Kevin Oberman wrote: >> Date: Sat, 27 Feb 2010 23:05:30 -0500 >> From: Danny Mayer <ma...@ntp.org> >> Sender: questions-bounces+oberman=es....@lists.ntp.org >> >> unruh wrote: >>> On 2010-02-10, David J Taylor >>> <david-tay...@blueyonder.delete-this-bit.and-this-part.co.uk.invalid> wrote: >>>> "David Woolley" <da...@ex.djwhome.demon.invalid> wrote in message >>>> news:hksmaf$1c...@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 _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
