[time-nuts] Steve's new QTH...

[jimlux]

Hal Murray wrote:
>> 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's a big problem..
look for variation in Refractive Index with temperature (and pressure 
and humidity)..

I was working on an 8 GHz antenna design (essentially an interferometer) 
that in order to be tested needed the source to be at least 27km away to 
meet the plane wave approximation. Can't do it outdoors (or indoors, 
even...).

CERN has similar problems aligning the beamline with a laser theodolite, 
and that's in a tunnel underground. I'm thinking they needed mm 
transverse precision over km distances (i.e. 1 ppm)


A 1ppm change in RI is caused by
1degree C
0.4kPa pressure (3mm Hg)
50% RH at 35C

It's pronounced enough that you get tropospheric ducting on VHF (e.g. 
sometimes, repeaters on Mt Wilson above Los Angeles can hear people down 
in San Diego, below the radio horizon)

http://emtoolbox.nist.gov/Wavelength/Documentation.asp

go for it...



> I think they do it with GPS now.
> 
> 
> 
>