Continuously adjusting clocks, even atomic clocks, to keep them within
a certain tight tolerance is, in general, not a good pratice. Clocks
will "keep" better time if left running. Rather, the offset of the
clock from the "standard" is measured and used as appropriate.
Performance levels of atomic clocks often assume that a linear rate
term has been removed.
-- Richard Langley
On 14-Jan-11, at 12:26 PM, Warner Losh wrote:
On 01/14/2011 00:22, Sanjeev Gupta wrote:
On Fri, Jan 14, 2011 at 13:47, Tom Van Baak <[email protected]>
wrote:
You really didn't expect 250 diffeent atomic clocks around
the world to all agree at the ns level at all times did you?
<tounge-in-cheek>
Why not? nano is 10E-9, and I see references to people trying for
clocks with 10E-12 on this list.
And what good is the "atom" part of an atomic clock, if it can't
even handle "nano"?
</foot-in-mouth>
Still waiting for the flying cars I was promised ...
A good Cesium standard is good to better than 1ns/day. This is
already 1e-12 or 1e-13 depending on the model. Hydrogen Masers are
also available commercially, and they push this down to 1e-15 or
1e-16, which is good to about 1ns/year in frequency error.
Experimental clocks can do even better, at least in the short term.
The problem is that Cesium standards are between $5k and $25k to
buy. Hydrogen Masers are more like $1M. It is a lot easier to have
a bunch of Cesium standards than HMs.
The BIPM collects time and frequency data for the different clocks,
measured against each other. Each clock then has an error in
frequency and time computed. These clocks are then weighted based
on assigned values (based on the time scientists best guest about
how good the clocks are). This value goes in to producing what's
called a 'paper clock' which is a historical look at what the best
guess at the actual time for each of these measurements. Based on
that, you can know how close your clocks are running, and can steer
them, if you wish.
When you are running a clock, one thing that might not be obvious is
that you can't have 'phase jumps' and keep the users of the clock
happy. If you have a phase error of .1ns and want to steer it out,
you have to adjust your frequency by 1e-10 / <steer-time>. The
steer time is how long you want the steer to take, and is usually
dictated by how much change in frequency the steering systems can do
and how much the users of the time signals can tolerate.
Warner
P.S. I'm not sure if I agree that this will one day be common
place. Having helped in a small way to run an ensemble of clocks at
a former job, I know there's a lot of fussiness that goes into it.
You need to calibrate the cable lengths, you need to adjust for
temperature, you need to review the data frequently to make sure
that everything is operating normally, etc. You also need to
calibrate it to NIST from time to time. It can be quite the
undertaking. I'm not sure that the ns level of accuracy and
precision will ever make it into many devices. On the other hand,
there's a lot of activity on the chip-scale atomic clocks pushing
the cost way down, so who knows.
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