[LEAPSECS] Looking-glass, through

Warner Losh imp at bsdimp.com
Fri Jan 14 11:26:07 EST 2011

On 01/14/2011 00:22, Sanjeev Gupta wrote:


> On Fri, Jan 14, 2011 at 13:47, Tom Van Baak <tvb at leapsecond.com

> <mailto:tvb at leapsecond.com>> 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.


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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