigation system defining the track an aircraft should follow to land on an airport runway is characterized by the provision of two surfaces defined in terms of a given signal intensity of one, or equisignal intensities of two, electromagnetic radiations; which surfaces intersect to define the landing. track. In the first embodiment described, the intersecting surfaces 22 and 26 are substantially flat planes formed by the points at which equal intensity signals are received from pairs of radiators 12, 13 and 14, 15. The landing track 19 formed by the intersection of surfaces 22 and 26 touches the landing runway 11 at 23. The radiators 12 to 15 may be arranged as shown well clear of the runway 11 so as to present no obstruction. The planes 22 and 26 are preferably perpendicular and inclined at 45 degrees to the ground. The radiators 12 to 15 may be parabolic reflectors with rectangular openings fed by wave-guides and each pair utilize the same radio frequency modulated by differing audio frequencies. On the craft the receiver may comprise two antennµ 63 and 64 for receiving the radio frequencies defining planes 22 and 26. Each channel comprises a receiver feeding two audio-frequency filters separating out the signals corresponding to each overlapping beam. These signals are rectified in bridge rectifiers and the outputs added in opposition. If desired, the two output signals from the two channels and corresponding to the distances the craft is off course in the two planes may be applied directly to a cross-pointer indicator whose pointers are inclined at the same angles as the equisignal planes. However, in order to provide the more usual horizontal and vertical displacement indication the two output signals are applied to double bridge circuit 72 from opposite corners of which outputs corresponding to the sum and difference of the applied signals are fed to a normally arranged cross-pointer indicator 81. In a second embodiment the surfaces defining the path are not planes but conical (Fig. 8), each conical surface being defined by two hollow conical beams of energy 135, 136 having the same axis and either slightly differing apex angles or differing apex positions so that one is substantially inside the other, but with an overlapping portion between them including the equisignal surface 137. The landing point 158 is well spaced from the radiators 131, 132 in this embodiment also. An array capable of radiating a conical beam may comprise dipoles fed in phase and spaced #/2 sec. #/2 apart, where # is the apex angle of the cone. Such an array will radiate equally backwards and forwards and in order to cut off back radiation further dipoles may be introduced into the array midway between the others, fed in phase quadrature with the ones preceding them. As may be seen from Fig. 8, slightly more than half of the cones are cut off.by the ground and of what is left only that portion of the cones reaching the ground closest the runway is useful in defining the landing track. The radiating array may be modified to direct the greater part of the radiant energy along the useful portion of the cone by angularly tilting the dipoles with respect to the directional axis of the array (Fig. 17). If the angle of tilt is a half of the cone angle of the pattern, radiation to the unwanted side of the cone is reduced to a minimum. By transmitting at two different frequencies over the same array it is possible to radiate two cones one inside the other as required to form a conical equisignal surface as, for a given array, the cone apex angle is dependent on frequency. In the landing path transmitter, Fig. 19, arrays 191 and 192 are each used to radiate two nested cones defining an equisignal conical surface. Generators 197, 198 of radio frequencies F1 and F2 modulated by audio-frequencies f1 and f2 respectively supply energy alternately to array 191 through mechanically keyed switches 195 and 196 to define one equisignal surface and generators 208, 209, of frequencies F3 and F4 modulated by f3 and f4 feed array 192 alternately to define the other. On the craft radiation from array 191 is received by antenna 221, mixed with a signal from local oscillator 223 at the mean of the two frequencies received so that components of the two signals pass through receiver 224 tuned to F1-F2/2 and their characteristic audiofrequencies are then separated, rectified and the direct current. resultant applied to one movement of cross-pointer meter 234. A similar set of units connected to antenna 238 derives information about the craft's position with respect to the other equisignal surface and displays it by means of the other meter pointer. A third embodiment is described also using conical surfaces to define the landing path, but in which the radiators are slotted wave-guides buried flush with the surface of runways. Such a system is particularly useful when a number of runways are involved as there is no obstruction to craft landing and the radiators for any particular landing direction may be readily switched into and out of use according to wind direction. The radiators each comprise a wave-guide extension 370 having a radiating slot 371 and flanges 374, a tapering portion 372 matches the impedance to the feeding guide 373. The radiation pattern is conical with the axis coincident with the longitudinal directional axis of the slot; one half of the cone being cut off by the ground. The apex angle of the cone depends on the velocity of the wave energy along the guide 370 relative to the velocity of the energy in space and is thus dependent on the dimensions of the guide, its dielectric filling, and the wavelength of the energy used. Each radiator may be supported by its flanges in a hollow conduit and covered by a thin sheet of dielectric material for protection. Two radiators of differing depths of radiating chamber may be mounted side by side and fed with energy of the same radio frequency but having differing audio-frequency modulations to define one equisignal surface and a similar pair used to define the other. It is stated that instead of using equisignal surfaces to define the landing path, single beams may be used and surfaces of constant intensity utilized instead.