[ntp:questions] offline machines' time synchronization

Chris Albertson albertson.chris at gmail.com
Sat May 26 02:17:13 UTC 2012

On Fri, May 25, 2012 at 2:51 PM, unruh <unruh at invalid.ca> wrote:

> On 2012-05-25, Chris Albertson <albertson.chris at gmail.com> wrote:
> > On Fri, May 25, 2012 at 12:29 AM, Terje Mathisen <"terje.mathisen at
> > tmsw.no"@ntp.org> wrote:
> >
> >> Chris Albertson wrote:
> >>
> >>> Next how to get the PPS to be at the top of the second.  Two methods
> >>>
> >>> 1) don't bother.  Linux PPS will log the time of both the GPS's PPS
> >>> and the Rb PPS.  Read the log file to determine the offset.  This give
> >>> offset with uSec precision.  To get better you need a hardware time
> >>> interval counter.  I happen to have a few of those and can read
> >>> sub-nanoseconds but this is gross overkill.  Microseconds are good
> >>> enough for NTP......
> >>>
> >>
> >> Much simpler would be to use a fudge command to specify the current
> phase
> >> offset between the Rb and GPS PPS signals, i.e. if the Db happened to
> start
> >> out 367 ms off, just let ntpd know that these pulses are supposed to
> have
> >> such an offset.
> >>
> >>
> > Yes, correct the "fudge" method is what I was getting at in #1 above.   I
> > assumed that every on here would know to use the measured difference in a
> > "fudge" but the key is measuring the offset.   Get that from the log
> file.
> > or a TIC if you have one.
> >
> > But there is a problem in the even with the best Rb the offset will drift
> > over time.
> Yes. but slowly (probably about 1 sec per millenium or less)

You are close.  My envelope back says 30,000 years to gain a second.

But this also means you can run a $40 Rb oscillator in cave, disconnected
from the Internet or GPS for 30 years and only see a 1 ms error.   The unit
is about the exact size standard computer hard drive.

When I said "it drifts" I was thinking of using this as a frequency
standard, not so much for a GPS backup.  In that case I'm concerned with
micro second level errors so it needs to be corrected daily for drift.

> Chris Albertson
> Redondo Beach, California

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Chris Albertson
Redondo Beach, California

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