RE: "Radar Scramblers"

[DeWitt Harrison <de@aztek-eng.com>]

At 11:46 AM 10/30/97 +0100, Aleksander Mierzwa wrote:
>DeWitt wrote:
>
>>It's really pretty simple. All passive, reflecting "jammers" are a hoax and
>>fail the common sense test as well. [ ... ] 
>
>Dewitt, what you've described makes perfect sense as far as we treat a
>radar only as a locating device. The police radar is, however, a speed
>measuring device, which uses a Doppler effect to measure the speed of
>the target. [ ... ] There are also high demands on police radars in terms of
>reliability and accuracy, otherwise they could not have been used as a
>proof. To catch up to these demands, a police radar makes a series of
>measurements. It won't display the speed until the results of 10 or so
>measurements are equal within 2 percent or so. A passive radar jammer
>mixes the radar signal with constantly varying FM noise. It does not
>makes you invicible to the radar, nor even prevents the radar from
>measuring your speed, it just makes these measurements less accurate.
>Actually most reputable manufacturers of the jammers don't claim that
>their products will make you invicible to radar, they just say that the
>jammer will prolong the time the radar needs to lock on your car,
>sometimes it will make the radar not lock at all (it will keep measuring
>your speed, but the results of all the measurements will not fall within
>specified range), it is not guaranteed, though. [ ... ]

Adding the additional complication of doppler shifted return signal does
not overcome the fundamental flaw in passive radar/lidar "jamming" devices.
Whatever sort of reflected signal is returned from a doohickey equipped
car will consist almost entirely of "car" and virtually nothing of "doohickey."
Signal-to-noise-ratio. All you have to do is measure the square meters
of the car's frontal area and compare it to the area of the passive reflecting
device.  That ratio is signal to noise.  If we ignore the windshield area which
may not be a good radar/lidar reflector, I think the car-to-doohickey-ratio
would be something like 2 square meters divided by 0.0005 square meters.
Fudge all you want, but the cop's supersensitive-califragilistic zapper will
see all car and no doohickey. What we have here is a modern day sequel
to "The King's New Clothes."  What is really needed is an inexpensive and
legal radar/lidar/ovloV homing tactical nuclear missile.  Cheers,

DeWitt Harrison    de@aztek-eng.com
Boulder, CO
88 5kcstq