[ntp:questions] Time synchronization

David Mills mills at udel.edu
Sat Feb 7 04:37:28 UTC 2009


That would work fine, but does not explain the large jitter reported, 
unless gpsd displays the raw data placed in the shared segment. That 
wouldn't be very useful; the ntpq rv data would be useful and in fact 
the ultimate performance calibration. Also, why is there a PPS claimi 
but the kernel PPS is not disciplining the clock?


Rob wrote:

>David Mills <mills at udel.edu> wrote:
>>Third, the data reported is not from ntpd, but another daemon called 
>>gpsd. Apparently it doesn'tt like the PPS signal. I don't know what 
>>grooming algorithm it uses, but both the kernel and atom driver use a 
>>trimmed-mean median filter, which is a rather heave dose of signal 
>>processing that probably would not be the first inspiration of folks who 
>>have not studied the detailed statistics.
>gpsd does not perform any filtering at all.  it puts the timestamp
>data from NMEA and PPS into shared memory and lets ntpd do all the
>filtering via the shm driver.
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