From: jimf@centerline.com (Jim Frost)
Subject: Re: Laser Detectors - How do they work?
Organization: CenterLine Software, Inc.
Date: Tue, 21 Jul 1992 17:16:32 GMT

wcscses2@alfred.carleton.ca (Geoff Hamer) writes:
>Laser speed detectors work on the same principles as a regular radar
>detector.. after all light is just a radio signal at a much higher
>frequency.

This is incorrect.  Laser speed detectors work by determining
range-to-target repeatedly and calculating a vector while radar guns
work by measuring doppler distortion of the reflected signal.

A radar gun is almost always pretty accurate in terms of velocity
(assuming it's tuned reasonably well), but because of the
characteristics of the beam it's hard to determine exactly what you're
measuring the velocity of.  The legal system neglects this
technicality.

A laser speed detector is accurate so long as it's measuring the
distance between the gun and the same point on the target.  The
problem I have with them is that you can "paint" the gun across the
car and get reflections from several points on the car which are
separated by a distance of several feet (eg the chrome around the
windshield followed by the headlights or the chrome on the bumper),
which gives an apparent velocity even if the vehicle is standing
still; this can cause a considerable delta between measured velocity
and true velocity because the measurement intervals are relatively
short.

One person on this group said that he'd had the opportunity to
personally test one of these guns and said that the painting technique
can be mastered in very little time (about an hour if I remember
right).  Thus the laser guns are ripe for abuse.

>If you were able to build a Laser-detector
>it would only be able to tell you when your specific car is being
>targeted... this of course is too late.

This isn't true, it can also pick up reflections.  It's been a matter
of debate as to how effective laser detectors really are (they do
exist).  I maintain my position that the 2-mile detection distance
quoted by Escort is under direct illumination, which should be rare in
the real world.

Anybody have any experience with intermittent transmission radar
traps?  In NH this was referred to as "shotgunning" -- the cop would
only turn on the transmitter when you were on top of him, so your
detector wouldn't pick him up until you were already caught.  Laser
speed detectors are inherently intermittent, thus making them even
harder to detect at a distance.

One last problem is that it's going to be easy and cheap to switch the
laser portion of the laser speed detector with one that operates on a
different wavelength.  There aren't any FCC rules to deal with and the
lasers are readily available in a number of wavelengths.  Thus a nice
shiny new laser detector will be obsolete quickly unless it can handle
multiband detection -- which I consider very unlikely with a $100
price tag.  I expect that the useful lifetime of the current crop of
detectors will be quite short, probably no longer than two years (I'd
bet closer to one) before we start to see laser gun diversity.

I haven't heard any real experiences with laser detectors yet so the
jury is still out, but don't bet on a lot of warning time (if any)
using a detector.

>One more plus, these systems are VERY expensive at this time.
>Therefore there aren't many states and provinces using it. This is
>bound to change in the near future.

There are a couple of other pluses.  It's rather difficult for the
laser guns to get enough returns on your car to get a vector (a
"lock") until your car is pretty close.  Since the policeman needs to
aim the gun manually (instead of just letting it sit on the dash while
he eats donuts as is common with radar) you probably have a good
chance of seeing the cop with the gun and slowing down before the cop
can lock the gun onto you.  Further, reflectivity of cars varies
depending on color.  Black cars are much harder to lock onto than
white, and cars with pop-up lights (when down) or very little chome
are also difficult to lock onto.  So the guns are not as bad as they
might be.

I consider the guns unreliable since it is quite easy to get an
incorrect velocity reading, either deliberately or accidentally.  I
consider it as a tool that is just waiting to be abused.

jim frost
jimf@centerline.com
Newsgroups: rec.autos.driving