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

Rob nomail at example.com
Mon Oct 6 06:54:22 UTC 2014

Rich Wales <richw at richw.org> wrote:
> I'm running ntpd 4.2.7p475 on a server in my home LAN, with refclock_acts.c
> patched to open the serial line at 1200 instead of 19,200 bits/second (see my
> earlier e-mail "Re: Adjust serial device speed in ACTS refclock driver?").
> The modem refclock, syncing to NIST, appears to be about 200 msec off with
> respect to a GPS refclock which has been running for a long time on another
> server in my home LAN.
> Any ideas why the modem refclock is so far off?  Is there anything I can do
> about it?

"so far off"??
you have just raised your expectations and now the actual results
disappoint you.  when that service was established, typical users would
probably have found the results extremely good, because the clocks that
they controlled using that method appeared to be on-time when they looked
at them and listened to the six-beeps time signal on the radio.

now, the times are different.  we can get millisecond accuracy easily,
and microsecond accuracy with some effort.  but then you should step
away from using phonelines, modulation methods in baseband audio, etc.

when you want serious backup for GPS, get another receiver that can also
receive one of the competing systems (glonass, galileo) or when you can
live with a little less accuracy a receiver for a longwave time signal.

here I have 2 GPS receivers and a DCF77 longwave receiver:

     remote           refid      st t when poll reach   delay   offset  jitter
 GENERIC(0)      .DCFa.           0 l 1058   64    0    0.000   -0.379   0.000
oPPS(0)          .PPS.            0 l   15   16  377    0.000   -0.001   0.001
*SHM(1)          .PPS.            0 l   13   16  377    0.000    0.005   0.002

The longwave receiver currently receives no valid data, and as you can
see the last time it did it was about .38 ms off.  So when I lose GPS
reception my system will be inaccurate as well.

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