Patent GB-604445-A (GB)
Title: Radio communication and guiding system for mobile units
IPC: H04B7/155
Filed: 1945-04-27
Abstract: 604,445. Radio navigation; radio signalling. STANDARD TELEPHONES & CABLES, Ltd. April 27, 1945, No. 10724. Convention date, May 5, 1944. [Classes 40 (v) and 40 (vii)] In a system for guiding a mobile unit, e.g. an aircraft, along a course line defined by a plurality of equi-signal course beacons, successive. beacons along the course transmit on frequencies F1, F2, F1, F2, &c. and a marker beacon transmitting at the same frequency as one of a pair of adjacent course beacons is located in the region of overlap of the directive patterns from the adjacent course beacons, the purpose of the marker beacon being to switch over the tuning of the receiver on the mobile unit from F1 to F2 or vice versa as the mobile unit moves out of the predominating field of one course beacon into the predominating field of the adjacent beacon. The system is described as combined with an intercalated pulse multiplex repeater system similar to that described in Specification 604,203 for communication between terminals of the course and between one control terminal and the aircraft via broadcast transmitters, the repeaters and broadcast transmitters being mounted on the same towers as the beacons. One channel of the communication system may be used for switching on the beacons and broadcast transmitters and this operation may also be effected by signals transmitted from the aircraft. Course and marker beacons. Each course beacon 26, Fig. 1, which may be of the doubletone or keyed A-N type is mounted on a tower and is provided with an angular reflector 26A, to produce unidirectional radiation patterns 225, 226, 227 ... shown in elevation and plan in Figs. 5 and 6, the patterns being radiated at frequencies F1, F2, F1 ... and extending eastwards along the course. The horizontal portion of the reflector 26A, Fig. 3 (not shown), also serves as an artificial earth. Marker beacons 25 are mounted on top of the towers, the beacons having vertical fan-shaped radiation patterns 230, 231 ... inclined slightly in the forward (eastward) direction and each marker beacon transmitting at the frequency of the preceding coursebeacon. In the aircraft receiver, Fig. 4 (not shown), the received signals of frequencies F1 and F2 are separated and fed into amplifying channels with crossconnected A.V.C. circuits so that only the signal due to the predominating frequency appears at the output and the purpose of the marker beacon radiation is to make the transition in field strength more abrupt so that the changeover takes place at substantially the same point irrespective of the direction of flight along the course. In a modification, Fig. 15 and Fig. 16 (not shown), each marker beacon transmits at the same frequency as the adjacent course beacon, but it radiates two fan-shaped patterns 600, 602 slightly displaced along the course, the one displaced in the direction of an F2 zone being, modulated at 900 c/s. and the one displaced in the direction of an F1 zone being modulated at 500 c/s. In the modified aircraft receiver, Fig. 17 (not shown), the 900 and 500 c/s. are separated from the output of a broadly tuned detector capable of receiving F1 and F2 and applied to operate separate relays which tune a sharply tuned detector to F2 and F1 respectively; thus on passing from an F2 zone to an F1 zone, the 500 c/s. tone switches the tuning to F and on passing from an F1 zone to an F2 zone the 900 c/s. tone switches the tuning to F2. Communication system. The pulse multiplex transmission is transmitted and repeated eastwards by the directive aerials 21, 22, Fig. 1, on successive frequencies F1, F2, F1 ... and the westbound transmission takes place on aerials 21A and 22A on successive frequencies of F4, F3, F4 .... The odd numbered eastbound channels from the West terminal control station are selected at each tower and broadcast to aircraft from a wide angle aerial 23 on the same frequency as the eastbound relay from that tower, the transmission from the aircraft being received on a wide angle aerial 24 at a frequency F5 and then inserted in appropriate odd numbered westbound channels. A system of push-pull time displacement modulation of adjacent pulse pairs is described, Fig. 8 (not shown), of the type disclosed in Specification -600,244, but other types of pulse modulation, e.g. amplitude modulation, could be used; the first channel is given a distinctive pulse width and is used as a synchronizing and order channel. At the control (west) terminal station the separation of the outgoing channels is controlled by a base wave generator which is fed through individual phase shifters to each modulator and the incoming channels are separated by selecting the synchronizing pulses by width discrimination and feeding them through individual delay circuits to channel gating circuits, a similar system being used at the repeater stations, Fig. 3B (not shown), for selecting the odd channels for transmission to the aircraft. The signals received at the aircraft comprise intelligence pulses 216, Fig. 4A (not shown), superposed on the course beacon modulation frequency 215 and these are separated in the receiver, Fig. 4 (not shown), by limiting and filtering; the order channel pulses are selected by width discrimination and fed through a demodulator to a standby earphone, the selected pulses being also fed through a variable delay circuit to a channel gating circuit for selecting another channel which is fed to a communication earphone. In communicating with an aircraft, the control station calls it up on the order channel and instructs it to select a particular communication channel. The time displacement modulated pulse channels transmitted by the aircraft are distinguished by their pulse widths and each aircraft transmits two channels controlled by a base wave generator which may be synchronized by the received order pulses, one of the channels having a particular pulse width corresponding to the order channel and the other having a selectable pulse width corresponding to the communication channel in use. These transmissions are separated by pulse width discrimination at the repeater stations, Fig. 3B (not shown), and injected into corresponding westbound transmissions under the control of synchronizing pulses separated from the incoming westbound repeater transmission. In a modification, Figs. 12-14 (not shown), all the channel pulses are the same width and a synchronizing frequency which is a subharmonic of the pulse recurrence frequency is injected into the first channel modulator. The channels are separated by a system similar to that described in Specifications 600,252 and 604,203, channel pulses being fed to a channel gating selector controlled by an oscillator running at a slightly lower frequency than the repetition frequency of each channel pulse train so that each channel is selected in turn and when the first channel is received the synchronizing frequency is filtered out and applied to lock the oscillator, the output from which is also fed through individual phase shifters to channel gating selectors. The different channels from the aircraft are amplitude modulated on to different sub-carriers of a common carrier F5 and at the towers these channels are separated by filters and injected into westbound pulse channels under the control of a base wave generator synchronized with the westbound channel separating oscillator. Control of tower transmitters. In the modification shown in Figs. 12-14 a switching pulse channel of particular pulse width is transmitted by the control station and this is selected at the relay towers and applied through a demodulator to trigger a thyratron which switches on the course beacon, marker beacon and broadcast transmitters. The thyratron may also be triggered by the rectified carrier of the received aircraft transmissions so that an aircraft switches on the transmitters located on adjacent towers. Suitable circuits for pulse demodulation and pulse width discrimination are given in Figs. 9 and 10 (not shown), of the types described in Specifications 592,789 and 600,291 respectively. At each tower the aerials for the marker beacon and the broadcast transmitter and receiver are mounted above a shielding sheet, Fig. 3 (not shown), which also serves as an artificial earth.