[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).


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