The lidar gun is an effective instrument for police. The lidar gun produces a radar which strikes the vehicle and records the speed as well as the distance between it and the police vehicle. The speed of the vehicle can also be computed with the help of lidar gun.
Police lidar gun description:
The lidar gun used by police is 904 nanometers, 5 nanosecond pulses of 25 Watt instantaneous power delivered into a 4 milliradian cone angle at 1 KHz repetition rate. Safety of eyes is ensured by the long wavelengths and low average power. The divergence angle on transit permits to use the units without a using a tripod. The flight time is multiplied by the light speed and the results are plotted as a function of time. A line is drawn by least square method. The car speed is represented by the slope and validity test is done by the variance.
lidar gun power:
The lidar width at 250 meters is about 1 meter^2. The license plate (Colorado) has about .001 meter^2 of retroreflective paint. – 30 dB loss. They are illuminating the retroreflective paint with 25 milliWatts
The return (from the retroreflective paint) is also about 1 meter^2 and the receiving aperture is about .001 meter^2 – 30 dB loss.
The return (Colorado) is thus 25 microWatts instantaneous power. The effective measurement bandwidth is 30 GHz.
lidar gun range The range of the lidar gun is the fourth root of target crossection. This is denoted by a very weak function and lies between extreme ranges and lowest ranges as measured by people in each state. lidar gun jammers: Two type of lidar gun jammers are available. They are:
- CW Jamming sources: Headlights focussing into .5 by .2 radian allot their power over 0.1 steradians, at 250 meters range, this elucidates 6000 square meters or 10^(-6) of the police encountering aperture. 200 Watt lights cast 200 microWatts into the lidar gun. It is assumed that the lidar gun has a narrow band filter passing about 10 nanometer of the spectrum, reducing this CW jammer by a factor of about 40, meaning that the light is now 5 microWatts. The detector is coupled with AC.
- Pulsed Jamming Sources:
- LEDs: With a 250 meter range, LEDs broadcast into .005 steradians (.5 radian horizontal times .01 radian vertical) would have to be 500 times brighter than the 25 milliWatts they hit you with to beat the retroreflective paint which broadcasts into only 10^(-5) = ( 4 milliradian times 4 milliradian) return. Jamming is a problem for short ranges. The police lidar power develops as 1/(Range^4) power as the range reduces, and the jammer power develops only as 1/(Range^2). Jamming is not practicable because one have to broadcast into every directions, minimizing the power focusing at the lidar gun.
- Laser diodes: The lidar gun uses the strongest laser diode. This is a very good jammer at ranges more than about 200 meters for a jamming signal broadcast into .005 steradians.
Thus the lidar gun is indeed an effective tool for the police!
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