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Surveillance systems

  • US 4,352,098 A
  • Filed: 05/16/1980
  • Issued: 09/28/1982
  • Est. Priority Date: 05/18/1979
  • Status: Expired due to Term
First Claim
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1. A method of detecting the presence and position in a surveillance zone of an electromagnetic wave passive receptor reradiator with signal mixing capability, comprising the steps of simultaneously radiating first, second and third energy fields through said zone for causing said receptor reradiator to radiate at least one reply signal which is a function of said energy fields, wherein said first energy field is produced by a microwave signal, said second and third energy fields are produced by low frequency signals relative to said microwave signal, said second energy field is produced by continuously radiating a first pair of closely spaced apart signals and said third energy field is produced by continuously radiating a second pair of closely spaced apart signals, said second and third energy fields being radiated into said zone at spaced apart locations adjacent the edges of said zone, and said reply signal is the function of the position of said receptor reradiator;

  • detecting in said zone the presence of said reply signal;

    indicating the position of the receptor reradiator in the zone; and

    triggering an alarm in response to detection of said reply signal.

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