[ntp:questions] Strange jumps in PPM

A C agcarver+ntp at acarver.net
Wed Aug 21 20:29:54 UTC 2013

On 8/21/2013 12:15, E-Mail Sent to this address will be added to the 
BlackLists wrote:
> A C wrote:
>> Ok, so I saw the below event in the loops on my system which
>>   is controled by ATOM, SHM, and several internet servers.
>> I suspect it has to do with the GPS PPS output glitching
>>   during extreme satellite events (bad constellation arrangement,
>>   fringe signal, etc.)
>> Is there a way to have ntpd ignore the PPS quickly if it
>>   spins out of control?  The system recovers fairly quickly
>>   and the clock doesn't go too far out but it would be nice
>>   to avoid these jumps.
> Some GPS receivers have a PPS holdover option,
>   for when the fix quality goes invalid,
>   where it uses the local oscillator to try to keep the
>   PPS in phase, while the satellite reacquisition occurs.

This receiver (Globalsat ET-102) in theory does that automatically but 
the local oscillator is probably not great.  I'd have to see if I could 
load up the crystal with resin or something to make it thermally stable 
but I don't have any further control options with the PPS output.

I can't complain too much, I got three of them for free. :) I just want 
to try and work around the quirks.  It does really well when everything 
runs smoothly but then these hiccups are pretty wild.  I think this is 
the main source of my old friend the out of control ntpd (the one that 
would spin off into oblivion).  I identified already one potential 
source of sync error which was the on-board SBAS controls.  It would 
lose lock on one of the SBAS satellites (135-139) which would cause all 
of the calculations to go awry for a several cycles.  I disabled SBAS 
which seems to have calmed the jitter in the serial data and also 
perhaps helped the out of control issues (though this other PPS issue is 
still there).

More information about the questions mailing list