[ntp:questions] Strange jumps in PPM
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
> 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
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