A person liable to be kidnapped carries a small transmitter- receiver capsule (2) which can be activated by pressing a button and then swallowed. The capsule sends an emergency call, switches to receive on the same frequency for 4 seconds and repeats the cycle until it receives an acknowledgement signal. Then it switches to receive on a second frequency and awaits a command signal, whereupon it transmits a homing signal on the first frequency. The sequence is designed to conserve the capsule battery while a rescue is organised. Signals are received from and sent to the capsule by a mobile relay station, (1) which may be installed in the person's own car. A group of monitoring stations, with land line connections to a central control (16), receive signals from the relay and locate its position. The monitors send acknowledgement and command signals to the relay when so instructed by the control centre.
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