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Police Lidar & Radar Jamming

From drip.colorado.edu!weverka Wed Jul 5 18:33:09 1995
id ; Wed, 5 Jul 95 18:33 CDT
Date: Wed, 5 Jul 1995 17:33:02 -0600
Subject: FAQ on Police Lidar

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FAQ on Police Lidar.

lidar is 904 nanometers, 30 nanosecond pulses of 3 Watt instantaneous
power delivered into a 4
milliradian cone angle at 1 KHz repetition rate.
The long wavelength aids eye safety. 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
velocity, and the variance gives a validity test.[1]

Liscense plates have
retrorflective material that returns light in a 4
milliradian cone angle (Colorado plate
measurement). [2]

The lidar beam width at 300 meters is about 1
The liscense plate has about .001 meter^2 of retroreflective paint.
– 30 dB
loss. They are illuminating the retroreflective paint with
3 milliWatts

return beam (from the retroreflective paint) is also about 1 meter^2
and the recieving
aperture is about .001 meter^2
– 30 dB loss.

The return is thus 3 microWatts
instantaneous power. The measurement
bandwidth is 30 MHz.


The liscence plate is most important. If you can’t remove it, paint the
material with glossy house paint to match the color. Check
your local laws first.

To find out what is next 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.

(and radar) range goes as the fourth root of return signal power.
The best you can hope for is
to reduce the range by a factor of 2 to 4,
by reducing your crossection by a factor of 16 to
This may help in combination with a detector, if the police target
cars at the
extreme of their range as defined by the reflectivity of
the average car.

Jamming feasible?

There are two kinds of jamming proposed pulsed LEDs and CW
My calculations indicate that neither of these work without combining them

with stealth measures. These calculations are specific to the range of
300 meters.

/> CW Jamming sources.
HeadLights aimed into .5 by .2 radian distribute their power over
steradians, at 300 meters range, this illuminates 10^4 square meters or
10^7 of
their recieving aperture. 200 Watt lights put 20 microWatts into the
lidar gun. Presumably the
lidar gun has a narow 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
caluclate the shot noise due to this background
ShotNoise =
SQRT[RecievedPower * PhotonEnergy * MeasurementBandwidth]
and get 2 nanoWatt equivalent
optical power.
This is small compared to the return from a liscense plate.
If someone
has the Car and Driver article on this jamming technique, I
would appreciate a check if they
say it only works with the liscense removed.

Pulsed Jamming sources

broadcast into .005 steradians (.5 radian times .01 radian) would
have to be 500 times
brighter than the 3 milliWatts they hit you with
to beat the retroreflective paint which
broadcasts into only
10^(-5) = ( 4 milliradian times 4 milliradian) return.

/> References
Some of the stuff I put in the FAQ is speculative.

904 nanometers,
30 nanosecond pulses, 4 milliradian cone angle,
4 milliradian cone angle
are from the
IEEE article, and the other referenced sources.

3 Watt instantaneous power is from a
904 nm laser diode manufacturer.
3 microWatts return is based on my calculation at the quoted
and my personal measurement of Colorado liscense lidar crossection.

30 MHz measurement bandwidth is speculative, and is only used for the
noise calculation. I am
assuming the pulse is filtered, and the leading edge
is detected with temporal resolution much
higher than the reciprocal
measurement bandwidth. This requires signal to noise ratios well in
excess of
unity. Perhaps I should have used 1 GHz to allow direct comparison of the

power levels of the shot fluctuations for CW Jamming sources to the power
level of the return
signal. This jamming noise goes as the square root of
measurement bandwidth, making the exact
number not that important.

[1] P. David Fisher, “Improving on police Radar”,
IEEE Spectrum July, 1992
p. 38

[2] personal measurement.

[3] P=
SQRT[RecievedPower * PhotonEnergy * MeasurementBandwidth]

o Improving on Police Radar;
IEEE Spectrum, Jul. 1992, pg 38. (good
general technical info on all speed measurement

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

o Radar Revisited; Road and Track, Nov. 91, pg 106. (excellent

/> summary of band and type differences)

o 7 Wide Band Radar Detectors; Car Audio and
Electronics, Mar. 93 pg
103. (good general info and detector range detail, laser too)

o Radar Wars ….; Car and Driver, Oct. 92, pg 153. (good general
info and detector
range detail)

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

/> info)

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