Tom Rogers wrote:
>
> OK, The Math:
>
> >This is how surveying equipment like EDMs (Electronic Distance
> >Measurement) work.  They are kind of expensive but I suspect the cost is
> >mostly in the optics, not electronics.  The unit needs a perfectly
> >reflecting target and medium-power telescope optics to obtain enough
> >return signal to take a reading.
> >
> >If you can resolve the phase to 1 degree, the wavelength of the
> >modulating signal needs to be (distance resolution / 720).  The factor is
> >720, not 360 because the wave travels to the target and back, doubling
> >the phase change for a given distance change.  Using a 50 MHz signal
> >(wavelength 6 meters), it could resolve about 1 cm.  But the measurement
> >repeats every 3m, so if the range is longer than that the measurement is
> >ambiguous.  So it is necessary to use additional modulating frequencies
> >to resolve which cycle contains the target.
>
> Lets see: 50 MHz resolved to 1 degree is about 56 picoseconds. Thats to get
> resolution to 1 cm (not very good, but OK for lots of stuff). Stable
> measurements of the order of 56 ps require a fairly fancy box. Thats where
> the money is, not the optics.
>
> --Tom Rogers

A few guys at my Alam Mater built a ground radar a few years ago for a
mine.
By sampling the incoming wave, they got a resolution of 1ps. This turned
out
to be far better than the commercial (state-of-the-art) radar the mine
bought.

--
Friendly Regards

Tjaart van der Walt
mailto:tjaart@wasp.co.za
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