GB888361A - Improvements relating to information exchange systems - Google Patents

Abstract

888,361. Radio-frequency systems; radar. ELECTRIC & MUSICAL INDUSTRIES Ltd. Feb. 15, 1957 [Feb. 15, 1956], No. 4799/56. Classes 40(5) and 40(7). A communication system comprises a radar transmitter having means for emitting repetitive radar pulses in different directions in sequence, wherein means are provided for modulating the radar pulses so as to convey a message to suitable receivers, the message being confined to radar pulses which can be received from one direction. The system may be used to transmit from one ship to another information about the position of a target. If the radar has a 2 degree beam rotating at 24 r.p.m. and transmits 2,000 pulses/sec., a group of 28 pulses will be received at any bearing at intervals of 2¢ seconds. A message is transmitted as a group of 11 pulses, Fig. 1, of which the odd ones scan regularly and the even ones are time modulated. Pulse 2 is the synchronizing pulse and is displaced the maximum amount, or alternatively it may be omitted. Pulse 4 indicates the sending ship and may have any one of eight positions. The sixth pulse may have four positions to indicate one of three scales for position coordinates or bearing only. The eighth and tenth pulses transmit N-S and E-W coordinates in analogue fashion. Fig. 2 shows a receiver, which is inoperative until a synchronizing pulse is recognised by unit 16 and used to reset binary counters 17, 18 and ring counter 25. The next pulse received, number 3, initiates in decoder 14 a sawtooth wave 26, Fig. 3, which is reversed by pulse 4 and arrested by pulse 5; the potential 27 which represents the signal is fed through gates 19 and 21 to the appropriate channel, gate 19 being opened by counter 18 and gate 21 of counter 25. Decoder 15 similarly decodes the sixth pulse, and so on alternately until the whole message has been decoded. As an alternative binary coding may be used; this requires eight pulses for each co-ordinate, but it may be unnecessary to leave alternate pulses unmodulated. In another alternative the position is represented by three co-ordinates sin #, cos # and I/R. Some protection from jamming once a synchronizing pulse has been recognised can be achieved by arranging a gate which only admits pulses during the expected maximum range of time displacement. Pulse width modulation may be used instead of time modulation.