New speed detectors catch traffic violators in the act with more accuracy and less work for officers.
September 01, 2005
How Lidar Works
Laser or Lidar, light detection and ranging, uses a laser, or concentrated light, to measure a vehicle’s speed. Specifically, it tracks how much time it takes for the light to bounce back to determine how far away it is. It does this several times to develop a tracking history of how quickly the target is moving.
A laser speed gun shoots a very short burst of infrared laser light and then waits for it to reflect off a vehicle. The gun counts the number of nanoseconds it takes to reach the car and back, and by dividing by two it can calculate the distance to the car. If the gun takes 1,000 samples per second, it can compare the change in distance between samples and calculate the speed of the car. By taking several hundred samples over the course of a third of a second, the accuracy can be very high.
To operate a laser speed detection device, an officer points and aims the laser gun like a rifle at a particular vehicle that appears to be moving quickly. Most models feature a heads-up display with crosshairs to help aim. Then he lets the device do its work and reads the display.
The advantage of a laser speed gun is that its “cone” is very small. It’s only about three feet in diameter at a range of 1,000 feet. The disadvantages of laser guns are their higher cost and that the officer operating the device must accurately aim at one vehicle to catch its speed.
As with most technology, both radar and lidar guns are tending toward smaller and lighter models. The industry has also come out with features that make the job of speed detection easier for the officer.
Carl Fors, president of Speed Measurement Laboratories, tests radar and lidar guns and instructs officers in the use of these speed measurement devices. He says the new industry standard for radar is Ka-band. Fors has seen a lot of innovation over the past 20 years, and says this is one of the many changes that has occurred within just the past five.
Other frequencies, especially K-band, are still used, but because Ka-band uses a much narrower section of the spectrum, it can operate with smaller antennas and on smaller devices. This makes it more attractive to officers who don’t want to lug a large handheld device or don’t have room for any more electronics than necessary in their already crowded cruisers.
Other innovations have reduced the errors made in calculating speed by streamlining the process for the operator.
With radar it can be difficult to find the fastest car in a group of vehicles. But now it’s easier with the function on many new models that finds the fastest vehicle in a group. Of course, with a laser you just aim at one vehicle and get a display of its speed.
“In the past, you had to manually tell the radar gun if you were moving or stopped by keying it into a remote control,” says Fors. “If you’re in hot pursuit, you don’t have time to think about it. New guns do it automatically.
“They use a VSS (Vehicle Speed Sensor) interface. The gun itself senses whether it’s moving or stationary. This is critical in terms of what new radar guns can do. It eliminates one of the greatest errors in radar guns: shadowing. Automatically recognizing whether the device is moving or stationary eliminates that.”
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