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

Hello Bob,

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

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

 I think he was just trying to baffle you with bullshit.  Not that they do
that sort of thing of course, being respected members of our community...
 (barf)

 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)

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

 Bob, sit down with a pen (or pencil, crayon or any other writing implement),
and paper and work it out.
 If the "speed gun" in question sends CARRIER PAUSE CARRIER PAUSE
and that is
reflected of an object that is approaching the speed gun, then the return
signal will look similar except "squashed up", that is, the carrier will be
of a higher frequency, and the pause between the carriers will be of a shorter
time.  Only a little difference mind you, but all proportional and all
measurable.
 For you to say the carrier changes but not the pause between is just silly.

 BTW, guess what?  That just happens to be called the doppler effect.  Wow.
 
 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. 

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

 Having never seen the equipment, and going on what I already know, (they
use an infra-red beam to take the actual speed sample), my guess would be the
sample laser is sent straight out, and a telescopic vision type arrangement
is used to point the thing in the first place.  There would be cross-hairs on
the visor, and it would be accurate, since the laser will hit the same spot
regardless of distance.  (Unlike rifles, where the bullet strays due to wind
and gravity).

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

... Gratuitous censorship - is a 'clever tool'... to mindless impotents.
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