[ntp:questions] false ticker after GPS coldreset
Gary E. Miller
gem at rellim.com
Fri Nov 3 19:08:13 UTC 2017
Yo Martin!
On Fri, 3 Nov 2017 14:27:52 +0100
Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> >> Anyway, gpsd seems to accept the raw GPS time as UTC time, feeds
> >> it to ntpd, and ntpd sets the Linux system time off by a number of
> >> seconds.
> >
> > Which is why gpsd should rarely be the only input into your ntp
> > daemon.
>
> Hm. According to my own observations, if you get most accurate time
> from your GPS receiver operating normally, as single time source, and
> then add some upstream NTP sources beside the GPS refclock the
> resulting accuracy of the system time becomes worse.
That can happen, but there are mitigation strategies. First I use
the prefer keyword on my PPS time. I'm not seeing that here:
remote refid st t when poll reach delay offset jitter
*SHM(1) .PPS. 0 l 1 2 377 0ns 722ns 968ns
+spidey.rellim.c .PPS. 1 u 25 32 377 254.35µs 57.594µs 63.995µs
-pi.rellim.com .PPS. 1 u 22 32 377 416.27µs -654.8µs 154.28µs
pi2.rellim.com .PPS. 1 u 9d 32 0 581.89µs 28.000µs 0ns
pi4.rellim.com .STEP. 16 u - 32 0 0ns 0ns 954ns
-catbert.rellim. .PPS. 1 u 30 32 377 376.48µs 455.34µs 49.887µs
-kong.rellim.com 204.17.205.8 2 u 59 32 266 347.37µs 196.00µs 30.736µs
-backup.rellim.c 204.17.205.8 2 u 22 32 377 412.37µs 158.38µs 48.486µs
+SHM(0) .GPS. 0 l 13 16 377 0ns 507.16µs 672.13µs
Note the 722 ns offset and 960ns jitter on mmy PPS.
> So it's not generally better to use other time sources beside a GPS
> receiver.
Until your GPS goes wacky...
> You can simply try this if you look at the smoothness of the loopstats
> generated with a GPS receiver only vs. the loopstats generated with a
> GPS receiver plus some NTP servers on the network.
I've got a dozen chimers up now with PPS, not seeing it.
> ntpd doesn't anyway accept the time from a source that indicates that
> its time is not synchronized. So it wouldn't accept the time from the
> GPS receiver (directly or via gpsd/SHM) if a "not synchronized" status
> was available from the receiver and passed to ntpd.
Which is NOT what the OP was complaining about. His GPS was saying synced,
but was off by the UTC/GPS offset which had not been downloaded yet.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem at rellim.com Tel:+1 541 382 8588
Veritas liberabit vos. -- Quid est veritas?
"If you can’t measure it, you can’t improve it." - Lord Kelvin
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