Patent GB-527937-A (GB)
Title: Improvement in radio navigation system
IPC: G01S1/02
Filed: 1939-04-24
Abstract: 527,937. Wireless signalling. WALLACE, M. April 24, 1939, No. 12232. Convention date, April 26, 1938. Addition to 525,393. [Class 40 (v)] Relates to the wireless system described in the parent Specification, in which different signals within a given wave-band are shown as separate traces on the fluorescent screen of a cathode-ray tube, the traces being spaced apart according to the frequency of each signal. The method is applied to show the direction of different transmitters in a radio-navigation system. According to the invention, a number of beam transmitters are used to radiate waves which vary in frequency cyclically, but at different rates, about selected datum values, so that each can be distinguished from the others without using modulation. The frequency variation may be synchronized with definite positions of the beam, as it rotates, so that the frequency of the maximum received signal is related to the bearing-line of an observer. The variation of frequency may be imparted by a continuously-rotated condenser with suitably shaped vanes. Or, as shown in Fig. 4, the frequency of a piezo-electric crystal 7 is varied cyclically by altering the gap between it and an electrode 8 by means of cams mounted on a rotating shaft 5, which also controls the tuning of the circuits 15, 20, 26 of the transmitter. The signals are radiated from the aerial system shown in plan in Fig. 6, in which an energized dipole 30 is associated with reflectors 31, 32, 33 and with " directors " 34, 35. The aerial assembly is rotated through belt-gearing by the same motor 4 that controls the tuning of the circuits. The movement of the aerial is co-ordinated with a variable condenser 36 to ensure that the same frequency is radiated at corresponding positions in azimuth. The condenser may be so arranged that, at the end of each sweep of 360 degrees, it jumps back to its initial frequency-setting. At the receiver, the visual signal-trace moves over the calibrated fluorescent screen in repeated cycles which pass through a maximum indicative of the bearing-line. The indicator scale may be arranged to give direct readings, of the shape shown in Fig. 15, over the full 360 degrees (Fig. 12, not shown). By radiating two beams of varying frequencies from two different points F, Fl, Fig. 10, a predetermined course L<SP>1</SP>, of any given shape, can be indicated to an observer between the two dotted lines L<SP>11</SP> and L<SP>111</SP> which correspond to the limits of off-course control. The lines L<SP>1</SP>, L<SP>11</SP>, L<SP>111</SP> are determined by the design and setting of the transmitter tuning-controls, and correspond to the intersection of lines P, 21, 31 &c. of " peak " frequency and lines 1<SP>11</SP>, 2<SP>11</SP> &c. and 1<SP>111</SP>, 2<SP>111</SP> &c. representing a constant difference in frequency. By varying the vertical, instead of the horizontal pattern of the radiated field, the method can be used to provide a path for " blind landing " or for guiding an aviator over hilly country (Fig. 16, not shown). Specifications 525,393 and 529,712 are referred to.