On Thu, 08 May 2008 09:00:23 +0100, John Williamson
wrote:
>For cheapness, they're usually just a cheap radar unit, so they do
>respond to both directions. The ones that don't cost more, as they need
>a sensing loop in the road.
That is incorrect. Radar measures speed by sensing a doppler shift in
the frequency of the reflected signal. The doppler shift will be
higher in frequency for an approaching car, and lower in frequency for
a car going away from the radar.
So what is needed is circuitry in the radar that can determine whether
the doppler shift is higher or lower in frequency. That is a little
more complex that determining the magnitude of the doppler shift
(which can be done simply by measuring the beat frequency of the
reflected signal with the transmitted signal), but certainly does not
require any sensing loops in the road and amounts to a £ or two of
additional electronics.
--
Cynic
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