March 9, 2007 By Jim McKay, Justice and Public Safety Editor
Radar has been used for decades with more than adequate results, and lidar has come onto the scene in recent years as a viable companion to radar.
Radar -- short for radio detection and ranging -- sprays a web of high-frequency radio waves in a cone shape, finds an object, and gauges its speed. Radar uses electromagnetic waves, or radio waves, to locate moving or fixed objects. A radar beam used for tracking the speed of motor vehicles is typically 12 feet wide and 100 feet long.
It uses the Doppler principle, which measures frequency change. The radar transmits a microwave frequency that bounces off the vehicle and returns to the initiator. The vehicle's speed is calculated by measuring the difference between the frequency that reached the vehicle and the frequency that returned.
The radar frequency, or beam, is conical in shape and reaches outward until it is reflected, refracted or absorbed. The range of the beam can be controlled by the operator. Radar disperses its beam and clocks any vehicle that enters that beam.
A benefit of radar is that it can be used in a moving vehicle, whereas a lidar operator must be stationary.
Much like the technology used by surveyors, lidar -- short for light detection and ranging -- shoots a laser at a target to measure its distance and speed. This laser beam is about 1 foot to 3 feet in diameter, and with its approximate 1,000-feet reach, lidar has a wider range than radar. Another advantage of lidar over radar is that it lets police target a specific vehicle.
Closing in on Tailgaters
Lidar's ability to measure the distance between moving vehicles is a relatively new feature of the technology that police increasingly use to bust tailgaters.
The officer sets the gun to measure the distance between himself and the center of a traffic lane. When two cars pass by, the gun tracks the speed of both cars and calculates the distance between them.
The Colorado State Patrol uses this function in heavy traffic to target aggressive drivers as well as speedsters.
"Lidar works extremely well in heavy traffic conditions," said Sgt. Kevin Ratzell. "The laser beam allows the officer to individually pick out a violator's vehicle even while in a group of cars."
The Colorado State Patrol also uses lidar to take accurate measurements at accident scenes by measuring skid marks, reference points and so forth. "This is great stuff that radar does not have the capability of doing," Ratzell said.
The Arizona Department of Public Safety recently purchased nine lidar units and uses them primarily to bust speeding motorists. But a side benefit, and one that the department sought when it made the purchase, was the feature that measures the distance between cars.
"That was one of the benefits we were looking for," said Tom Mason, public information officer for the Department of Public Safety. "Lidars are like a blender; they come with different functions within them. You can get upgraded versions. We elected to purchase them with that extra [tailgating] feature on it. People don't think [tailgating] is dangerous. They don't understand why it's illegal, and we've definitely gotten the message out by using those instruments. It's been a really good tool."
About two years ago, the Newark, Del., Police Department had grant money to spend and considered either lidar units or radar units.
"The decision was to give lidar a chance," said Master Cpl.
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