Patent US 4,124,848

A system for detecting intruder motion in selectable zones of a defined area is disclosed. The system is principally used as a line protector to detect intruders in the vicinity of a fence or wall and may be used as a virtual fence by employing a narrow-beam antenna and deep range coverage to detect an intruder crossing its line of sight. The system utilizes a transmitter for periodically sweeping the area to be protected and a receiver incorporating a narrow band filter which is variable to select a plurality of overlapping zones within the protected area, the presence of a reflective target in a selected zone resulting in a corresponding output signal. Means are provided to store such output signals from each zone and to compare received signals with previously-stored signals to detect differences in the target pattern of each zone. Differences exceeding a predetermined threshold level produce event signals which may result in a rescanning of the same zone, the system producing a zone trip signal only if an event is detected in a rescan. Upon completion of a complete scan of all zones, the system responds to selected alarm criteria to produce an alarm indication if there has been a preselected change in the target pattern.