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

Rich Wales richw at richw.org
Mon Oct 6 02:49:55 UTC 2014


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.

Here is the output of "ntpq -c 'hostnames no' -c peers" on the server with the
modem:

     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 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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