BL> I' ll have two minutes even if the cop is mobile... and by then
 BL> I' ve turned the flash function off.

 JT> You may be able to get away with it if you don't get got in the
 JT> same area too often.

  That's a thohght... you wouldn't want to drive down the F$ every day
and flash the same cop each time! ROFL!

 JT> You have to be careful, I've seen cops in groups of two, the
 JT> second soon after the first, just when you thought you could
 JT> speed up again, zap. 

  I've seen these on long weekends when they're running a special on
speeding, but not normally... and I don't think it's planned, so much
as two cops working the same stretch of road.

 JT> Oh, ok, then explain to me how they DO work. Well stuff me if
 JT> that technique isn't based on the doppler effect.

 BL> They measure the range differential over time.

 JT> In english?

  Doppler sends a broad wave at the moving object, and the return wave
is "compressed" so that it comes back as a higher frequency. They
measure the difference in frequency as "Doppler-shift." The problem is
that the return wave may have reflected more than once, off more than
one surface, and the final speed measurement can be wrong, especially
if other things (like other cars or wheels) are moving in the area.
They try to avoid this by taking the "LOW" speed reading, but what if
*ALL* readings are too high? It is simply not foolproof.

  A laser sends a short pulse of light on a specific frequency and
measures the elapsed time for the pulse to return. The frequency is
ignored. The elapsed time at the speed of light measures the return
distance. The car is moving, but a short pulse makes the distance
measurment quick and accurate. They send another pulse and measure the
new (shorter) distance. The change of distance between pulses is the
velocity of the car (the range differential).

  The speed of light is 100m per uS. A car at 70mph is 20m per second.
The possible accuracy is very good, but you need a very short pulse of
light to get good precision. You need to send a pulse every 20mS to
measure a millimetre at 70mph.
 
  The beauty of the laser is that it can be pointed. The cop can
simply aim it like a gun, put the red dot on the car he wants, and it
reads the relative speed in *THAT* direction. All other reflections
are *slower* than the real one. The first pulse back is the real one,
and the pulse is narrrow enough to only touch one car, or even pick
out the car behind it in a line of traffic, or going the other way, or
whatever...

  It's a total bastard!

Regards,
Bob
  
___ Blue Wave/QWK v2.12
@EOT:

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