[ntp:questions] National time standard differences

Danny Mayer mayer at ntp.org
Sun Feb 28 15:34:57 UTC 2010


Kevin Oberman wrote:
>> Date: Sat, 27 Feb 2010 23:05:30 -0500
>> From: Danny Mayer <mayer at ntp.org>
>> Sender: questions-bounces+oberman=es.net at lists.ntp.org
>>
>> 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.
> 
> A slight exaggeration, I believe. While the elevation of the clock must
> be taken into account to deal with general relativity, it is hardly
> "halfway up a mountain".
> 
> It is located in Boulder, Colorado, USA. While I failed to find the
> exact elevation of the clock, Boulder is at 5430 ft. (1655 m.) above sea
> level. While this ay sound like it is halfway up a mountain, it is at
> nearly the same elevation as Denver (5280 ft.) and is actually at the
> base of the Rocky Mountains.
> 

Yes, that was something of an exaggeration but it's not at sea level.

> The clock should remain accurate to within a second for about 20 million
> years (assuming no adjustment is made). When the clock was moved down a
> floor a year or two ago, the difference in elevation and the strength of
> the gravitational field had to be adjusted for. Even if it was at the
> USNO, elevation would need to be taken into account.

The reason that they have to apply general relativistic corrections is
that their clocks are far more precise than anything that even a cesium
clock will give you. Their current uncertainty is about 5 x 10-16 with
the NIST-F1 clock. There's a discussion here:
http://tf.nist.gov/cesium/fountain.htm about their current clock though
for some reason I don't see any discussion about the relativistic
corrections for not being at sea level. That memory may have been from a
discussion I had with Judah.

Danny




More information about the questions mailing list