BL> They try to avoid this by taking the "LOW" speed reading, but
 BL> what if *ALL* readings are too high? It is simply not
 BL> foolproof.

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

  I had a long argument with a cop in Armidale when he timed me at
88kmph (or something ridiculous) when I was only just over 60. I'd
pulled out to overtake another car that braked suddenly, so there was
a complicated scenario or me turning a little sideways, the other car
braking and me accelerating, and then pulling back in again.

  His story was that his radar "locked" on the *fastest* return, and
then took the lowest reading of three. According to him, he'd seen
me accelerate to in excess of 88kmph and then braked dramatically. I
tried to point out that Ayrton Senna would have had trouble hitting
88 kmph in 10 metres in his F1, not to mention the sudden braking. In
fact, I hardly accelerated and didn't brake at all... until I saw the
cop. In fact there are no individual returns with doppler radar, just
a continuous wave that varies in phase and frequency all over the
place!

 BL> They send another pulse and measure the new (shorter) distance.
 BL> The change of distance between pulses is the velocity of the
 BL> car (the range differential).

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

  No. The *carrier* frequency will be higher, but not the pulse
pacing. Each pulse will be treated the same by the moving relective 
surface and spacing is constant. 

 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,

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

  I would be surprised it they used a laser sighting system. My guess
it that it's the same as a camera; just a dot in the field of view. As 
for the laser frequency, it could be anything from infrared up. But the 
principle is the same.

Regards,
Bob
___ Blue Wave/QWK v2.12
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