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Speed Guns

  • 01-09-2009 2:53pm
    #1
    Murphy09
    Closed Accounts Posts: 20 ✭


    hi all,

    just wondering if anyone knows if the speed guns can catch you speeding while driving away from the speed gun. i know that they get you while your going towards them because there programmed as how quick the laser comes back to the gun at and that determaines your spped, but can they work the other way round???

    often see garda up on hills outside carrickmacross which is blind to cars that would be drivin away from them, just wondering if there been sneeky that way or are they just checking the cars comin towards them?

    thanks


Comments

  • #2
    Hooch
    Registered Users, Registered Users 2 Posts: 4,957 ✭✭✭Hooch


    betafrog wrote: »
    Of course it can...

    +1

    It works on same principle as car travelling towards the gun....expect when the detected vehicle is travelling away it shows a minus (-) before the speed.


  • #3
    johnl
    Closed Accounts Posts: 207 ✭✭johnl


    The car moves closer
    And the gun measures how fast
    We are left behind.


  • #4
    Murphy09
    Closed Accounts Posts: 20 ✭Murphy09


    i also read somewhere else on the internet the laser guns send out a scatter, which on a good str8 road apparently can get about 200yards away making a 'v' like laser scatter from the gun.

    is this true?

    if so how are the garda using the gun sure as to which car it is that is speeding, say if there was 10 cars in a row and only 1 was speding, like the last car as it is catching the end of the pack?


  • #5
    Murphy09
    Closed Accounts Posts: 20 ✭Murphy09


    was just bored there and googled how a laser speed gun works, makes for quite intrestin readin, here ya go,

    Laser (or lidar, for light detection and ranging) speed guns 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 gun measures the round-trip time for light to reach a car and reflect back. Light from a laser speed gun 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 gun shoots a very short burst of infrared laser light and then waits for it to reflect off the vehicle. The gun 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 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 or so, the accuracy can be very high.
    The advantage of a laser speed gun (for the police anyway) is that the size of the "cone" of light that the gun 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 gun to target a specific vehicle. A laser speed gun is also very accurate. The disadvantage is that the officer has to aim a laser speed gun -- normal police radar with a broad radar beam can detect doppler shift without aiming.


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