[ntp:questions] How to determin hardware latency for PPS offset given simple tools.

Paul G bodosom at gmail.com
Mon May 27 02:06:36 UTC 2013

On Saturday, May 25, 2013 1:06:48 AM UTC-4, unruh wrote:
> GPS receivers are not particularly good time sources UNLESS you use PPS.

All of my clocks use PPS and in fact none of them use GPS to number the seconds
except the one that uses refid GPS.  That clock is a purpose built NTP-equivalent server called a Laureline which uses PPS and numbers
seconds via serial input.

Per the refclock document the PPS/NMEA (20/22) drivers use time1 to correct for
PPS offset while 22 uses time2 to correct for serial latency.  Since the clocks
differ by O(100 microsec) without time1 correction I assumed that was internal rather than network latency.

> If you use PPS they should all agree to the 1micro second level.

They do per ntpq given resp. 124, 151 and 85 microsecond time1.

> grabbing times via the net from the those computers
> will be dominated by network issues. 

I've been assuming that since I get fairly consistent offsets (fudged or not)
and jitter that network latency is not the culprit.

But I'm clearly confused about something.

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