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How Do Radar Detectors Work?

American Government Special Collections Reference Desk

How Do Radar Detectors Work?

Jennifer Bailey
March 1, 2006

Police have long used radars to track the speed of drivers and to help maintain safe roads. Meanwhile, many drivers have begun using radar detectors to help avoid costly fines for speeding violations. Despite the popularity of radars and radar detectors, many people do not understand the science or technology behind them.

Police radar guns work by transmitting radio waves from the radar to the target vehicle and back. Because radio waves move through the air at a constant speed (the speed of light), radars can calculate how far away an object is based upon how long it takes to the radio signal to return. When an object such as a vehicle is moving, there is a change in frequency in the radio waves. Radars detect this change and convert it into miles per hour to determine the target's speed. Further, radar guns also use different types of bands to determine the target's speed, including X band, K band, and Ka band. Traditional radar detectors alert the driver to the presence of radars.

Law enforcement officers also use lasers (concentrated light) to determine a vehicle's speed. Laser radar guns measure the time it takes for infrared light to leave its origin, reach a car, bounce off, and return. These tools can determine how far away an object is by multiplying this time by the speed of light. Because this type of radar sends out many bursts of light to determine multiple distances, the system can determine how fast the car is moving by comparing these samples. However, because laser speed detectors have a much more focused beam and detect lasers over great distances, police laser guns are generally more difficult to evade. While modern radar detectors often include a light-sensitive panel that detects these light beams, the detector (and thus vehicle) is usually already in the beam's sights.

While traditional detectors worked by alerting the driver to the presence of the radar or laser, the last few years have seen a rise in radars that also emit a jamming signal. This signal duplicates the original signal from the police radar gun and mixes it with additional radio noise, which confuses the radar receiver, and prevents the police officer from preventing an accurate speed reading. Some detectors also use a laser jammer, or light emitting diodes (LEDs), that produce a light beam of their own. This beam prevents the receiver from recognizing any reflected light to get a clear reading on the vehicle's speed.

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