[ntp:questions] GPS Offset problems

utahcbs at gmail.com utahcbs at gmail.com
Sat Jan 31 01:09:27 UTC 2015

I built a GPS (HP 58503) based NTP server about 10 years ago.  It worked fine but eventually died, and I built a replacement.  But I am having no end of problems with long offsets.

The new machine is Linux-based with a Z3801 and PPS.  It consistently shows an offset as compared to other NTP clocks. 

After about a week of trying to figure it out, I got desperate and bought a used Datum Tymserv 2100 as a stopgap until I could figure out the problem.  It worked great until recently, and now it has a 1-second offset as compared to the world.

Both servers share a common antenna (Symmetricom GPS LT) on a HP/Symmetricom splitter.  Obviously I make certain that I have good GPS signal and everything is locked.

I recently purchased a SECOND Datum 2100 and it had the exact same offset.  So I ran a new wire and hooked it to a different antenna of the same model, same offset.

So I changed to a different antenna, same offset -1 second as compared with known good network servers.

Here is what I have running in this example:

Freiburg = Datum 2100
Augsburg = Linux server with HP Z3801, shared with Freiburg's antenna
Shaun = Datum 2100 on different antenna

When I go into Augsburg, all seems good (PPS currently disabled), but it is off by nearly a second in the other direction, +1 second from known-good servers so +2 seconds from my other servers.

Clearly I am doing something wrong.  I've been beating my head against a wall for quite a while trying to figure it out.  I have another system with a Z3805 that has an offset of 16000 with a good lock.
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================     2 u   33   64    7   33.777   10.353   5.371
 time.utah.edu   3 u   35   64    7    0.880    1.186   0.182
 utcnist2.colora .ACTS.           1 u   31   64    7   13.350    1.439   0.042     .ACTS.           1 u   32   64    7   31.841    5.743   1.930
 freiburg        .GPS.            1 u   31   64    7    5.336  -999.51  40.219
 shaun           .GPS.            1 u   31   64    7    5.072  -999.65   1.103
 augsburg        .3801.           1 u   32   64    7    4.599  970.238   2.261

This is obviously right after starting NTP, but this is the trend that stays
even after NTP is locked on. 

Any idea what I am doing wrong?

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