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

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat May 25 07:05:13 UTC 2013

On 24/05/2013 21:28, Paul G wrote:
> I have four GPS receivers into four different computers.  I've assumed that I
> should be able to get small offsets between them but to do so I need
> to set time1.  If that's the correct approach how do I pick the "right"
> system to be the benchmark.  So far I've just been assuming that
> the purpose built M. Tharp server should have the lowest latency but I'm not sure -- in any case, at present, I can't adjust it.
> Each machine in the truncated table below (except 210) has a time1
> value derived in an ad hoc fashion.
> l     r    refid       st   delay   offset  jitter
> ==================================================
> 1   o22.0  .PPS.        0   0.000   -0.001   0.001
> 2   +244   .PPS.        1   0.068    0.000   0.006
> 2   *210   .GPS.        1   0.514   -0.008   0.136
> 2   +192   .PPS.        1   0.455    0.008   0.032
> I feel as if this topic should be clearly explained somewhere but I've been
> unable to find it.


Those PCs with PPS feed should have zero offset, and time1 should be set 
to zero unless you /know/ that the GPS PPS signal is offset (e.g. you 
are feeding it via several microseconds delay of cable, which is 
unlikely).  What NTP is actually syncing to is an internal PPS (if you 
like), i.e. the external PPS signal delayed by whatever processing is 
taking place inside the PC, but any external things you measure will be 
subject to the same delay.  What happens with the fudge set to zero?

Please show the table for each machine.

For the PC with just GPS sync, you might get it a PPS feed somehow, use 
PPS as the reference, set GPS/NMEA to noselect in the ntp.conf, and 
record the peersstats.  I wrote this up with help from others here:


and although the results in that case are wrong, the method is, I 
believe, correct.
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