[ntp:questions] National time standard differences

unruh unruh at wormhole.physics.ubc.ca
Thu Feb 11 18:38:29 UTC 2010


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. 

>




More information about the questions mailing list