[ntp:questions] project ntp.br - discrepancy from UTC
David L. Mills
mills at udel.edu
Tue Oct 9 21:34:31 UTC 2007
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.
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.
> David L. Mills wrote:
>> 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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