Radio channel alarm system - central station monitors alarm points using small number of channels overall

摘要

An alarm system having a number of mobile stations which may originate alarm calls, and each radiating a carrier wave frequency modulated by a first code identifying the source and a number of relay stations which can recognise the reception of an alarm signal and relay it with a second code identifying the relay station. A number of fixed stations are able to recognise an alarm situation on receipt of an interrogation signal on a second frequency modulated with a third code identifying the fixed station which excites a response signal on a carrier frequency. The interrogation signal is radiated periodically by a central monitoring station to interrogate each of the fixed stations. The central station may comprise a search unit able to recognise the frequency, another to identify the source, relay station, and fixed station carrying the alarm frequency and a logic unit which can change the interrogation sequence to give preferential attention to any source raising the alarm, as well as if needed to a adjacent points.