Patent ID: 7725026

Claim:
A detection apparatus comprising: at least one signal source, coupled to ends of a waveguide at least partly defining two signal paths for carrying signals in each of two counter-propagating directions, wherein at least one of said signal paths for each of the two counter-propagating directions traverses a detection zone; wherein a disturbance to be detected and located, occurring at a point in the detection zone that can be nearer to one or another of opposite ends of the detection zone can cause a time change in a phase relationship between the signals of said two signal paths, for each of said directions, originating at the point of disturbance and propagating over a distance toward opposite ends of said two signal paths, which distance can be longer or shorter depending on a distance between the point of the disturbance and respective said opposite ends; a phase responsive receiver coupled to each end of said signal paths, comprising at least one beam combiner for combining said two signals for a respective said end, and a detection device coupled to said beam combiner for each said end, wherein said detection device derives at least two mutually independent detector signals, wherein the detector signals developed by the detection device are further processed to obtain the phase relationship between said two signals for each of said two opposite directions; a processor coupled to the said phase responsive receivers, operable to determine a time difference between receptions at said phase responsive receivers for said two opposite directions of said time change in said phase relationship, said time difference varying with a location of the point of the disturbance, which time difference identifies said point in the detection zone at which the disturbance occurred.