A radar detector detector (RDD) is a device used by police in areas where radar detectors are declared illegal. Radar detectors are built around a superheterodyne receiver, which has a local oscillator that radiates slightly. It is therefore 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).

VG-2 Radar Detector Detector
VG-2 Interceptor was the first device developed for this purpose, although more current technology such as the Spectre III (Stalcar in Australia) is now available. 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 the local oscillator frequency of the detector could be altered to be 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.

Spectre III / Spectre IV
The Spectre III (Stalcar in Australia) RDD alerts its user when it senses the presence of a radar detector. It does this by detecting the radio frequency “leakage” emitted by the local oscillators of all radar detectors, as they are based on superheterodyne receivers. In fact, only 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 and Beltronics STI-R.