[ntp:questions] How to tell which clock is wrong?

John Pettitt jpp at cloudview.com
Sun Apr 3 02:40:23 UTC 2005


A man with two clocks never knows the right time.

I’ve been trying to track down a persistent offset in my stratum 1
clock and I’ve reached a point where I’m stuck and open to suggestions.

The system runs FreeBSD 5.4BETA1 on Soekris 4801 hardware with a
Garmin GPS18LVC as a refclock and pps provider using the NMEA driver.
 I have this GPS hooked to two machines with a daisy chain cable.  The
second box is a Celeron 2.9Ghz on an Intel chipset also running
FreeBSD 5.4BETA1  (upgrading from 5.4 to 5.4 didn’t change any of the
symptoms).  Both machines show time within 50us of each other.

The symptom is this – my clock is persistently 2ms off from a basket
of other stratum 1 clocks.    The spec on the GPS18LVC says the pps
output is +/- 1us so at least for the moment I’m not blaming the GPS
(although I could be persuaded).

My first assumption was that the asymmetry in my DSL line was causing
the offset.    However I no longer believe that for two reasons.
First the calculated asymmetry is about 1.2ms not 2.  Second I slaved
machine in a server farm at my ISP to my stratum 1 and it tracks
within 100us (and show the same offset to the basket of S1’s).

My next thought was that I had a weirdness in the de-glitch in the
serial port that was messing with the PPS (2ms =~ 1 char @ 4800bps) –
however changing the baud rate of the GPS to 38400 didn’t change the
apparent offset.

So where I’m at now is either

1) my GPS is off by 2ms (why?)  or

2) my IPS (sonic.net) has a sufficiently asymmetric connection to the
rest of the world that it skews all external reference clocks.

How do I tell if my clock is wrong or if everybody just appears wrong?
Any thoughts, short of buying another GPS, on how to prove one or the
other would be most welcome.

John



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