Cynic wrote:
> 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.
>
The one near me that does this seems to use a pair of loops to detect
which lane is occupied & (probably) which way the vehicle is travelling.
Unless the loops are just there by coincidence to count vehicles &
record speeds for analysis.
Blimey, things *have* moved on a bit.....
I'd have thought one part in about ten million would have been harder to
filter out.
--
Tciao for Now!
John. |