[Index][Thread]

Frequently Asked Questions about Police Lidar



With the Lidar detectors on the market, you can detect lidar pointed
at your car, but lidar measures your speed so fast you don't have 
time to react.

What can you do?  Details are in the FAQ, here's the summary.

Summary:
A detector will save you sometimes
   If they target a car ahead of you, and you are ideally situated
   to detect the lidar, your detector might go off.
Stealth in combination with a Lidar detector will save you at long range.
   If they are targeting cars at 1000 feet, and your car has reduced
   reflectivity, they don't get a speed reading until you come into range.
   Your detector goes off and you have time to slow down.

Archive-name: PoliceLidar-faq
Version:  0.9
Last-modified: August 9, 1995


Frequently Asked Questions about Police Lidar
==========================================================

                         Table of Contents                         
-------------------------------------------------------------------
1. Lidar Gun Description
2. Lidar power estimates
3. Lidar Range
4. Stealth
  A. Lidar targets on your car.  (what is most important)
  B. Stealth for these Lidar targets.  (what to do about them)
5. Jamming Police Lidar
6. Caveats
7. References
8. Product Reviews.


1. Lidar Gun Description

Police lidar is 904 nanometers, 5 nanosecond pulses of 25 Watt instantaneous
power delivered into a 4 milliradian cone angle at 1 KHz repetition rate.
The long wavelength and low average power aids eye safety.  The divergence
angle on transmit allows the units to be used without a tripod.  The time of
flight of the pulses are multiplied by speed of light and the resulting
distances are plotted as a function of time.  A least square fit is
used -the slope gives the car speed, and the variance gives
a validity test.[1]

2. Lidar power estimates

You need to know this power if you want to build a jammer.

The lidar beam 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 beam (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.  Other states have stronger
returns, with Georgia 14 times as strong.


3. Lidar Range

Lidar range goes as the fourth root of target crossection.  This is a
very weak function, and so the range for your state probably falls
between these extremes of measurment range to the best and worst
liscense 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.

The Uniden detector ad (Aug 95 Road&Track p31) claims that:
   Uniden detector is good to about 8000 feet.
   Lidar guns are effective to about 2000 feet.
   Average operating range of police lidar is about 800 feet.

Craig Peterson has written alot on police radar and lidar, and
  he makes the argument in AUTOtronics, March 95  that 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.

4. Stealth

Stealth is not invisibility, it is just reduction of the range.
This may help in combination with a detector, if the police target 
cars at the far half of their range as defined by the reflectivity of 
the average car.  You get the warning before they get the speed measurement.
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.

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

To find out what is most important on your car,  stand with the
light source behind you (park in the sun at dawn or dusk), and look at
the region of the car right next to the shadow of your ear.  You are looking
at light reflected back towards the source.  Alternately, you can at night
use a flashlight held against your ear and pointed at the car.  Stand back
by at least 30 feet to see light returned at an angle close to straight back
at the source.  Another technique is to park your car on a dark street,
stand >30 feet away and take a flash photo.  Most car parts have similar
properties in the visible and near IR.  Some special materials absorb
in the near IR and transmit in the visible and hence this test using
visible light is not indicative that a special police-lidar countermeasure
is working.

The license plate is the strongest target on the front of the car.
License plates have retroreflective material that returns light in a 4
milliradian cone angle (Colorado plate measurement)[2].  Different States
have a very large difference in the lidar return from the plates.
Colorado is one of the lowest, using retroreflective paint on only
the letters and a thin border.  Most large states[3] have better
retroreflective 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.
These are just like a bicycle reflectors.  You can distinguish 
these retroreflectors from the tail light lens by the array pattern.
The tail light lenses are in a rectangular array, and this corner
cube array is in a hexagonal pattern.
The second strongest target on the rear is the rear liscense 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 retroreflection mechanism is interesting.  Light
enters one-half of the light and is returned from the other half.
I suspect  that the light enters one half, hits the back reflector, 
is focused near the filament, expands to the other side back reflector,
and is recollimated returning towards the original source.  This phenomena
happens over a moderately narrow range of angles directly in front of
the car.

Grills, forward facing chrome, and any rounded specular material
are the next strongest targets. Look for a bright glint
from any rounded surface which always presents one small region facing the
source.  Flat regions are almost
always pointed away from the source and hence do not contribute to the return.


B. Stealth for these Lidar targets.  (what to do about them)

  The license plate is the most important on the front of the car,
unless you are blessed with a plate without retroreflectors
(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 retroreflective paint
with glossy house paint to match the color.  Check your local laws first.
Many states use 3M retroreflective material, which would be difficult to
paint over inconspicuously.  For these states it is preferable to
use a lidar crossection 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.
The only thing I can figure is to get one of the liscense plate covers
that block IR, and cut out a piece and glue it over the corner-cube array.
  (expect new products for this application to come out soon)

Head lamps, turn signal lamps, and fog lights.  Get retractable headlights
or glue a section of IR absorbing liscense plate cover over your headlights.
Remove the fog lights,  glue liscense plate cover over your turn signal
indicators

The most extreme stealth includes taking car of all other possible glints.
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, cornercube array, and lamps.


Lidar (and radar) range goes as the fourth root of return signal power.
By eliminating the _single strongest return_, the best you can hope for is
to reduce the range by about a factor of 2 to 4,
from reducing your crossection by a factor of 16 to 256.  This is because
even if you reduce the strongest return by a larger factor than 256, you 
probably have something else which returns the lidar signal by at least
1/256 as much as this strongest factor.  When you eliminate the strong
return you still have the other source of returns.  To get a range reduction
of more than a factor of 4, you probably have to apply stealth measures to
multiple reflectors on your car.  If you get the liscense 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.


5. 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 $ophisticated 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."
I am not a lawyer and the above should not be considered legal advice.

There are two kinds of jamming proposed and on the market - 
pulsed LEDs and CW Headlights.
My calculations indicate that neither of these work without combining them
with stealth measures.  These calculations are specific to the range of
250 meters.

CW Jamming sources.
Headlights aimed into .5 by .2 radian distribute their power over 0.1
steradians, at 250 meters range, this illuminates 6000 square meters or
10^(-6) of the police receiving aperture.  200 Watt lights put 200 microWatts
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 microWatts.  The detector is AC
coupled so we calculate the shot noise due to this background 
ShotNoise = SQRT[RecievedPower * PhotonEnergy * MeasurementBandwidth]
Sqrt[5. Micro Watt PlanckConstant SpeedOfLight/(900 Nano Meter)*30 Giga Hertz]
= 0.200 microWatt equivalent optical power.
This is small compared to the 25 microWatts return from a license plate.
The Car and Driver article indicates that this jamming technique works,
contrary to this calculation.

Pulsed Jamming sources

LEDs:
At 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.

This is 12 Watts, well beyond the power of an LED.  At shorter range,
the problem of jamming is worse.  The police lidar power grows as
1/(Range^4) power as the range decreases, and your jammer power grows
only as 1/(Range^2).  The reason jamming is not feasible is that
you have to broadcast into all directions, reducing the power aimed
at the lidar gun.

Laser diodes:
The strongest laser diode you can buy is the one they put in the
lidar gun (see product data sheet below).  Unless you actively steer
this jamming signal towards the police lidar gun, it will only
be an effective jammer at ranges farther than about 200 meters for
a jamming signal broadcast into .005 steradians.


6. Caveats

Some of the stuff I put in the FAQ is speculative.
The numbers I assigned to the lidar are estimates.
The calculations are 1 significant digit.
Exact numbers are not important because the lidar problem is
one where the signal power falls as 1/(Range^4), and consequently
a factor of 2 error in estimate of signal power leads to a small
error in estimate of range.
The jamming noise goes as the square root of measurement bandwidth,
making the exact number not that important.  The use of 30GHz gives
a 10 millimeter range resolution.



7. References
[1]  These numbers are best guess averages from the technical references
     (below).

[2]  personal measurement.

[3]  personal parking lot survey of out of state cars in a university town.


Technical References:

  This reference is highly technical, but it describes the design of
  the actual lidar, and hence gives the best data.
o Samuels, M.A., Patterson, S.W., Eppstein, J.A., "Low-cost hand-held
  lidar system for automotive speed detection and law enforcement." Laser
  radar, 1992 v 7  p.147

  I suspect that this is the actual diode used in police lidar.
o Manufacturers Data sheet 1994: PowerSpectra, 919 Hermosa Court,
  Sunnyvale, CA 94086, (408) 737-7977, Fax (404) 732-1832:
  25 Watt peak power, 904 nanometer, 5 nanosecond pulse, up to
  10 KHz rep. rate laser diode.  Fully integrated driver.
  part number POS 904-01 Pricing $645 (qty 1-4), $549 (qty 5-9),
  $339 (qty 100)

  Includes some lidar information as well as radar
o P. David Fisher, "Improving on police Radar", IEEE Spectrum July,
  1992 p. 38

o Shortcomings of Police Radar; IEEE Spectrum, Dec. 1980, pg 28.
  (radar accuracy and usage)

Popular Magazine References:
o Radar Revisited; Road and Track, Nov. 91, pg 106.

o 7 Wide Band Radar Detectors; Car Audio and Electronics, Mar. 93 pg. 103.

o Radar Wars ....;  Car and Driver, Oct.  92, pg 153.

o They have Lasers; Car and Driver, Apr. 92, pg 87.

o Car and Driver, Apr. 94,  (~$300 dollar detectors reviewed)

o Car and Driver, Sept. 95, (~$100 dollar detectors reviewed)

   Craig Peterson Articles:
o The Little Radar Jammer that Didn't, Automobile Magazine 1993 

o Driving Stealth Part 1. The New Combination Radar/Laser Detectors

o Driving Stealth Part 2. Road Testing the Latest in
  Radar/Laser Jammers, AUTOtronics, March 95 p.36

o Don't Get Zapped: 4 Radar Detectors, Car Audio and Electronics,
  March 1995,  p.114

8. Product Reviews.

-----Review of the Taylor-Bell "Laser-Guard" license plate cover 8/2/95-----
reviewed by Robert T. Weverka
  Company Contact: tombell@ix.netcom.com
    Taylor-Bell Technologies, Inc.
    P.O. Box 420002
    Atlanta, GA 30342

    Order line:     800-945-1141
    Business phone: 404-751-5787  

  I got this plate cover and I am impressed.

Visual inspection:
  In transmission in the visible it is indistinguishable to the eye from
clear plastic.  To see that it is not just clear plastic you must hold it
to directly reflect a light source and looking at the reflection you see
a very slight color change.

Optical measurement:
  I measured 2.8% one-way transmission at 900 nanometers, and
the transmission spectrogram shows the transmission staying within
a factor of two of this value from about 830 to 960 nanometers.

Effective Stealth protection analysis:
  The one-way transmission is 0.028.  The two way reduction in lidar return
from the plate should be (0.028)^2 = .00078.  If the plate were the
only target on the car, this would reduce the range at which speed could
be measured by a factor of 0.16, since lidar range goes as the fourth
root of the signal strength and .00078^(1/4) = 0.16