A radar detector detector (RDD) is a device used by police in areas where radar detectors are declared illegal.

Because radar detectors are built around a superheterodyne receiver, which has a local oscillator that radiates slightly, it is possible to build a radar detector-detector which detects such emissions (usually the frequency of the radar type being detected, plus about 10 MHz for the intermediate frequency).

Some police radar guns are equipped with such a device. VG-2 Interceptor was the first device developed for this purpose, current technology uses the Spectre III (Stalcar in Australia). [1] This form of electronic warfare cuts both ways and since detector-detectors use a similar superheterodyne receiver, many early "stealth" radar detectors are equipped with a radar detector-detector-detector circuit, which shuts down the main radar receiver when the detector-detector's signal is sensed, thus preventing detection by such equipment. This technique borrows from ELINT surveillance countermeasures. In the early 1990s, BEL-Tronics, Inc. of Ontario, Canada (where radar detector use is prohibited) found that merely the local oscillator frequency of the detector could be modified out of the range of the VG-2 Interceptor. This resulted in a wave of detector manufacturers changing their local oscillator frequency. Today, practically every radar detector on the market is immune to the VG-2 Interceptor[citation needed].

The Spectre III detects every radar detector certified for operation in the United States by the Federal Communications Commission as of December 2004. Currently, the only radar detector able to defeat all radar detector detectors (RDD) is the Beltronics STI Driver. [1]

However, as radar detector detectors are based on the same principle, some radar detectors can detect the detector detectors, and shut down the primary receiver, evading detection by the police equipment.