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 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 mounted on a fixed object, such as a traffic signal.
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), that operate in the same part of the electromagnetic spectrum as radar guns.
In recent years some radar detectors have added GPS technology. This allows users to manually store the locations where police frequently monitor traffic, with the detector sounding an alarm when approaching that location in the future (this is accomplished by pushing a button and doesn't require coordinates to be entered). These detectors also allow users to manually store the coordinates of sites of frequent false alarms, which the GPS enabled detector will then ignore. Some GPS enabled detectors can download the GPS coordinates of speed monitoring cameras and redlight cameras from the Internet, which are contained in the Trinity database. An interstate traveller could receive an alarm when approaching the location of a speed monitoring camera.
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 
The superheterodyne receiver in radar detectors has 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 
It is illegal in many countries to sell or possess any products that actively transmit radar signals intended to jam radar equipment. In the United States, actively transmitting on a frequency licensed by the Federal Communications Commission (FCC) without a licence is a violation of FCC regulations, which may be punishable by fines up to $10,000 and/or up to one year imprisonment.
LIDAR detection 
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 centre 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). This has resulted in some manufacturers producing LIDAR jammers. Unlike radar, LIDAR's frequencies and use are not controlled by the FCC. These jammers attempt to confuse police LIDAR into showing no speed on the display. Many times they are successful, and therefore many LIDAR manufacturers are producing LIDAR guns that have "jam codes" which show when they are being jammed. They will work on some LIDAR jammers and not others. In spite of this, police can often tell when they are being jammed when they get no reading on their LIDAR. Many jammer equipt motorists using jammers now try to counter this by reducing their speed to legal limits before they turn their jammer equipment off or "kill the equipment" referred to as "JTK" or "Jam to Kill." Officers can often tell this is going on by just watching for their LIDAR equipment not being able to lock in a speed properly along with visual indication of sudden deceleration of the targeted vehicle. They will then pull the offending vehicle over and look for LIDAR jammers on the front of the vehicle and can hit the motorist with an obstruction of justice charge should they be found. Some states also have laws about any police radar or LIDAR jamming. These are CA, UT, CO, OK, TX, MN, IL, TN, VA, SC. In these states the penalties can be severe.
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. In addition, in order to use LIDAR the officer must be stationary and cannot be moving at all on account of the need for a very precise aim.
||This section needs additional citations for verification. (November 2007)|
Using or possessing a radar detector or jammer is illegal in certain countries, and it may result in fines, seizure of the device, or both. These prohibitions generally are introduced under the premise that a driver who uses a radar detector will pose a greater risk of accident than a driver who does not. However the 2001 Mori report in the UK suggests that radar detector users posed a 28% less risk of accident. The table below provides information about laws regarding radar detectors in particular nations. In 1967 devices to warn drivers of radar speed traps were being manufactured in the United Kingdom; they were deemed illegal under the Wireless Telegraphy Act 1949.
|Australia||Illegal in all states besides Western Australia||They are illegal to use in a moving vehicle (as stated by the traffic laws in) SA, NSW, ACT, NT, Queensland and Tasmania. In the state of Victoria it is an offence to sell, use or possess a radar detector, and police may confiscate such equipment if found in a vehicle. Heavy fines apply, ranging from $200 – $1200 AUD, up to 9 demerit points, and confiscation of the Radar Detector. However, importing a unit is permitted under Australian customs regulations. Queensland = up to 40 penalty units (approximately equal to $3000) for being in or on a vehicle whether or not the device is operating or in working order.||K, Ka, Laser|
|Belgium||Illegal||In July 2006, a provisional seizure of a vehicle worth over €75.000 and destruction of the radar detector was ordered by the courts. The driver's licence was suspended for 3 months.|
|Brazil||Illegal in all states.|
|Bulgaria||Detectors legal, jammers illegal.||Radar jammers are illegal.||X, K (fixed camera+radar)|
|Canada||Regardless of whether they are used or not, police there may confiscate radar detectors, operational or not, and impose substantial fines in provinces where radar detectors are illegal. Quebec penalizes $500 CAD for use of a radar detector, along with confiscation of the device.|
|Croatia||Detectors and jammers are illegal to use or to have in a motor vehicle.||HRK 2000 (€ 270) fine.|
|Czech Republic||Detectors legal, jammers illegal.||Fine up to CZK 200,000 (€ 7750), penalty points.||Ka|
|Egypt||Detectors and jammers are illegal.|
|Estonia||Illegal to use or have in a motor vehicle.||Fine up to 18000 EEK (€ 1150) as well as confiscation of the device.|
|France||*Illegal||Regardless of whether they are used or not, police there may confiscate radar detectors, operational or not, and impose substantial fines.|
|Finland||Illegal to use or have in a motor vehicle on a public road.||The fine depends on income. The detector will be confiscated.||Ka|
|Germany||Legal to own, illegal to use in a moving vehicle||75€ fine, 4 Points, destruction of the radar detector.|
|Greece||Illegal||2000€ fine, 30 day driver's license suspension, 60 days car registration license suspension and 5 SESO penalty points|
|Hungary||Detectors: legal. Jammers: legal to own, illegal to use.|
|Iceland||Legal||Radar jammers are illegal.|
|Ireland||Illegal (Republic of Ireland, Éire)||Law Refers to 'Speed Meter Detectors', Introduced 1991. No fixed penalty for possession, expect confiscation and Hefty fine.|
|Israel||Legal||Some cellular providers such as Pelephone actually provide radar detecting as a service powered by GPS|
|Jordan||Illegal||Regardless of whether they are used or not, police there may confiscate radar detectors, operational or not.||Laser (possibly others)|
|Kazakhstan||Legal||X, K, Ka, X POP, Ka POP, Laser|
|Latvia||Legal to own, illegal to use in a vehicle||40 LVL (57 EUR) fine and the detector will be confiscated.||K, POP Ka, Laser|
|Lithuania||Illegal||X, POP K, Laser|
|Malaysia||Illegal to possess, purchase, sell or use. Heavy fines apply.||Radar detectors are also prohibited items under customs laws.|
|Netherlands||Illegal||420€ fine and seizure of the device (since 2004)|
|New Zealand||Legal||Ka and Laser used also fixed speed camera and speed camera vans (Low Powered K Band)|
|North Korea||Illegal||Confiscated in border controls along South Korea and China borders, and in airports.|
|Norway||Illegal to use. (Legal to own, sell, and buy)||8000-10000 krone fine and seizure of the device.|
|Poland||Legal to own, illegal to use in a moving vehicle||Jammers illegal||X, K, Pulse K, Ka Narrow, Laser|
|Romania||Legal since 2006||Radar jammers still illegal||Instant-On K|
|Russia||Legal||The national system for measuring the speed of vehicles and cars. National ranges work radars in X and Ku.||Instant-On (Pulse) X(10,2-10,65GHz), Ku(13,56-13,62Ghz) POP K, Laser (0,8-1,1 mkm)|
|Serbia||Illegal to use, possess, sell or advertise||Illegal to use or have in possession in a vehicle in traffic, to sell or advertise: an apparatus or any other means of detecting or interfering with operation of vehicle speed measuring devices, or any other apparatus used for discovering and reporting traffic violations|
|Singapore||Illegal to possess, purchase, sell or use. Heavy fines apply.||Radar detectors are also prohibited items under customs laws.|
|Slovenia||Legal||Laser jammers illegal (fine up to 500€ and confiscation of the device).|
|South Africa||Illegal to use in a moving vehicle||Importing a unit is permitted under South African customs regulations.|
|Spain||Radar Detectors legal, jammers illegal.||Radar and Laser jammers fine up to 6000€||Ka narrow, laser, Autovelox|
|Switzerland||Illegal||660SFr fine. Radar detectors may be confiscated and destroyed. The use of any GPS-based device to locate speed cameras is also illegal.|
|Turkey||Illegal||Radar jammers are illegal|
|United Kingdom||Legal||Legal to own, but technically illegal to use under the Wireless Telegraphy Act 1949 until 27 January 1998, due to legal loophole causing them to be legalized suddenly.|
|United States||Law varies from state to state, but detectors are generally legal in private vehicles under the Communications Act of 1934 and illegal in commercial vehicles by DOT regulation (49 CFR 392.71). Exceptions:
||Confiscation and/or destruction of the detector was once a common practice but lawsuits raised by drivers arguing violation of property rights have resulted in temporary removal while a citation is written, then return of the device after its description (make, model and serial number) has been entered on the ticket - usually for speeding and possession/operation of detector. Use of a radar/lidar detector on a military installation is prohibited. Persons entering a military installation with one visibly mounted to the sun visor or windshield will be asked to remove it and put it away. Those who refuse will be denied entry onto the installation at that time. Military law enforcement may not stop a vehicle solely for a detector being in use, but along with being stopped for another moving violation, active duty personnel using a detector may be ordered to report the violation to his/her unit commanding officer.
Radar jammers are illegal under federal law, but laser jammers are not specifically banned. Nine states have state laws specifically banning laser jammers: California, Utah, Texas, Colorado, Minnesota, Illinois, Tennessee, South Carolina, and Virginia.
See also 
- "Radar Detector Detector (RDD)". Radar.757.org. Retrieved 2010-10-17.
- "FCC Ban". Radarjammer.com. Retrieved 2010-10-17.
- "Communications Act of 1934" (PDF). Retrieved 2010-10-17.
- "Radar Speed Trap Warning Apparatus (Licence)". Hansard. 1967-03-02. Retrieved 2010-04-25. "a pocket-sized instrument is being produced by Marchant House Limited of New Street, Oadby, Liecester, to be used in motor vehicles to give instant warning of a police radar speed trap ... Anyone wishing to use this type of apparatus would require a licence under the Wireless Telegraphy Act, 1949. The firm in question has recently been informed that I am not prepared to issue a licence for such purposes"
- "Road Traffic (Miscellaneous) Variation Regulations 2003". SA State Government. 2003.
- "Traffic Infringement Notice". Department of Infrastructure, Energy and Resources.
- "De Standaard 2006-07-20". De Standaard. 2006-07-20.
- "United States Department of State: Consular Information Sheet for Canada". Travel.state.gov. Retrieved 2010-10-17.
- "Promet". mup.hr (in Croatian). Ministry of the Interior. Retrieved 2010-01-30.
- German Bussgeldkatalog[dead link]
- "proxeiro_kok.indb" (PDF). Retrieved 2010-10-17.
- "Irish Statute Book S.I. No. 50/1991 — Road Traffic (Speed Meter Detectors) Regulations, 1991". Irishstatutebook.ie. Retrieved 2010-10-17.
- "Dutch Ministry of the Attorney General - Speed and violations".
- "Ofte stilte spørsmål om fart" (in Norwegian). politiet.no (Norwegian Police). 21 August 2009. Retrieved 24 November 2009.
- "OUG 63/2006 Art. 44 Al.4-5".
- "Zakon o bezbednosti saobraćaja na putevima, SLUŽBENI GLASNIK: 41-09 (Art. 29)" (PDF).
- "Mobile Scanner & RADAR-Detector Laws In The United States". Afn.org. Retrieved 2010-10-17.
- DoD Traffic Safety Program
- "Windshield Mount Law". Garmin.com. Retrieved 2010-10-17.
- "Connecticut Repeals Detector Ban".
- "USA Laser Jammer Laws".