[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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