LASER-Light Amplification by StimulatedEmissions of Radiation

LADAR-LAser DetectionAnd Ranging

LIDAR- LIght Detection And Ranging

    Laser

            Laser radar guns are pulsed infrared light instead of continuous microwaves to register a target. This pulsed infrared light measures target distance call "time of flight." The time it takes for a laser light pulse to travel from the gun to the target and back again , is used to compute the range of target. The change in target range over time equals target velocity. Laser has an distinct advantages over radar guns. At five hundred feet the Laser's elliptical beam is a mere 18" wide  compared to X band radar at 155 feet.

                        .  Laser Gun's Infrared Signature                                    A Laser Gun               .        Lidar Gun Description
     
         -Police lidar is 904 nanometers, 5 nanosecond pulses of 25 Watt instantaneous power delivered into a 4           milli-radian cone angle at 1 KHz  repetition rate. The long wavelength and low average power aids eye safety. 

Models LTI 20-20 Marksman                         Pro Laser II/ browser Range                                30 to 2000 ft.                                         20 to 2000 ft. Speed                                  0 to 200 mph                                        5 to 299 mph       Time                                   0.3sec +/-                                              0.3 to 0.6sec +/- Accuracy                            +/-1mph                                                +/- 1 mph  

    Lidar Range
 
          Range for the Laser Gun is about 2600 feet to 2000 feet depends on reflectivity and weather. A Supreme
     Court ruling in New Jersey states that the must be used 1000 feet or less.
                 -The best and worst license plates measured by Tom Bell:
                            Georgia license plate, by itself = 2577 ft.
                            •Colorado license plate, by itself = 1339 ft.
                            •With no plate- detection range of a Mazda RX7 with headlights retracted is around 800 ft.
                            •No plate, and the lights covered - detection range < 500 ft.
                            •Lidar guns are effective to about 2000 feet.
                            •Average operating range of police lidar is about 800 feet.

                 -A white car at 1000 feet has 10 times the reflectivity of a black car at the same distance.
        "Typical radar ambushes are 600-800 feet because the police must identify the car and make a visual
       estimate of speed as well." This argument may not apply to lidar with its narrower beam, since the beam
       itself  identifies the car. ~Craig Peterson

      Problems with Laser
 
             1. Laser cannot be used:
                    - in moving mode, it must be stationary
                    - rain, snow, or high dust environments

            2. Interference
                If  for any reasons intended signal returns are interrupted, it may not be able to determine target
           velocity. Bright lights (such as Halogens) beaming directly into a laser aperture may desensitize
           or entirely masks  target's echoes. The brighter and closer the light source the greater the chance of
           interference.

             5. Reflection Error
                On very hot days with low humidity a visible mirage/reflection of the target vehicle is created. In
             many cases, when the laser is aimed at the target vehicle  the infrared beam also receives readings from
             both the target vehicle and the mirage causing a Sweep Error.
 
             6. Overexposure Error
                When a laser gun receives an extremely powerful reflective signal, such as a sun flare off a vehicle,
            the computer's timer can't see return of the 904 nanometer signal it sent. It can't compute a speed
            reading. In general, the laser gun is looking for the strongest return reflection of its own emitted beam
            for speed computation.

    Stealth
 
         Stealth is not invisibility, it is just reduction of the range. Without stealth, in most circumstances, lidar detectors only go off when you have been targeted, and you don't have the time to slow down. With some range reduction due to stealth, they might target you before you come into range. They are trained to point at your license plate (or headlights.) If you have a stealth plate cover, your detector goes off, and you have a chance at slowing before they retarget another part of your car.

      A. Lidar targets on your car. (what is most important)

        The license plate is the most reflective part of the vehicle (if ones in place.) Different States have a very large difference in the lidar return. Colorado is one of the lowest, using retroreflective paint on only the letters and a thin border. Most large states [3] have better retro-reflective material covering a larger area of the plate.

        The strongest reflection of the rear of the car is an array of little corner cubes in the red tail light cover. All cars have a hexagonal array in the plastic tail light covers, like a bicycle reflectors. The second strongest target on the rear is the rear license plate.

        There is also a corner cube array on the side front turn signal indicators. These do not present much of a target unless the police target you from the side in which case the cosine factor in speed measurement will give them too low a speed to worry about.

        Next in strength are head lights, brake lights, turn signal indicators, and fog lights. Of these, headlights are highest, followed by turn signals and fog lights. The retro-reflection mechanism is interesting. Light enters one half of the light and is returned from the other half.

        Grills, forward facing chrome, and any rounded specular material are the next strongest targets. Flat regions are almost always pointed away from the source and hence do not contribute to the return.

    B. Stealth Techniques. (What to do about these Lidar targets)

        The license plate is the most important on the front of the car, unless you are blessed with a plate without retro-reflectors (The only example I know of this is the old California yellow on blue plates) In some states you can legally remove the front plate. In some states it may be sufficient to paint over the retro-reflective paint with glossy house paint to match the color. Check your local laws first. Many states use 3M retro-reflective material, which would be difficult to paint over inconspicuously. For these states it is preferable to use a lidar cross-section reducing license plate cover. One such cover , is reviewed at the end of this document.

         The array of corner cubes in the tail light cover. These are on all cars, and are probably required by law. If you paint them black, a car driving by would not see your car on the side of the road as easily.

         Head lamps, turn signal lamps, and fog lights. Get retractable headlights or glue a section of IR absorbing license plate cover over your headlights. Remove the fog lights, glue license plate cover over your turn signal indicators. The "smoke" headlight covers that are on the market do a fine job of reducing retro-reflection from headlights. The down side to these things is that they really aren't legal in most states.

         Get a flat black car bra, paint over flat surfaces with glossy paint, paint over rounded surfaces with flat paint. These returns are small compared to the Plates, corner cube array, and lamps. If you get the license plate, the corner cube array (back of car only) and the lamps I think you'll get your visibility under the average range at which police target cars.

 
Jamming Police Lidar
 
-Craig Peterson wrote reviews of radar and lidar jammers in which he tested products that use these jammer techniques and found they didn't work.

     Is Jamming feasible? The two techniques outlined here do not appear to be viable on technical grounds. Assumptions about these techniques are built into the descriptions below, and a more Sophisticated jammer might work.

     Is it legal? Jamming lidar is not illegal under FCC rules since they don't regulate this part of the spectrum, but most jurisdictions have a law which makes it illegal to "interfere with the duties of a police officer."

 
      CW Jamming sources.
 
     -Headlights aimed correctly distribute their power, 200 Watt lights put 200 micro Watts into the lidar gun. Presumably 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 micro Watts. This is small compared to the 25 micro Watts return from a license plate. In many reviews this technique works but only down to a certain feet dependent reflective of the vehicle.