(forw) When stratum zero Just. Won't. Do.

[Hal Murray]

> "Obtain your time signal from The Very Fabric Of The Universe - an array of
> 30 meter dishes on the roof of your building receives the unwavering echo of
> the Big Bang, the Cosmic Microwave Background.  ...

Don't laugh too hard.

The astronomers are very close to GPS accuracy.  VLBI is the magic word.  
They have antennas scattered around the earth.  In order to do the math, you 
have to know the locations of the antennas.  I think they calibrate things by 
looking at a known-good strong signal and back computing the locations.  They 
got good data after the big Alaska quake of 2002 from an antenna that was 
conveniently located on the other side of the fault.

Note that there are tides in solid earth.  They are about a foot.  There was 
an experiment at CERN where the data got much better after they corrected for 
the "phase of the moon".  It was extremely sensitive to the diameter of the 
ring.  That changed slightly with earth tides.

For a while, people were working on using pulsars as clocks.  Atomic clocks 
are (much) better.  Neutron stars have things like star quakes that change 
the timing if you are looking that carefully.


Atomic clocks are riding a curve similar to Moore's Law.  There is a fun 
paper on it:
  Time Too Good to Be True, Daniel Kleppner
  Physics Today, March 2006, page 10
  http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_59/iss_3/10_1.shtml
It's 2 magazine pages.

There is a very good paper on practical uses of GPS for timing.
  Timing for VLBI
  Tom Clark and Richard Hambly
  http://www.gpstime.com/files/TOW/tow-time2015.pdf
It's 47 pages of slides.  It's great background for GPS timing.  They start 
at the microsecond level so don't expect any discussions of errors from USB.  
It's actually a series that gets updated every year or two.

I think an older version of that talk covered Hanging Bridges.  They seem to 
have dropped it.  There are good graphs here:
  http://www.leapsecond.com/pages/m12/sawtooth.htm
(Time sink warning if you start following links.)

Hanging bridges are important.  They basically mean that you can't guarantee 
that you will get the right answer if you average for a reasonable time.  
That's a real monkey wrench for testing.

We should put together a web page to collect info like that.


-- 
These are my opinions.  I hate spam.