[ntp:questions] project ntp.br - discrepancy from UTC
John Ackermann N8UR
jra at febo.com
Tue Oct 9 19:26:23 UTC 2007
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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