[ntp:questions] Atheros AR9331 w/GPS + PPS

Dan Drown dan-ntp at drown.org
Wed Sep 9 14:47:02 UTC 2015


Quoting Joachim Fabini <Joachim.Fabini at tuwien.ac.at>:
:
> I'm really surprised, that's excellent. Do you have any figures how ntpd
> performs when using this PPS source? My earlier comment was relying on
> the assumption that the diagram displays the ntp log (loopstats)
> statistics. Until now, with Linux, desktop computers and rather
> expensive GPS equipment I couldn't get the system clock's offset better
> than +-5µs over some hours. Publications point out that the CPU
> performance has major influence on the ntp timesync accuracy, too. The
> PPS reporting is a different story but still a main prerequisite. Can
> you share some results on ntp loopstats based offset?

Here's some data from my systems.  I'm using chrony, but this is from  
the equivalent logfile to ntp's loopstats.  These are histograms of  
all local clock offsets over the past 7 days.

Beaglebone Black + NS-T GPS + pps_gmtimer.ko:  
http://dan.drown.org/bbb/run23/offset-histogram.png

50% of the time, the local clock's offset is between -0.03us and  
+0.02us.  98% of the time, it's between -0.34us and +0.36us.  The  
maximum offsets were -3.28us and +2.91us


Raspberry Pi + Navspark-GL GPS + pps_gpio.ko:  
http://dan.drown.org/rpi/run6/offset-histogram.png

50% of the time, the local clock's offset is between -0.12us and  
+0.17us.  98% of the time, it's between -1.63us and +1.04us.  The  
maximum offsets were -5.69us and +3.18us


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