SURVEILLANCE SYSTEMS

摘要

Improvements in or relating to surveillance systems A surveillance system transmits a microwave carrier signal through a surveillance zone (14). Two low frequency enemy fields are also established in the zone being produced by respective pairs of closely spaced apart low frequency signals radiated from opposite sides of the zone. A receptor reradiator in the zones receives these signals and reradiates a reply signal to a receiver the reply signal consisting of the microwave carrier signal amplitude modulated in accordance with the instantaneous value of the low frequency fields at the location of the receptor reradiator. The receiver separates out preselected intermodulation products and activates an alarm only when the preselected intermodulation products are present.