Patent GB-594530-A (GB)
Title: Improvements in radio navigational systems
IPC: G01S1/40
Filed: 1944-08-04
Abstract: 594,530. Directive wireless systems. STANDARD TELEPHONES & CABLES, Ltd., EARP,C.W., and STRONG, C. E. Aug. 4, 1944, No. 14971. [Class 40 (v)] In radio-compass, glide-path and like systems, phase modulation is imposed upon the carrier by means including an aerial array at either the receiving or the sending station A direction-finding or homing system, Fig. 1, includes an array of two or more receiving aerials such as A1, A2, A3, which are connected in turn to an amplifier 1 through a commutator comprising detectors Dl, D2, D3. These are rendered conductive in turn by rectangular pulses from a pulse-generator 7 timed by a crystal-controlled oscillator 6 whose period is one cycle of the commutator. The pulses, which differ in phase, are passed through a limiter 2 and discriminator 3, the output of which is converted by a filter 4 into a sine wave at a subharmonic of the pulse frequency, whereafter its phase is compared at 5 with that of the oscillator 6, the comparison giving an indication of direction. In a modification, pulses may originate at 7 and pass through a frequency-divider at 6. When three or four aerials are used and the output of the discriminator 3 bears a non-linear relation to the phase of the signal, octantal error arises if the aerial spacing exceeds one-fifth of a wavelength, but is avoided, Fig. 2 (not shown), by the use of the discriminator described in Specification 591,668. In this case, part of the output of the limiter 2 at a radio frequency f is modulated with a local frequency F to produce side-bands (f+F), and these are demodulated with the remainder of the output of the discriminator, the demodulation product being applied to an ordinary discriminator after being filtered. The distributer for connecting the aerials in turn to the amplifier may comprise, Fig. 2 (not shown), a multivibrator feeding a delay network which has evenly spaced tappings 23, Fig. 4, connected to the detectors D1, D2, D3. Rectangular pulses are applied to these detectors, rendering them conductive in turn and so connecting the aerials Al, A2, A3 in turn to the transmission line T. Timed rectangular pulses for application to the detectors at 23 may be produced by the arrangement shown for an eight-aerial system'in Fig. 3, instead of being produced by the parts 6-7. Squarewave oscillations are generated at 17 and differentiated at 18 to form positive peaked pulses at intervals of 30 microseconds. A frequency-divider 19 reduces the frequency eight-fold so that peaked pulses at 4.16 kc./sec. are applied to the phase-comparer 5 to serve as a phase standard, and also to the first K1 of a series of Kipp relays or flip-flop devices K1 ... K8 which tend to return to the stable state somewhat more than 30 microseconds after being triggered into the unstable state by the controlling impulses. Each subsequent relay is triggered into the unstable state by the restoration of the previous relay, and the reversion to the stable state is timed exactly by impulses applied to all the relays through the paths 1 from the differentiator 18. Each relay preferably comprises a multi-electrode valve with suitable interconnected electrodes. The detectors D may be crystal detectors for very short wavelengths and are connected between each transmission line and its aerial and also between it and other transmission lines. For longer wavelengths diodes are used. In order that aerials may not act as reflectors while disconnected by the detectors D from the transmission lines 21 they are constructed as folded monopoles, Fig. 4, so that when disconnected they are open-circuited quarter-wave lines. A beacon system defining a glide path can be set up by substituting a sender for the receiver, Fig. 1, and making the feeders 21 of appropriately different electrical lengths. The radiation received at a receiver which is not on the glide path will be phase-modulated and can be converted into indications by phase-modulation technique. Each switching detector D may comprise a triode whose grid and anode are connected through an impedance which becomes negligible at radio frequencies. Cathode-follower and grounded-grid amplifiers may be used for the lower and higher frequencies respectively, and the aerials may be directive. In a beacon system of this kind, Fig. 5 (not shown), two alternately active aerials may send dots and dashes respectively, to enable sense to be distinguished, and at the receiver, Fig. 6(not shown), a single aerial feeds a phase demodulation system operating a glide-path indicator and also feeds an amplitude demodulation system for a speech channel. Alternatively, the beacon may use three aerials acting in turn for equal intervals of time, Fig. 5 (not shown). The manipulation of the received signals for course-indication purposes may be effected as described with reference to Fig. 7 (not shown), and in Specification 559,062. In a five-aerial beacon for indicating a course line, Fig. 8, the aerials TA11 ... TA15 are fed by a transmitter 33 through phase-adjusters PN11 ... PN15 and switching rectifiers TD11 ... TD15, the phases being adjusted so that all the aerials are excited in the same phase. They first emit dots 39 in succession from left to right, and then emit dashes 38 in succession from right to left. A pulse 41 from the end of a delay network 34 triggers a Kipp relay 37 which times a dash 38 and applies it to the switching detector TD14 and also to the beginning of the delay network 35. Pulses from the tappings TP3, TP2, TP1 then reach switching detectors TD13, TD12, TD11 in order at dash intervals. At the end of the last dash the Kipp relay 36 is triggered from the tapping TPO and times a dot which is applied to the switching detector TD12 and also to the input of the delay network 34. The latter applies pulses at dot intervals to the switching detectors TD13, TD14, TD15, in order, and the cycle then repeats. (Alternatively, the networks 34, 35 may comprise series of Kipp relays). The radiation reaching a receiver from the beacon is uniform in phase when the receiver is on the course line, but otherwise the phase changes in steps as successive aerials come into play, and the output of a discriminator at the receiver is according to graph (a), Fig. 9, for deviation to one side of the course and according to an inversion of graph (a) for deviations to the other side. If many aerials are used, a smooth graph (b) is obtained, and the output may be differentiated to yield graph (c) or its inversion according to the sense of the deviation, for the purpose of controlling a course indicator. When few aerials are used differentiation yields an output conforming to a graph such as (d), which may be integrated through a low-pass filter and applied to produce an output as at (c). A glide-path beacon comprises a similar array of horizontal radiators close to the ground. An omnidirectional beacon analogous to the direction-finder shown in Fig. 1 is described with reference to Fig. 10 (not shown), and the corresponding receiver with reference. to Fig. 11 (not shown), and a comparison wave, corresponding to that applied by the oscillator 6 to the phase-comparer 5, Fig. 1, is in this case supplied by radiating the synchronizing amplitude pulses from the sender, or by modulating the radiation at half the com- Mutation frequency, or by radiating a separate modulated carrier which may heterodyne the main transmission. Specifications 533,782 and 579,346 also are referred to.