Patent ID: 7884717

Claim:
A method for detecting an intruder of predetermined size using passive reception of electromagnetic radiation, comprising: passively receiving a first electromagnetic radiation signal at a first receiver, said first receiver including a first directional antenna at a first location, a signal value at a first frequency of said first passively received electromagnetic radiation signal being indicative of a temperature of a source of said first electromagnetic radiation signal; passively receiving a second electromagnetic radiation signal at a second receiver, said second receiver including a second directional antenna at a second location different from the first location, a signal value at a second frequency of said second passively received electromagnetic radiation signal being indicative of a temperature of a source of said second passively received electromagnetic radiation signal, wherein said first frequency of said first passively received electromagnetic radiation signal and said second frequency of said second passively received electromagnetic radiation signal are within and are not outside a range of frequencies protected from active electromagnetic radiation signal transmission in a geographic area encompassing said first and second antenna locations; and comparing the signal value of said first frequency with the signal value of said second frequency using a signal processor to determine a distance of the intruder of the predetermined size from one of said first antenna and said second antenna.