Patent ID: 6049301
Filing Date: 2000-04-11
Classification: G01S

Abstract:
An electromagnetic surveillance system for detecting the presence of receivers having non-linear demodulation elements in the input channels thereof within a predetermined surveillance region comprising:signal generation means for supplying an electrical signal including signal components at a first and second predetermined frequency, said signal generation means including means for modulating said electrical signal;means responsive to said electrical signal supplied by said signal generation means for establishing an electromagnetic surveillance field within a predetermined surveillance region, said electromagnetic surveillance field having signal components at said first and second predetermined frequencies;means for detecting electromagnetic waves at frequencies which correspond to at least one intermodulation frequency of said first and second predetermined frequencies, said detecting means including means for excluding signals at said first and said predetermined frequencies; andranging means responsive to said means for detecting electromagnetic waves for determining the distance from said electromagnetic surveillance system to a receiver unit causing said electromagnetic waves at said intermodulation frequencies.