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

Rich Wales richw at richw.org
Mon Oct 6 06:53:17 UTC 2014

Replying to William Unruh:

> 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,
> when that is far smaller than the time it takes to deliver even one
> character.

Understood.  I can't really do anything about the speed of the NIST ACTS
(it appears to be fixed at 1200 bps as best I can tell) -- and I realize
I'm not going to get really high accuracy here (but that's OK, this is
just a fallback time source for my home LAN in the event my local GPS
and my Internet connection both fail simultaneously).

> You could try fudging the ntpd refclock source to correct the time.
> . . . and you fudged time1 to .021 why?

I used a fudged time1 of 0.021 because I thought the "021.3" part of the
NIST ACTS data lines meant my delay should be set to around 21.3 msec.

I changed the time1 fudge to 0.222 just now, and my modem refclock has
an offset of -17 msec now -- not nearly as good as my local GPS, or even
my link over the Internet to a collection of stratum-1 servers on my
university's campus -- but given what I wanted the modem refclock for,
it's OK.  Thanks for suggesting that I fiddle with time1 some more.

Rich Wales
richw at richw.org

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