[ntp:questions] ACTS refclock is way off w.r.t GPS refclock
unruh at invalid.ca
Mon Oct 6 05:52:10 UTC 2014
On 2014-10-06, 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
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
You could try fudging the ntpd refclock source to correct the time
> 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.
> Here is the output of "ntpq -c 'hostnames no' -c peers" on the server with the
> remote refid st t when poll reach delay offset jitter
> 127.127.18.0 .NIST. 3 l 83 512 1 0.000 -204.85 0.000
> *10.0.229.163 10.0.229.2 2 s 27 32 17 0.169 -0.056 0.036
> +10.0.229.2 .GPS. 1 s 10 32 17 0.119 0.056 0.024
> 10.0.229.117 10.0.229.2 2 s 60 64 3 22.133 0.553 0.116
> Here are the relevant refclock configuration lines from the "ntp.conf" file:
> server 127.127.18.0 minpoll 9 maxpoll 16
> fudge 127.127.18.0 stratum 3 time1 0.021 flag1 1 flag2 1
> (I fudged the stratum to 3 in order not to risk having the modem refclock take
> over my home LAN while I'm still experimenting with it.)
and you fudged time1 to .021 why?
> And here is the last line from the "clockstats" file:
> 56936 8840.501 127.127.18.0 56936 14-10-06 02:27:19 28 0 -.4 021.3 UTC(NIST) *
> In case it matters, I'm using a TrendNet TFM-561U USB modem.
> Any ideas why the modem refclock is so far off? Is there anything I can do
> about it?
> Rich Wales
> richw at richw.org
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