[ntp:questions] Large and apparently permanent step in PPM error - confused?

David Cureton david.cureton at dcureton.com
Sat Nov 1 00:47:59 UTC 2008


Hi,
    I am syncing a Linux 2.6.26 kernel to a Serial DCD PPS GPS source.
The kernel time  has been running for quite some time with a ~160ppm 
frequency error however inexplicably the ppm error over night simply 
jumped to ~105ppm. The system  has for the last couple of day tracked in 
a stable manner at that ~105ppm rather than the ~160ppm offset it used 
to be.

There has been no change to the system that I am aware of. The machine 
was not rebooted. There has been no dramatic change in temperature 
although I think temperature change would be very dramatic cause 55ppm jump.
(In fact you can see the daily temperature variation in the ppm error 
which is in the order of a couple of ppm over time)

I have a plot of the kernel offset and ppm error as well as the time 
offset to a stable GPS steered internet peer in my area. 
http://dcureton.com/ppm_jump.jpg

I am at a loss to explain why this jump occurred and whether to expect 
it to occur in the future.  Does anyone have any ideas to the cause of 
this jump in PPM error. System is on a UPS so unlikely to be a power 
supply issue. I am stumped?

Thanks,
David











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