[ntp:questions] National time standard differences
unruh at wormhole.physics.ubc.ca
Mon Mar 1 18:43:22 UTC 2010
On 2010-02-28, Danny Mayer <mayer at ntp.org> wrote:
> 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:
>>>>> "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
>>>> 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.
The relativistic correction is approx 10^-16/m (or 10^-10PPM/m)
If the clock has an accuracy of 5 10^-16, a 5 m change in height will be
larger than that the uncertainty (or course it depends on what 5 10^-16
Certainly the 1600m above sea level will make a very very noticeable
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