[LinuxPPS] Strange offset behavior

[Andrew Hills]

Hi all,

I'm observing some strange behavior of the clock offset. I set up two 
machines, nearly identical on the software side (same patched kernel 
version, same version of NTP), and monitored their offsets overnight by 
polling `ntpq -p` once per second. I graphed the results here (sorry for 
the PDF): http://people.umass.edu/ahills/ntpq_offset.pdf

The top graph represents the entire data set; the bottom graph is the 
first twenty thousand samples or so (to display more detail).

The green one, L13, is a machine that has been working fine for the past 
several weeks. The red one, L11, is one that I've just recently revived; 
I'm told that it worked before. However, the offset behavior seems very 
strange. Its offset jumps a lot at the beginning and exceeds the usual 
15us limit I observe with the other machine by a factor of four. It 
takes several hours to finally synchronize with the PPS signal, and 
there's also that strange peak at about 5am GMT.

Can anyone think of an explanation for this behavior? The GPS hardware 
has been confirmed to work in the expected manner on another machine.

--Andrew Hills