unruh wrote:
On 2010-02-10, j...@specsol.spam.sux.com <j...@specsol.spam.sux.com> wrote:
unruh <un...@wormhole.physics.ubc.ca> wrote:
On 2010-02-10, David J Taylor
<david-tay...@blueyonder.delete-this-bit.and-this-part.co.uk.invalid> wrote:
"David Woolley" <da...@ex.djwhome.demon.invalid> wrote in message
news:hksmaf$1c...@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).
Probably because the clocks came out of the HP Palo Alto office?
But if they are comparing time standards, it does not matter where the
clocks were manufactured, but where they are synchronized with the time
standard, and that is surely in Washington, not Palo Alto.
You do not try to synchronize a clock in Washington via phone lines or
microwave links with Palo Alto surely.
ISTR that NIST has facilities in both Colorado and Hawaii. The
Washington DC area has the U.S. Naval Observatory.
_______________________________________________
questions mailing list
questions@lists.ntp.org
http://lists.ntp.org/listinfo/questions
