How Radar Detectors Work

Radar stands for Radio Detecting And Ranging. The system operates by transmitting radio waves at certain frequencies which reflect off objects and are picked up by the receiver. When the beam reflects off a moving object, a measurable frequency shift occurs which is then converted into miles per hour to determine the object's speed. There are 3 main sets of frequencies used by the manufacturers of speed trap equipment. These are known as X, K & KA. The majority of traps use K band (e.g. GATSO cameras and most hand held guns). X band is the oldest and is used by some older gun based systems, it is also the band where most false alarms occur since other pieces of equipment use this frequency range. KA band is used by the new 'Stalker' radar guns and is one of the most popular, having been extended twice since 1991. GATSO is the name given to the Dutch made "photographic trap" system used in the UK and Europe. The GATSO traps are unmanned and take a photo of the rear of the speeding vehicle. GATSO traps operate on K band and are therefore detectable by most good detectors. The majority of GATSO cameras are inactive - the average ratio is one "live" camera site for every ten boxes. Even "inactive" sites will appear to take photographs of passing vehicles by flashing at them, but since no photographic equipment is installed no photographs can be taken. If the GATSO system is fully loaded it is transmitting K band signals constantly monitoring the speed of every vehicle that passes. The laser speed Detection System uses a gun that emits infrared light pulses just outside the spectrum of visible light. Each pulse measures the distance to any object that reflects the laser. The speed of the object coming towards the gun is measured using a very narrow beam of light so that it can pinpoint a speeding car in the traffic. A radar detector will pick up the signal due to "splattering" caused by the beam hitting warm and cold pockets this "splattering" makes the beam appear much wider. VG-2 is a microwave receiver used by some police forces to detect signals radiated by the local oscillator of a radar detector, because of this VG-2 has become known as a 'radar detector' detector. It is primarily used to identify radar detector equipped vehicles, as, in the past driversfaced losing their detector if caught. VG-2 immunity prevents electronic detection of your unit. VASCAR is an acronym for Visual Average Speed Computer And Recorder. This is little more than a glorified stopwatch, whereby the vehicle is timed over a set distance (for example over two white markings in the road surface or over two bridges) an average speed is then automatically calculated. No radio waves or beams of light are emitted and thus this system is unable to be detected by any form of electronic detector. Radar Facts RADAR: Acronym for RAdio Detection And Ranging. A remote sensor that emits electromagnetic waves on order to measure reflections for the purpose of detection. X Band Radar: Frequency tolerance 10.525 GHz25 Mhz Frequency range 10.500-10.550 GHz. X band radars have been around since the 1960s and operate on a single frequency. Typically their operational range was 20 mph- 90 mph or more. U.K. and Australia ceased using X Band Radar many years ago when the frequency was licensed out to other industries that required access to Microwave transmitters (alarm systems etc). K Band Radar: Frequency Tolerance 24.150 GHz100Mhz Frequency Range 24.050-24.250 GHz. K Band radars have been around since the 1970s and operate on a single frequency. With K BAnd operating in the limits of the water vapor absorption band (centered at about 22.24 GHz) signals in the absorption band tend to become absorbed by moisture in the atmosphere and do not have the range that other frequency bands offer. Primarily this is why the FCC allocated this frequency for short range Police use. The most well known Radar devise operating on this frequency is the HAWK. Ka Band Radar: The available bandwidth allocated to Ka Band traffic radar is 2.6 GHz operating between 33.4GHz-36GHz. Most Ka traffic radar have a frequency tolerance of 100Mhz (200MHz band width). Therefore 2.600 MHz (available band width) divided by 200MHz (Channel Bandwidth) equals 13 channels. A traffic radar in the Ka band with a frequency tolerance of 100MHz may have more channels, but some or all the channels will overlap. Some models transmit on a single frequency only. Others may allow the operator to select one of the several fixed frequencies. Some can hop from one frequency to the next in a Phase Loop. The Doppler Principle: Everyday life has a multitude of examples of the doppler effect with sound. The whistle from a train is a good example. As the train approaches a stationary listener, the pitch (frequency) of the whistle sounds higher than when the train passes by, at which point the train and the person standing are technically stationary. Electromagnetic waves radiated by the traffic radar obey the same principle, although electromagnetic waves travel at the speed of light and audio waves at the speed of sound. The Doppler Effect that enables police radar to work is a frequency shift that results from relative motion between a frequency source and the listener. The Doppler shift is proportional to speed between source and listener, frequency of source, and the speed the waves travel at (speed of light for electromagnetic waves). Instant ON (Pulse Radar): Intended to defeat radar detectors. Instant ON radar allows the operator to control the radar transmission. The operator only transmits after selecting the target, and only long enough to get a speed reading. In practice most police find this a difficult mode to operate in and are more likely to have the radar on all the time unless two officers are present in the car, one driving and one working the radar. Cosine Effect on Moving Radar: Moving Radar measures closing speed between the radar and target. The radar also measures patrol car speed (from the ground echo) to calculate the target speed. (Target speed=closing-patrol car). This introduces additional sources of cosine error. In most situations the angle between the radar and target is the major error source and favours the target (measure too low). However if the antenna is misaligned (off patrol car direction) the patrol car speed may measure low resulting in target speed measured too high. Moving Radar Variables: Target speed will only measure higher than true speed when the target is approaching the patrol car AND the cosine angle between radar and target are small, (typically less than 5%) AND the angle between the patrol car and the ground is large, (typically greater than 5%). Patrol car and target speeds are significant, patrol car speed greater than target speed increases the error. (The greater the difference the larger the error and the higher the measured speed). Shadowing: Radars identify ground echoes as the strongest signal (most of the time). The ground echo cosine angle is a function of the radar antenna alignment and beam width. More reflective terrain in only part of the beam could change the angle of the ground return (shadowing) which can change the measured speed of the patrol car. Large and or reflective objects such as overpasses or billboards and road signs may have a momentary effect on radar. Guardrails, bridge trusses and construction zones may have a longer effect. Ka Band Radar: Photo Radar: Automatic unattended photo radar started appearing in the late 80s and came to U.K. in 1993. With Photo radar systems a human operator does not observe any speeding violation, but is replaced by electronic circuits and a photo-recording device. No one has to see the alleged violation; the process is automatic. The registered owner of the vehicle usually receives a ticket in the mail. Photo radar is across the road radar and designed to point a narrow beam of radar (typically 5 degree horizontal beam width) across the road at an angle of 22.5 degrees. Speed measurement is then adjusted for the angle.Some units operate with an amber (orange) flash filter. This is not as bright to the human eye and causes minimum disruption to a driver even at night. Power output is very low (2.5mW typically) which makes detection for radar detectors difficult, but not impossible.