Radar detectors
A radar detector is an electronic device used by motorists to detect if their speed is being monitored by police or law enforcement using a radar gun. Most radar detectors are used so the driver can reduce the car's speed before being ticketed for speeding. Only doppler radar-based devices can be detected — other speed measuring devices including those using ANPR, piezo sensors, and VASCAR technology cannot be detected. LIDAR devices require a different type of sensor, although many modern detectors include LIDAR sensors. Most of today's radar detectors detect signals across a variety of wavelength bands: usually X, K, and Ka. In Europe the Ku band is common as well.
One device law enforcement use to measure the expected speed of a moving vehicle is called Doppler radar, and it uses the Doppler effect to measure the relative speed of a vehicle. Namely, Doppler radar works by beaming a radio wave at the vehicle to then measure the expected change in frequency of the reflected wave (that bounces off the vehicle). Many times, law enforcement employ Doppler radar in their hand-held radar guns, on their vehicles, as well as on fixed objects such as traffic signals.
Radar detectors use a superheterodyne receiver to detect these electromagnetic emissions from the radar gun, and raise an alarm to notify the motorist when a transmission is detected. False alarms can occur however due to the large number of devices, such as automatic door openers (such as the ones at supermarkets) and adaptive automotive cruise control, that operate in the same part of the electromagnetic spectrum as radar guns.
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