[LEAPSECS] Looking-glass, through

[Warner Losh]

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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