From: jimf@centerline.com (Jim Frost)
Subject: Re: IRD?...Infra-Red-Detector?
Organization: CenterLine Software, Inc.
Date: Wed, 15 Apr 1992 21:14:40 GMT

bphdarcy@ubvmsb.cc.buffalo.edu (SJ Roc D'Arcy, MBP) writes:
>In article <1992Apr14.163907.19746@wpi.WPI.EDU>, mishaal@wpi.WPI.EDU (Mashal Abdulra Almashan) writes...
>>          In School with a new sophisticated speed measurement device called
>>the IRD. It is an Infra-Red (heat detection) device that measures speed accur-
>>ately to .1 MPH. It is a small round device that is being installed in grey/
>>silver/blue 1992 &93 unmarked Mustangs and a limited number of State Cruisers.
>>It was rumored to appear on April 1st. First known siteing was yesterday 
>>4/8/92.  There is presently NO KNOWN DEFENSE against it OTHER THAN obeying the 
>>speed limit. Feel free to pass this note around to others as a precaution. 
>>Please strip headers before doing so. 

>Time to whip a a few IR jammers.  

This note got forwarded to one of our internal mailing lists too.  I
had this reply to it (which is mostly assuming they meant laser speed
detectors and not IR).  Most of the reply has to do with whether or
not I feel it's possible to make reliable external detectors for such
a gun.

Note that at the time I had no information whatsoever on laser speed
detectors.  I was happy to hear reports that the actual laser speed
detectors are limited even more than I would have suspected, working
well only on light-colored cars of particular geometries and only if
the person controlling it is paying strict attention.

Drive on,

jim frost
jimf@centerline.com

-- cut here --
|FYI - Don't waste your money on a radar detector!!
[...]
|I know the following is really about LADAR, not RADAR, but I thought the term
|'radar' woud mean more in a subject line
|
|================================================================
|Where have all the MA state police been for the last 2 months?
|
|In School with a new sophisticated speed measurement device called the IRD. It
|is an Infra-Red (heat detection) device that measures speed accurately to .1
|MPH. It is a small round device that is being installed in all their new grey /
|silver / blue 1992 & 93 unmarked Mustangs and some of the State Cruisers.  It
|was rumored to appear on April 1st. First known siting was yesterday 4/8/92.
|There is presently NO KNOWN DEFENSE against it OTHER THAN obeying the speed
|limit.

FYI, the poster's information is at least partially bogus if not
completely so.

Heat detection techniques can be used for angular location of heat
sources but are very unreliable at determining range even if the exact
temperature of the target is known.  Without exact temperature you
can't even begin to determine speed by doppler effects (although you
can determine velocity delta assuming near-constant temperature), and
without range you can't determine true velocity from angular velocity.
There's no way you can get .1mph accuracy out of such a device.

On the other hand, it's possible that they will start using the new
laser speed detection guns, which are available although very
expensive (circa four-hundred percent the cost of radar guns).  There
are several companies working on detectors for these although it's my
opinion that detectors will me very unreliable because:

- Most chemical lasers are very tunable and there are few FCC
regulations against tuning them anywhere in the visible and
near-visible range.  Thus a detector would need to detect coherent
light in a very wide frequency range, which is difficult to say the
least.

- Lasers are highly directional.  If you're in LOS (line-of-sight)
it's pretty easy to detect laser radiation, but at that point the
laser wielder already has you (in the Army's terms, if you're being
radiated there's probably a bomb on the way).  If you're not in LOS
then you're seeing reflected radiation which will be:

  1. Low power.  Most non-LOS radiation will be seen along an
     expanding wave rather than a coherent beam, resulting in pretty
     much the inverse square power loss rule rather than a near-linear
     rule with totally coherent light.  Distancing lasers don't start
     with much power (usually less than that of a penlight and far
     less than that of a radar transmitter) so there won't be much
     left after even short distances.

  2. Diffracted or heavily phase-shifted due to differing reflective
     characteristics along the body of your average car and the normal
     motion of the car.  These will tend to fluctuate the observed
     frequency of the signal.

  3. Highly intermittent -- the angular motion of the target versus
     the transmitter along with the varying surface of most vehicles
     will lead to widely variant reflective characteristics even if
     the laser is constantly transmitting (which is unlikely since
     you'd get better accuracy, simpler construction, and lower power
     use by pulsing the laser).

Thus these detectors will have to be both very sensitive and very
flexible, which equates to very expensive and very unreliable.

Depending on the exact type of laser being used, a lot of cars will be
nearly immune to laser ranging -- visible range lasers won't get good
signals off of colors widely variant from their original frequency (eg
a violet laser doesn't reflect well off a red target and nothing
reflects well off of flat black) and the current trend towards smooth
car lines will tend to randomize the reflected signal along with the
normal phase-shift due to doppler effects.  All of these problems can
be partially overcome by ramping up the power of the laser except that
lasers are a health hazard at anything but very low intensities.

In my opinion it's likely they'll start using laser speed finders, but
that there won't be many of them around until the cost drops
significantly.  I don't believe that there's a lot anyone can do about
it except hope they have a car that isn't very reflective in the
frequency range in use.  Note that we nicely fixed Steve's car in this
respect :-).

Happy driving,

jim

PS
Lasers were a hobby of mine at one time :-)
Newsgroups: rec.autos.driving