[ntp:questions] National time standard differences

Richard B. Gilbert rgilbert88 at comcast.net
Thu Feb 11 19:45:22 UTC 2010


unruh wrote:
> On 2010-02-10, jimp at specsol.spam.sux.com <jimp at specsol.spam.sux.com> wrote:
>> unruh <unruh at wormhole.physics.ubc.ca> 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).
>> 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.




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