From: sykes@donner.uucp (25816-sykes)
Subject: IEEE Spectrum article on Police Radar (Laser operation)
Organization: Bellcore, Livingston, NJ
Date: Mon, 27 Jul 92 19:21:14 GMT
Keywords: Laser

Hi,

I've been reading the thread on "how do Laser radar guns work" and
I am reproducing a portion of an article from the IEEE Spectrum magazine
for July 1992 written by P. David Fisher who is a professor of electrical
engineering at Michigan State.

Down-The-Road Lasers

Recently, two companies- Laser Technology Inc. of Englewood, CO. and
Kustom Signals Inc. of Lexana, Kan.- have begun to market world wide 
a new class of speed detection device that attemps to overcome the
inherent deficiences of down-the-road Doppler radar. These new devices,
which emit a 3-to-4 milliradian infrared laser beam, operate on a totally
different princple from Doppler radar and are much less likely to display
spurious target-speed readings because of interference effects.

Their narrow beams enable speed enforcement officers to isolate single 
target vehicles at distances exceeding 300 meters. And the infrared laser
beam is not detectable by radar detectors currently on the market. Moreover,
while it is conceivable that such a laser detector could be produced, it
would not be very effective since the laser is an instant-on device with
little propagation beyond the target vehicle if the laser is properly
aimed. In most cases, therefore, the user of a laser detector would get
little or no warning that a laser speed measurement device was being operated
nearby.
 
These laser devices operate on the time-distance (pulsed-radar) principle	
which uses nanosecond-wide pulses of infrared energy to measure the range
to the target vehicle at 10-ms intervals. The change in range during the
interval divided by the interval itself is a measure of the vehicle's speed.
That sequence of measurements establishes a history of the speed of the
target vehicle, which can be displayed on a numerical readout.

Since a vehicle traveling at a typical speed limit of 90 km/h will cover
only a quarter of a meter in 10ms, the round-trip delay difference between
two consecutive readings will be approximately 1.7ns. Clearly, subnanosecond
accuracies are required in measuring this distance to ensure that target
vehicle speeds in the range of 8-240 kilometers per hour can be gauged
with an error not exceeding 1 km/h.

The requisite accuracy is achieved by transmitting a burst of pulses over a 
few tenths of a second and then performing a regression analysis on the
measured delays. If the results pass a pre-established accuracy test, the
speed is accecpted and displayed.

In this example, two principal assumptions were made:  first, that the laser
unit was held stationary while the readings were being made (since all vehicle
speed measurements are actually made relative to the speed of the detection
device itself) and second, that the light pulses would all strike a flat
surface perpendicular to the path of the light wave. 

While the first assumption presents no problem (the instrument is simply used
in the stationary mode), the same cannot be said of the second. In practice,
the waves strike the front or rear of the target vehicle, both of which are
irregular surfaces. The result is a reflected light pulse that is dispersed
in the time domain due to the different transit times for different portions
of the reflected beam. It is also dispersed at an angle. Hence, multiple  
reflections from neighboring objects can further confuse the measurement.

Dealing with the possibility of errors caused by beam dispersion places
extra requirements on the optical and signal-processing portions of the laser
system. Basically, the system must detect invalid measurement conditions and
not display a speed reading under those conditions.

In fact, laboratory and field tests conducted on laser systems indicate that 
they are accurate and simple to operate. Moreover they do indeed overcome
a number of shortcomings of traditional police radar. 

For instance, laser systems greatly simplify the business of target 
identification since thier beams can be directed to an individual's vehicle
with only that vehicle's speed being measured. Such precision does have
one drawback, however: for distances of more that a couple of hunderd
meters, it is difficult to keep the beam trained on a target vehicle without
a tripod or other steadying device. Of course, that does not compromise the
accuracy of the device, only its ability to acquire target vehicle speeds.

Very short distances also  present a problem because of the cosine effect.
To operate the device, an operator must be off to the side of the road or
an overpass above the roadway. For short distance, thu cosine of the angle
between the vhe direction of travel and the infrared beam may become
significant, and the measured speed my be significantly lower than the 
actual speed. For laser units positioned 4 meters off to the side of the
road, that source of error can be kept below 1% by restricting operation
to distances greater than about 30 meters(angles less than 0.13 radian). 

end of article.


If anyone wants a hardcopy of this article just e-mail your mailing address 
and I'll send you one.

Michael.