[ntp:questions] Time synchronization

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


Rob,

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?

Dave

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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