office: Radar detector

Description
One of the technologies that law enforcement agencies can use to measure the speed of a moving vehicle uses doppler radar to beam a radio wave at the vehicle, and then infer the vehicle's speed by measuring the Doppler effect-moderated change in the reflected wave's frequency. Radar guns can be hand-held, vehicle mounted or statically mounted.
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, that operate in the same part of the electromagnetic spectrum as radar guns.

Counter technology
Radar guns and detectors have evolved alternately over time to counter each other's technology in a form of civilian electronic "warfare". For example, as new frequencies have been introduced, radar detectors have initially been "blind" to them until their technology too has been updated. Similarly, the length of time and strength of the transmissions have been lowered to reduce the chance of detection, which in turn has resulted in more sensitive receivers and more sophisticated software counter technology. Lastly, radar detectors may combine other technologies, such as GPS-based technology with a point of interest database of known speed trapping locations, into a single device to improve their chances of success.

Radar Detector Detectors
Main article: Radar detector detector
The superheterodyne receiver in radar detectors have a local oscillator that radiates slightly, so 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). The VG-2 Interceptor was the first device developed for this purpose, but has since been eclipsed by the Spectre III. This form of "electronic warfare" cuts both ways - since detector-detectors use a similar superheterodyne receiver, many early "stealth" radar detectors were 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 detector manufacturers responding by changing their local oscillator frequency. Today, practically every radar detector on the market is immune to the VG-2 Interceptor.

Radar Scrambling
Although some companies advertise that their radar detectors can passively 'scramble' or 'absorb' radar (such as Rocky Mountain Radar), many or all of their products do not affect radar and laser equipment due to the low power intake from the device versus the high power that bounces off a vehicle.
It is illegal in many countries to sell or possess any products that actively transmit radar signals intended to jam radar equipment.Actively transmitting on an FCC licensed frequency without a license is a violation of FCC regulations and a felony in the USA.

LIDAR Detection
Main article: LIDAR detector
Newer speed detection devices use pulsed laser light, commonly referred to as LIDAR, rather than radio waves. Radar detectors, which detect radio transmissions, are therefore unable to detect the infrared light emitted by LIDAR guns so a different type of device called a LIDAR detector is required. LIDAR detection, however, is not nearly as effective as radar detection because the output beam is very focused. While radar's radio waves can expand to 85 feet (26 m) across at 1,000 feet (300 m) from their source, LIDAR's light beam diffuses to only about 6 feet (1.8 m). Also, a police officer targeting a car will most likely aim for the center mass or headlight of the vehicle and, because radar detectors are mounted on the windshield away from the beam's aim, they may not alert at all. Lastly, with such a focused beam, an officer using a LIDAR gun can target a single car in close proximity to others at ranges of up to 3,000 feet (910 m).
Despite the advent of LIDAR speed detection, radar remains more prevalent because of its lower price relative to LIDAR equipment and the amount of radar equipment already in service.