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From: zhahai@nbires.UUCP
Newsgroups: net.auto
Subject: Re: clogged traffic and other stories - (nf)
Message-ID: <188@nbires.UUCP>
Date: Thu, 23-Jun-83 22:57:15 EDT
Article-I.D.: nbires.188
Posted: Thu Jun 23 22:57:15 1983
Date-Received: Fri, 24-Jun-83 23:38:58 EDT
References: <1147@fortune.UUCP> <583@rocksvax.UUCP>
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There are some problems with the simple "transmit the output of a tunable
oscillator" approach: any difference between your Xtal and theirs should
show up as a speed reading.  Given the difference between the speed of light
and car speeds, it should take a very small difference in frequency to do it.

Also, if your frequency was right on, you would still have to take the shift
caused by your movement into account. (Only weighted by half, however, since
a transmitter travelling at 80mph should have approx the same shift as a
reflection on something moveing 40mph).

I recall seeing a suggestion some years ago (in 73 maybe?) for using a passive
modulator in a shaped horn.  Radar comes in, stimulates something (gunn diode?)
and reflects back.  Trick was to modulate it with an audio level signal while
it was there - but not to generate an rf signal yourself.  Said audio signal
was supposed to be stronger than the frequency difference, get through the filters, and make "dial a speed" possible.  I do not understand how this would work,
but the author claimed to have experimented with it.

I do know of a clever way which an acquaintance told me about, claiming to have
tried it with the help of a police friend.  I wonder if I should mention it?
It is probably legal, should work, is totally passive (no RF involved).

Sure, why not.  Build yourself a rotating cup device, similar to an anerometer.
Use well constructed corner reflectors, which can have a much bigger radar image
than their size would indicate. (metal of course).  A corner reflector is 3
planes all at right angles to each other - a signal comes in and reflects off
all three surfaces then heads back where it came from (directed, not dispersed).
You may have seen them near airports (at least military).  Anyway, these 
reflectors have large radar images one way, small the other.  Spin it at a
controlled rate with a motor.  (the prototype tested was spun by the wind).
If it has a bigger image than your car - you can set the rate the radar sees.
One problem is that if you subtract 40mph from a frontal reading (70-40=30)
you will add 40mph to the rear (70+40=110!).  Or vice versa, you can slow the
rear reading and speed up the front.  Better know which way the radar is from!
Now, such a thing would be rather obvious, but I figure one could hide one
in a plastic luggage rack.  Not so hot for your sports car?  I didn't say it
was magic, just clever.

Maybe adding and subtracting 50mph would do it: if the reading is vastly over
the rate your car can go it might be ignored or thrown out of court.  Not a
game to play if you have a very fast car.

Now I'm just waiting for the teardrop shaped dual rotor (one shielded from
rear, one shielded from front) commercial model...

Colorado like passive means of speed detection, by the way.  No way to beat a
plane.