[ntp:questions] National time standard differences

Kevin Oberman oberman at es.net
Sun Feb 28 06:19:36 UTC 2010

> Date: Sat, 27 Feb 2010 23:05:30 -0500
> From: Danny Mayer <mayer at ntp.org>
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
> unruh wrote:
> > On 2010-02-10, David J Taylor <david-taylor at blueyonder.delete-this-bit.and-this-part.co.uk.invalid> wrote:
> >> "David Woolley" <david at ex.djwhome.demon.invalid> wrote in message 
> >> news:hksmaf$1cm$2 at news.eternal-september.org...
> >>> David J Taylor wrote:
> >>>
> >>>> I remember the flying of caesium or other atomic clocks round the 
> >>>> world, and that folks had to invoke relativistic corrections.  Were 
> >>>> these better than microseconds as well?
> >>> That's called Navstar (GPS) and GPS position solutions do have to 
> >>> include a general relativity correction to the satellite clocks.
> >> Not today's GPS, but some forty or more years ago:
> >>
> >>   http://www.hp.com/hpinfo/abouthp/histnfacts/timeline/hist_60s.html
> >>
> >> 1964:
> >>
> >> "The highly accurate HP 5060A cesium-beam atomic clocks gain worldwide 
> >> recognition as the "flying clocks" when they are flown from Palo Alto to 
> >> Switzerland to compare time as maintained by the U.S. Naval Observatory in 
> >> Washington, D.C. to time at the Swiss Observatory in Neuchatel. The atomic 
> >> clock was designed to maintain accuracy for 3000 years with only one 
> >> second of error. The cesium-beam standard becomes the standard for 
> >> international time."
> >>
> >> I had wondered what accuracy was obtained - i.e. how far was each nation 
> >> out - and whether relativistic corrections had been needed for these 
> >> "flying clock" tests.
> > 
> > 1 sec/3000years is 1 part in 10^-11. The gravitational redshift is
> > gh/c^2 (g is gravity acceln on earth, h the height of the flight, and c
> > vel of light) which is 10^-12 -- ie below ( but not by much) the
> > accuracy of the clock. The velocity correction is 1/2 v^2/c^2 which is
> > again about 1 part in 10^12. Ie, both corrections are smaller (but not
> > much)  than the uncertainty in the clock rate. If the plane flew at Mach
> > 2, rather than well below Mach 1, you could get that velocity correction
> > up the accuracy and one would have to take special relativity into
> > account. 
> >  
> > 
> > Since the flight probably lasted say 10 hr, which is 100000 sec, th
> > eclocks would have been out by about 1usec. Assuming that the clocks
> > could then have been synchronized, that would mean that US and
> > Switzerland time have been out by about 1usec. (Why they would fly from
> > Palo Alto when the time standard is in Washington DC I have no idea).
> Actually the Time Standards lab for NIST are half-way up a mountain in
> Colorado. As a result they have to make corrections to the time to
> account for the difference between where they are and sea level. It's
> not USNO.

A slight exaggeration, I believe. While the elevation of the clock must
be taken into account to deal with general relativity, it is hardly
"halfway up a mountain".

It is located in Boulder, Colorado, USA. While I failed to find the
exact elevation of the clock, Boulder is at 5430 ft. (1655 m.) above sea
level. While this ay sound like it is halfway up a mountain, it is at
nearly the same elevation as Denver (5280 ft.) and is actually at the
base of the Rocky Mountains.

The clock should remain accurate to within a second for about 20 million
years (assuming no adjustment is made). When the clock was moved down a
floor a year or two ago, the difference in elevation and the strength of
the gravitational field had to be adjusted for. Even if it was at the
USNO, elevation would need to be taken into account.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

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