[LEAPSECS] Looking-glass, through

Warner Losh imp at bsdimp.com
Fri Jan 14 13:36:21 EST 2011


On 01/14/2011 09:40, Richard Langley wrote:

> 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.


Yes. That's why most people I've seen that keep their ensemble in sync
do it by steering a DDS or similar device to the paper clock that's
computed from the inputs of mulitple atomic clocks.

Some

Warner


>

> -- 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 <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.

>>

>> 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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