Richard.Prosser@POWERWARE.COM wrote:
>
> IIRC we measured the elongation of optic cable as it was being ploughed by
> injecting a 200MHz modulation onto a laser & measuring the phase of the
> reflecttion from the far end. Got better than 5mm resolultion over several
> km of cable length. Of course, we had the advantage that the reference and
> return signal were both available at the same location but there are ways
> around this I think. (e.g multiple transmitters )
> As above, all we used was a 200MHz sig gen and a vector voltmeter (& PC &
> software), along with the required optical components. I think the part we
> had the most trouble with was depositing a silver mirror onto the far end
> of the fibre!
> At 200Mhz,(in fibre), the wavelength is about 1.07m. 10mm (i.e 5mm increase
> in cable length) then represents about  3.4 degrees of phase difference so
> it is pretty easy to measure..
>
> However, doing it 2400 times asecond for 3 different signal sources it a
> somewhat different proposition!

Not at all difficult depending on whether one can place
the receiving antennas optimally.

It's ONE signal source and 4 antennas (to get 3-D)
4, 5, or 6 phase comparators (for error reduction by RMS fitting)
and a low end A/D board (2400 hz is barely audio rate).
The only hairy part is getting all 4 receivers to
use a single LO signal for the downconversion, and to calibrate
out the various phase errors. The rest is simple trigonometry.

Think of a sheet of paper, with antennas at the 4 corners.
A signal source on the surface of the paper puts out a spherical
wavefront.
You can see how the wavefronts will arrive at the various
pairs of antennas with phase difference depending on where
it is located in 2-D. As long as the source is on the plane
of the paper, you can solve for it's location with just 3 antennas.
To solve for 3-D, you need a forth antenna, and phase measurements.
By using redundant measurements for all pairs of antennas you can
compute an RMS fit for your source. Apply kinematic filtering
(filtering constrained by the physics of your model) and
you compensate for noise influences.

Given the starting wavelengths, and working volume, one has
to design in methods to compensate for lost cycle counts
as the source moves more than one wavelength. This is done
by using a second nearby frequency and suitable filtering/tuning
so that there is only ONE unique solution for BOTH sets of
phase information.

If I could change the frequency, I would drop down to the
FM radio band, where radios are cheap and readily available
for modification, and where sub degree phase comparisons are
easily done.

Any royalties can be directed my way <G>.

Robert

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