[ntp:questions] PPS from an external OCXO source. Correcting drift is it possible?

Kat Schwar3Kat at gmail.com
Fri May 1 23:30:50 UTC 2009


> How far are you willing to go? You might be able to build a clock
> based on the PPS that is amenable to disciplining through NTP, but
> if it's only for holdover during Internet outages, perhaps you'd
> be more helped with a way to keep the offset down (or at least
> known) between Internet outages.

Thanks Martin

Probably I would be well served with a way to keep the offset down at
least as a starting point.  This would take care of one of the two
issues, but ideally for long term accuracy some kind of drift
compensation would be nice.

For drift compensation, logically I would be expecting NTP to handle
pulses that are not exactly 1 second long, perhaps this is easy to
program, however I'm not a C programmer or a unix guru so there will
be a very steep learning curve for me to get into modifying the PPS
code.  (Unfortunately I have a tendency to want to make things work
correctly rather than just fix the problem but I will resist this
temptation for now).

I have conflicting and confusing info on the fudge commands so I have
tried using the fudge time1 setting to eliminate the offset at start
up.  I didn't get it to work, so I tried fudge time2 and this also
didn't work.  The documentation that I have isn't clear on how to use
these settings and what units are used, I tried both milliseconds and
microseconds and neither worked.  What units are used, and can they be
dynamically adjusted through ntpdc?

A hint at why the offset may not be working may come from this
fragment from - External Clock Discipline and Local Clock Driver.
http://web.mit.edu/Source/third/xntp/html/extern.htm

"The external clock driver controls the system time and clock
selection in the following way. Normally, the driver adjusts the
kernel time using the ntp_adjtime() system call in the same way as the
daemon. In the case where the kernel discipline is to be used intact,
the clock offset is provided in this call and the loop operates as
specified. In the case where the driver steers only the frequency, the
offset is specified as zero".

Please indulge me if I am way off here - I am finding the mountains of
NTP documentation exceptionally confusing and the more I wade through
it the more confusing I find it.

Does anyone know of an authoritative source of instructions for using
PPSAPI with a PPS only source?

I thought that if I could get the fudge to work then perhaps I could
script something to use ntpdc to keep the offset under control.

I'm also not to clear on how to get the authentication for ntpdc
working.  Can anyone point me to clear instructions or a tutorial for
current NTP versions?

Kind Regards
Kat




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