[time-nuts] Steve's new QTH...
Joseph M Gwinn
gwinn at raytheon.com
Tue Sep 7 22:12:18 UTC 2010
time-nuts-bounces at febo.com wrote on 09/07/2010 02:57:07 PM:
> From:
>
> Hal Murray <hmurray at megapathdsl.net>
>
> To:
>
> Discussion of precise time and frequency measurement
<time-nuts at febo.com>
>
> Date:
>
> 09/07/2010 03:15 PM
>
> Subject:
>
> Re: [time-nuts] Steve's new QTH...
>
> Sent by:
>
> time-nuts-bounces at febo.com
>
>
> > Many years ago I ran into a combined group on Mt. Wilson, our local
> > broadcast farm in the mountains, from Cal Tech and MIT that
> was measuring
> > the movement between Southern California mountains using
> lazers. While
> > this was scientifically fascinating, it gave me the willies.
>
> I'm in Silicon Valley. There is a big USGS group here.
>
> They used to have a laser setup between Black Mountain and Mt
> Diablo which
> are on opposite sides of the fault, roughly 50 miles apart.
> They used to fly
> a helicopter along the beam, measuring the temperature so they
> could get a
> more accurate answer.
>
> Fault motion is ballpark of 1 inch per year, the same as your
> fingernails
> grow. So they would want to measure the distance to a (small)
> fraction of
> that.
>
> I did a quick search, but I didn't find the speed of light as a
> function of
> temperature. 50 miles is 3E6 inches so 1 PPM would be a big deal.
It is about -0.9 ppm per degree Kelvin at 20 C, for 1310 nm radiation.
This comes from the NIST calculator at <
http://emtoolbox.nist.gov/Wavelength/Abstract.asp>, which calculates phase
velocity (but not group velocity yet).
This is also discussed in Appendix C of ASME B89.4.19-2006 (Performance
evaluation of laser-based spherical coordinate measurement systems), which
is available gratis on the web. Both phase and group velocity are
discussed. Google to find the electrons.
Joe Gwinn
> I think they do it with GPS now.
>
>
>
>
> --
> These are my opinions, not necessarily my employer's. I hate spam.
>
>
>
>
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