Patent GB-638342-A (GB)
Title: Improvements in or relating to radio navigational systems
IPC: G01S1/08
Filed: 1944-08-18
Abstract: 638,342. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd., STRONG, C. E., and ARMSTRONG, L. J. HEATON-. Aug. 18, 1944, No. 15862. [Class 40 (vii)] Radiations from beacons for use in navigation are pulse-modulated at constant frequency and are also used as communication channels and radiations from a given station may have the same carrier frequency and the same or different pulse-repetition frequencies. Automatic volume-control at the receiver, which is carried on an aircraft or the like, is governed by pulse-modulated signals from an omnidirectional aerial, so that the navigational aids shall not respond to weak unwanted signals emitted by the directional beacons in subsidiarylobesof the radiation pattern. Theinvention may be applied to the glide-path beacon system described in Specifications 577,276 and 577,527. In the embodiment shown in Fig. 1 (sender) and Fig. 3 (receiver), an oscillator 1 (e.g. of the type described in Specification 587,940) produces rectangular pulses with a duration of 2 microseconds and a frequency of 8,000 per second. These are fed back to the oscillator through a delay line 2 to stabilize the pulse frequency, and also modulate the radio-frequency sender 5, with omnidirectional aerial 6, through a device 3 which converts each pulse p into a pair of pulses f, thereby marking it for use as a synchronizing signal. The delay device or distributer 2, which may be of the kind described in Specifications 587,939 and 635,472 feeds several channels a, b, c, d, the pulses in each channel being delayed by 25 microseconds behind those in the preceding one. Channel c is shown as in use for communication, the pulses being duration-modulated at 4 and sent out from the omnidirectional radio sender 5. Channel d is shown as modulating a radio sender 7 which feeds two directional antennae 9, 10, alternately through a dot-dash switching device 8, the antennae having overlapping field patterns so as to define an approach course or the like. At the receiver, Fig. 3, which may be carried on an aircraft or the like, the receiver 11 feeds a pulse-selector 12 which converts each synchronizing pulse, distinguished from the signal pulses by its being double, into a single pulse. The latter controls a multivibrator or like pulsegenerator 13 which feeds a delay unit 14, whence a series of selector pulses are applied to a set of gating circuits such as that 15 for the channel a. The timing is such that the selector pulse for the a channel opens the gating circuit 15 for the signal pulse passing in that channel from the receiver 11 to the detector 16, and similar arrangements are made for the other channels. Voltage for automatic volume control is derived from the synchronizing-pulse selector 12 as shown, or from a communication channel, and is amplified at 17. The device for converting the synchronizing double pulse into a single pulse may comprise a valve with two grids, Fig. 4 (not shown), so arranged that the signals reach one grid directly and the other through a delay circuit, the valve being biassed below cut-off except when both grids are positive. In another embodiment, Figs. 5, 7 (not shown), the carrier is modulated by a pulse-generator with a frequency of 5,000 pulses per second for application alternately in dot-dash rhythm to two course-defining aerials through a switching device which may comprise either an electronic gating circuit or a mechanical switch such as that described in Specifications 580,481, 580,484 or 579,873. The same carrier is modulated with 8,000 pulses per second and these pulses are phase-modulated or duration-modulated with speech and applied to an omnidirectional aerial. At the receiver the pulses are filtered after demodulation and the 5 kc. pulses are applied to an integrating course-meter, while the 8 kc. pulses are demodulated to give speech and to provide automatic volume-control for the common receiver. Pulses may be relayed back from receiver to sender for determining the distance between these installations.