[ntp:questions] project ntp.br - discrepancy from UTC

David L. Mills mills at udel.edu
Tue Oct 9 21:34:31 UTC 2007


John,

Thanks for the headsup. I have three Cs clocks, all of which predate 
GPS. However, two of the three have expired tubes and I nurse the third 
by only occasionally switching the beam on. For awhile I was buying used 
tubes, but now GPS sans SA is so good I don't need the Cs clocks. I do 
have a Rb oscillator, but I use that primarily to calibrate the 
frequency synthesizers for my radios.

If I were running a cesium farm I wouldn't want to adjust the clocks 
either and would rather run a paper timescale. This means that GPS, 
LORAN-C and simiilar services have to run their own cesia and microsteppers.

Once upon a time it was tough to get a really good local UTC lab 
standard. Before GPS I used LORAN-C; before that I used WWV, CHU, WWVB 
and Omega. The first NTP servers used the power line, which is 
synchronized within a few seconds east of the Rockies except Texas. My 
how things have changed since then.

Good cheer.

Dave

John Ackermann N8UR wrote:

> One possibly relevant note is that a lot of standard/metrology labs 
> don't ever adjust their oscillators, but instead monitor the rate and 
> deal with that in data reduction.  There are a bunch of good reasons 
> (some of which were recently discussed over on time-nuts) such as 
> maintaining continuity of data, hysteresis and other bad effects from 
> making adjustments, etc.
>
> Modern Cs and Masers have synthesizers that can adjust frequency, 
> rather than brute-force adjustment of magnetic fields that older units 
> require.  But even in that case, there may be reasons to leave the 
> thing alone once it's put into service.  (There are also nifty devices 
> like microsteppers that can slew the phase of an input signal at rates 
> of pico (or femto) seconds per second).
>
> So the lab may be very happy with those Rbs, even if their raw PPS is 
> off by a microsecond a week.
>
> John
> ----
>
> David L. Mills wrote:
>
>> Guys,
>>
>> To be accurate, there are two national timescales in the US, 
>> UTC(USNO) kept by the US Naval Observatory in Washington, DC, and 
>> UTC(NIST), kept by the National Institute of Standards and Technology 
>> in Boulder, CO. I am told the holy grail is to discipline these 
>> timescales with each other and other standards laboratories within 
>> one nanosecond. Twenty years ago the grail was one microsecond and 
>> may still be in some parts of the world.
>>
>> With an ageing rate of 5e-11, the residual error after one day is 4.3 
>> microseconds, somewhat more than would ordinarily be expected of a 
>> national timescale. Our dedicated public NTP primary servers here 
>> typically keep within this nominal offset and jitter relative to a 
>> GPS with PPS.
>>
>> Other timescales derived from UTC(USNO) include UTC(LORAN) and GPS. 
>> GPS does not run on UTC and has no leap seconds. It runs on 
>> International Atomic Time (TAI) with a 5-s constant offset. However, 
>> what you see in your GPS receiver is UTC as corrected by the GPS 
>> navigation message.
>
>




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