[ntp:questions] National time standard differences

Danny Mayer mayer at ntp.org
Sun Feb 28 04:05:30 UTC 2010


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.

Danny




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