Intrusion detection system

Abstract

In an intrusion detection system comprising an open transmission line (10), for example a "leaky cable", and a receiver unit (12S,12T,14S,14T,16) for processing a radio frequency signal received by the line to detect perturbations caused by an intruder (28), problems of nonlinear sensitivity due to line attenuation are resolved by combining perturbation signals (R.sub.S, R.sub.T) from both ends of the open transmission line. A processor unit (18) computes logarithmic values of the perturbation signals from the opposite ends of the line and combines them to detect presence of an intruder. The sum of the logarithmic values may be used to determine presence of the intruder and the difference between them may be used to determine the location of the intruder relative to the end of the line. The processor may also use phase information from the two perturbation signals to facilitate detection in the presence of multipath effects. Reference signals (LO,IF.sub.T, RF) and power may be transmitted between receiver units (12S,14S, 12T,14T,16) by way of the open transmission line (10) itself. The system may employ several transmissions having different frequencies, in which case the processor (18) will sum perturbation signals for all transmission frequencies for each end of the line, combine the two sums, and compare the result with a predetermined threshold T.

Dependencies are inferred from the claim text.