time-nuts-boun...@febo.com wrote on 09/07/2010 02:57:07 PM: > From: > > Hal Murray <hmur...@megapathdsl.net> > > To: > > Discussion of precise time and frequency measurement <time-nuts@febo.com> > > Date: > > 09/07/2010 03:15 PM > > Subject: > > Re: [time-nuts] Steve's new QTH... > > Sent by: > > time-nuts-boun...@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. > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/ > listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.