A raar detector, sometimes called a fuzz buster, is an electronic device used by motorists to determine if their speed is being monitored by a radar unit. To do so, police bounce a radio wave off of a moving vehicle with a radar gun or other radar device that determines the vehicle's speed by the Doppler-effect-moderated change in the wave's frequency. Most of today's radar detectors detect signals across a variety of wavelength bands – usually X, K and Ka (as well as Ku, in Europe, also recently approved for use in the U.S).