[ntp:questions] National time standard differences
unruh at wormhole.physics.ubc.ca
Wed Feb 10 16:46:22 UTC 2010
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:
> "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
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).
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