| 摘要 |
Method and apparatus for the detection of a target receiver unit such as an enemy radar or radio receiver is disclosed. The surveillance system of this invention includes a transmitter unit coupled to an antenna to establish an electromagnetic surveillance field that includes signal components at two predetermined surveillance frequencies. When a target receiver is located within the surveillance field, the target receiver produces a second electromagnetic field having signal components at the intermodulation products of the surveillance field signal components due to a nonlinear mixing process that occurs within the target receiver. Signal components at predetermined intermodulation frequencies are detected by a receiver circuit within the surveillance system that has a passband excluding signal components at the two predetermined surveillance frequencies. Since signals at the intermodulation frequencies are present only when a target receiver is presents detection is accomplished without the necessity of distinguishing between signals reflected from a target receiver and signals reflected from other obstacles.
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