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From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: Fooling Radar Detectors
Message-ID: <28601@netnews.upenn.edu>
Date: 18 Aug 90 07:37:40 GMT
References: <2835@mindlink.UUCP>
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Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Organization: University of Pennsylvania
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In article <2835@mindlink.UUCP> a143@mindlink.UUCP (Ed Meyer) writes:

> reflectors caught the airstream as the car went forward and as expected the
> radar facing the oncoming car detected a much lower speed due to the relative
> retrogressive motion of the reflectors.  However, a radar (like the so-called
> radar camera) would see the reflectors receding at higher speed than the car.
> So, if they don't get you coming, they get you going.

Maybe I'm not picturing it right, but if you had reflectors moving toward
the rear of the car, a radar gun pointed at the front of a moving
vehicle would display a lower speed, and likewise, a radar gun
pointed at the rear would also display a lower speed.

When pointed at the front, the reflectors are approaching the radar
gun at a slower rate than the car is because they are moving away
from the radar gun.

When pointed at the rear, the reflectors are moving away from the
radar gun at a slower speed than the car, because they are moving
toward the radar gun.

Either way, it should yield a lower radar reading, unless I'm missing
something.

Remember that a radar gun only measures absolute difference in frequency.
In the oncoming car case, the doppler shift is positive, making
the reflector-induced shift less than the normal shift.  In the
receeding car case, the doppler shift is negative, making the 
reflector-induced shift more than the normal shift.  But in both
cases, the absoute value of the shifts is less when reflector-induced.
Did that make sense?  Probably not...it's 3:45 AM.

Anyone planning on testing this theory?  ;-)

							--- Jeff
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Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104