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Might be thinking of bistatic radar. Most radar systems use the same antenna for transmitting and receiving, real low duty cycle on the transmitter and listen for the echoes. Bistatic radar uses a remote CW source (Analog TV transmitters, carrier on cell phones (although this is difficult to filter out with the digital noise, and not as powerful as ATV), FM radio) and looks at the doppler shift at the receiver to detect an object's motion. Benefit to this system (and why the UFO/conspiracy theorist nerds like it so much) is that the receiver does no transmitting, and is therefore almost impossible to detect.I seem to remember a "radar" system, that keyed in on cell phone frequencies, to see holes in the atmosphere (stealth planes), and reflections of low flying craft bounced off the upper atmosphere. Same idea I think.