Method and apparatus is disclosed for detecting a security threat in a zone. A transmitter within the zone directs a first microwave signal of frequency f.sub.0 to a reflector. The reflector doubles the frequency to 2f.sub.0 and reradiates it to a receiver. The receiver correlates the reflected signal...http://www.google.fr/patents/US4684929?utm_source=gb-gplus-shareBrevet US4684929 - Microwave/seismic security system
