Re: Re: Radar detector/jammers (fwd)

[John Greenstreet <jgreenst@motown.ge.com>]

> From quattro-owner@swiss.ans.net Wed Nov 16 16:22:35 1994
> From: drobbins@wellfleet.com (David Robbins)
> To: quattro@swiss.ans.net
> Subject: Re: Re: Radar detector/jammers (fwd)
> Date: 16 Nov 1994 20:34:50 GMT
> Organization: Bay Networks, Inc.
> Reply-To: quattro@swiss.ans.net
> 
> Ernest Wong wrote:
> 
> > I read some time ago ( C&D, I think ) that a lot of the reflected signal
> > going back to the radar gun was from the large vertical (metal?) surfaces
> > in a car. The article stated that the headlights, license plate and the
> > radiator were pretty good at bouncing the signals back.
> > just what i remember . . .
> 
> I think you are confusing radar with laser, which depends on a "good
> surface".  Radar is far less directional.  In that same article, C&D tested
> the best ways to beat laser.  The bottom line was that because laser uses
> reflections of IR light, it is actually possible to defeat laser by
> overpowering the gun with too much light.  In their test, the laser gun had a
> harder time reading a speed when their were more lights on (headlights/high
> beams/fog lights).  They even went so far as to mount hi-powered off-road
> beams and cover them so only (non-visible) IR light could get through, and
> the laser was basically rendered useless.
> 
> Do you remember which issue that was?  I think it was 12-18 months ago.
> 

The report Ernest is citing did concern itself with radar, as I
remember seeing it myself.  A motor vehicle is quite a good reflector
of radar, with plenty of rounded metal parts that return the beam back
from whence it came.  But the amplitude of a radar return varies
chiefly with the target's radar cross-section and the decreases with
the fourth power of its distance.  That's why the signal returned by a
Corvette at 1/4 mile can be roughly the same as that of a
tractor-trailer at 1 mile.  Sure, the radar gun is aimed directly at
the 'vette, but its beamwidth is so wide that it envelops the entire
highway by 1/4 mile.

So the best radar jammer is really a large, slow-moving vehicle behind
one's smaller vehicle.  Of course, if one has the misfortune of being
in front of a rapidly closing truck, one could be cited for speeding
when one actually wasn't, and this is indeed the cornerstone of most
courtroom radar defences.

Because a car reflects so much signal, police radar relies on '50's 
defence radar technology, i.e. the speed of the object is determined
solely by the doppler-shift evident in the return.  So crude is this
method that police radars are actually calibrated by using a tuning
fork, because the amount of frequency shift is in the audio range.

Contrast this environment to that of a sophisticated phased-array
radar that can track hundreds of objects at one time in a heavy
active-jamming environment, several of which could be jammer-equipped
cruise missiles or planes closing radially.  Now one knows why DOD
engineers would not be too daunted if tasked with jamming police
radar.  But keeping a lawyer on retainer would probably be cheaper!

--------------------------------------------------------------------
John Greenstreet, Senior Engineer           (jgreenst@motown.ge.com)
Martin Marietta Government Electronic Systems    Moorestown NJ 08057
WPI Class of '75, Temple Class of '94

My new car history:
  1975    1978    1982       1986        1989      1992      1995
   VW ->  Audi -> Audi  -> Mercedes -> Mercedes -> Audi -> Mercedes
Scirocco Fox GTI  4000S    190E 2.3    190E 2.6    100CS     S320

POSSLQ's* new car history:
         1978       1981       1985      1988        1990     1993
       Triumph ->  Toyota ->  Toyota  ->  VW    ->   Audi  -> Audi
       Spitfire    Tercel     Corolla   Jetta GL      80      90S

*POSSLQ = Person of Opposite Sex Sharing Living Quarters
Note: All Audis and Mercedes above were sold to friends or family.
--------------------------------------------------------------------