Patent GB-1476224-A (GB)
Title: Emergency communication system
IPC: H04L29/00
Filed: 1974-11-27
Abstract: 1476224 Emergency communication system INTERNATIONAL BUSINESS MACHINES CORP 27 Nov 1974 [28 Dec 1973] 51368/74 Heading H4L In a communication system, e.g. for vehicles on a highway, for transmitting emergency messages to a control station, a number of relay stations are positioned along a predetermined route in order to receive and relay coded distress signals from one or more mobile transceivers situated on or travelling along the route. In use, a relay simultaneously transmits a relay identification code. A terminal station receives each signal group comprising a relayed signal and identification code, interprets it to determine the location of the relay station nearest to the originating transceiver and transmits an answer-back signal through the nearest relay station to the originating transceiver. Each transceiver when operable transmits a coded distress signal repeatedly with increasing power at each repetition up to a maximum power level, and then repeats the transmision starting from a low power level and increasing until it receives an Lnswer back signal. Mobile ransceiver (Fig. 2).-When a motorist in distres wishes to send his message he switches on power to the transceiver (15), sets SW18 to an appropriate message and SW17 to his direction of travel and presses push-to-send switch 22. Thereafter the transceiver functions automatically. Coded data relating to the message is set up in the character store 19, which also holds coded data identifying the overall system and the particular vehicle. This data is used to modulate a 75 MHz transmitter at periodic intervals determined by a tuning generator 24, each successive transmission in a group of transmissions, e.g. three, being at greater power, as determined by a sequence control 25. When eventually a transmission is detected by a relay station and identified as being appropriate to the system, an acknowledge signal is returned to the transceiver and can be received in the intervals between transmissions. This acknowledge signal is decoded at 29 and used to cause the control logic 23 to hold the transmitted power at the smallest level necessary to ensure satisfactory communication. Thus the operation is such as to minimize transmitted power, thus only the relay stations nearest to the vehicle will receive signals and the position of the vehicle can be approximately established. Detailed circuit diagrams of the control and logic arrangements in the transceiver are provided (Figs. 3, 3A, not shown). Relay stations (Fig. 4).-These stations receive from and transmit to the mobile transceivers at 75 MHz (unit 200) and to the terminal station at 960 MHz (unit 400). The relay is on permanent standby; when a 75 MHz squelch tone is detected (at 302) from a mobile transceiver, control logic 351 is activated to operate an appropriate timing sequence, in which the received signal is applied to the call number decoder 305 to identify that the signal is valid for the system and then the relay station code, held in buffer 308, is transmitted to the base station via transmitter 404 and a first acknowledge signal is returned to the transceiver from an acknowledge generator 252. Thereafter, the base station, having determined that the relay is nearest to the mobile transceiver, transmits to it its identification code, which is decoded at 306. This switches the relay to a "transparent mode" where transmissions between the mobile transceiver and the base station pass through the relay unchecked, by appropriate settings of switches 310 and 251. Base station (Fig. 5).-At the base station, when a coded signal from a relay is received it is decoded and an acknowledge signal transmitted. This second acknowledge signal is retransmitted to the mobile transceiver, and turns it off. The signal from the relay contains its own identification code which is decoded in a buffer 511, where output is connected to relay selection logic 600. Powers and distances in the system are such that no transceiver will illuminate more than three relay stations simultaneously, thus not more than three of the output lines from the decoder 511 will be energized. The decoded outputs are fed into an array of several shift registers, which are swept through by a clock and tested by comparators to decide w hich one, two or three consecutive relays were illuminated. From this result, appropriate logic redetermines the code of the nearest relay so that transmission to and from the transceiver can be established by it.