-=> Quoting Bob Lawrence to John Tserkezis <=-

Hello Bob,

 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?

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

 As far as I am aware of microwave type radars, they use different techniques
to get the same result.  The older ones used to have a tenency to pick up the
strongest signal, while the newer ones pick up the fastest signal.  I don't
know if they can pick different techniques on the same radar unit.

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

 And if you look at the train of pulses, you'll find that they are closer
together than how they were transmitted.  (grin)

 BL> The speed of light is 100m per uS. A car at 70mph is 20m per second.
 BL> The possible accuracy is very good, but you need a very short pulse of
 BL> light to get good precision. You need to send a pulse every 20mS to
 BL> measure a millimetre at 70mph.

 BL> The beauty of the laser is that it can be pointed. The cop can
 BL> simply aim it like a gun, put the red dot on the car he wants,

 I thought it was infra-red?  Unless the guide was a standard laser pointer.

 BL> and it
 BL> reads the relative speed in *THAT* direction. All other reflections
 BL> are *slower* than the real one. The first pulse back is the real one,
 BL> and the pulse is narrrow enough to only touch one car, or even pick
 BL> out the car behind it in a line of traffic, or going the other way, or
 BL> whatever...

 The sense laser itself has a broader beam, this is so they don't accidently
shoot through the windshield and miss a reading altogether.  With a broader
beam, they risk a lower return signal, but have a better chance to get a
return signal in the first place.  Still narrow enough to pick out a car in
the distance though.

 BL> It's a total bastard!

 The pricks.  Almost as bad as slant radar, where they can hide around corners.
They are using standard radar units, but mounting them on their dash boards
now.  So you can't look for the radar unit hanging off the side of the car
anymore.

John Tserkezis, Sydney, Oz. Fidonet: 3:712/610  Internet: jt{at}suburbia.com.au

... Where's my brain? Ah yes! The drawer, as usual...
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