Yo Martin! On Mon, 6 Nov 2017 22:16:12 +0100 Martin Burnicki <martin.burni...@meinberg.de> wrote:
> sorry for the late reply. I've been mostly offline for some days.
Yeah, we both have real lives too.
> I'm mostly working with GPS PCI cards, where the kernel driver reads a
> high resolution, accurate time stamp from the PCI card, plus an
> associated system time stamp, plus some TSC counts. The calling
> application can check the TSC values to see if reading the time stamp
> pairs has taken longer than "usual", and repeat the call, or feed both
> time stamps into SHM.
Coll? Anyhting cheap I could afford?
> On the other hand, with external GPS receivers connected via NMEA or
> some binary serial messages you need PPS to compensate the serial
> jitter. If you enable kernel PPS support then the kernel evaluates the
> PPS time stamps by itself, but the kernel is unable to determine the
> mean PPS latency.
We assume it is zero. Not valid, but what else can we do? PPS is
already way down in the noise.
> So here we have to look at what happens to the PPS if the GPS goes
> wacky. If you have a GPS with good, disciplined oscillator the it
> makes sense to keep on evaluating the PPS even after GPS reception
> has just failed.
Scary, but if a GPS could tell gpsd that its PPS is good, but PTV is
not good, some new code could handle that.
> > 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.
>
> That's exactly what I try to say. The GPS said "synced" but probably
> only because a navigational fix could be achieved, which doesn't need
> the UTC correction parameters. If there was a "time sync" status flag
> that could be evaluated then the time from the GPS wouldn't be
> accepted before the UTC correction parameters were available, even if
> a position fix could have been achieved.
Thecnically, a GPS should not output a 'valid' indication until it has the
current ephemeris for the sats in the fix.
The almanac may take 12 to 24 mins to download, but the almanac is just
for predicting future GPS sat locations.
I'm sure there a few small cases where my simplification is not right,
but I've not seen PPS shift from the almanac.
RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
g...@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
pgp2XwDviOTCa.pgp
Description: OpenPGP digital signature
_______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
