Patent ID: 8212671

Claim:
A method for detecting a body crossing a boundary, the method 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 proximate the boundary, 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 passively received 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 proximate the boundary, the second location being 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 detect the body crossing the boundary from one of the first passively received electromagnetic radiation signal by the first directional antenna and the second passively received electromagnetic radiation signal by said second directional antenna.