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Behind-the-Wheel

Dear Cop - Radar

How does Radar work? Is it possible to get the wrong car in high traffic? How do police make sure there isn't a wrong reading? How often are there
mistakes made and a ticket given when none should have been?

A police officer uses radar and laser speed instrument as a confirmation of a vehicles speed. The police officer will first visually estimate a vehicles speed to be in excess of the posted speed limit and then activate the speed gun to confirm the estimation. There is little room for error.
A normal speed radar instrument sends out a radio pulse and waits for the reflection. Then it measures the Doppler shift in the signal and uses the shift to determine the speed. It is possible to obtain two different readings on to vehicles in close proximity, however if one vehicle is passing another vehicle it is safe to say the higher obtained speed reading is on the passing vehicle. These speed radar instruments are calibrated both externally and internally as per the manufacturers specifications. If the instrument is not functioning properly, it is not used and sent in for repairs.

The new laser speed instruments use a more direct method that relies on the reflection time of light rather than Doppler shift. You have probably experienced the reflection time of sound waves in the form of an echo. For example, if you shout down a well or across a canyon, the sound takes a noticeable amount of time to reach the bottom of the well and travel back to your ear. Sound travels at something like 1,000 feet (300 meters) per second, so a deep well or a wide canyon creates a very apparent round-trip time for the sound.

A laser speed instrument measures the round-trip time for light to reach a car and reflect back. Light from a laser speed instrument moves a lot faster than sound -- about 984,000,000 feet per second (300,000,000 meters), or roughly 1 foot (30 cm) per nanosecond. A laser speed instrument shoots a very short burst of infrared laser light and then waits for it to reflect off the vehicle. The instrument counts the number of nanoseconds it takes for the round trip, and by dividing by 2 it can calculate the distance to the car. If the instrument 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 or so, the accuracy can be very high.

The advantage of a laser speed instrument (for the police anyway) is that the size of the "cone" of light that the instrument emits is very small, even at a range like 1,000 feet (300 meters). The cone at this distance might be 3 feet (1 meter) in diameter. This allows the instrument to target a specific vehicle. A laser speed instrument is also very accurate. The disadvantage is that the officer has to aim a laser speed instrument. The normal police radar with a broad radar beam can detect Doppler shift without aiming.

Laser Speed instruments are also calibrated for distance and accuracy as per the manufactures specifications and if the instrument is not functioning properly, it is not used and sent in for repairs.

Constable R.A.(Richard) ASELTON
Central Okanagan Traffic Services - Media Liaison
Kelowna R.C.M.P. Detachment


More Behind the Wheel articles

About the Author

Tim Schewe is a retired constable with many years of traffic law enforcement experience. He has been writing his column for most of the 20 years of his service in the RCMP.

The column was 'The Beat Goes On' in Fort St. John, 'Traffic Tips' in the South Okanagan and now 'Behind the Wheel' on Vancouver Island and here on Castanet.net.

Schewe retired from the force in January of 2006, but the column has become a habit, and continues.

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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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