[ntp:questions] ACTS refclock is way off w.r.t GPS refclock

Harlan Stenn stenn at ntp.org
Mon Oct 6 08:02:43 UTC 2014

William Unruh writes:
> 1200 bps is 120 characters per second. Thus the time it takes for the
> clock to deliver a line of characters representing the time to the
> computer is about a second. Makes it pretty hard to get ms accuracy,

I believe the NIST hardware expects echo to be enabled and expects the
transmit and receive delays to be the same.  In that case it adjusts the
timing of the OTM (on-time mark) so that when the OTM changes from * to
# the timing of that OTM compensates for the delay correction.  At
least, that used to be the case.

See http://www.nist.gov/pml/div688/grp40/acts.cfm

Yes, there will still be limits to this technology, but it's not as bad
as one might originally think.


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