[ntp:questions] Pi 4 and Ultimate hat weirdness
jimp at gonzo.specsol.net
Mon Jan 25 22:04:58 UTC 2021
On Mon, Jan 25, 2021 at 10:11:03AM -0800, Gary E. Miller wrote:
> Yo Jim!
> On Sun, 24 Jan 2021 18:06:12 -0800
> Jim Pennino <jimp at gonzo.specsol.net> wrote:
> > I got a Pi 4 and Adafruit ultimate gps hat to play with and decided
> > to see how good it was as a timekeeper.
> You will find it pretty good. With a bit of work it can keep
> system time stable to about 100 ns.
> > First weird thing; xgps does not show a skyview but both xgps and cgps
> > show at least 10 satellites in use.
> Before that:
> What versions of gpsd and xgps?
gpsd: 3.17 (revision 3.17)
ntpd 4.2.8p12 at 1.3728-o (1)
> From your distro of from source?
Distro, totally up to date.
> Did xgps give any warnings on the command line when started from the
> command line?
Gtk-Message: 13:45:00.966: Failed to load module "canberra-gtk-module"
TypeError: Couldn't find foreign struct converter for 'cairo.Context'
Makes me think the distro is missing a dependancy.
> > I don't care too much about this one as AFAIK xgps is broken on Pi.
> Nope. Works for me.
> > Second weird thing; I started the 20 driver in mode 0. ntpq showed
> > lots of syntax errors on the $GPGGA message.
> Before that:
> How are you running gpsd? Root? With what command line?
sudo systemctl start gpsd
sudo systemctl start gpsd.socket
> What version of ntpd? From where?
> How are you running ntpd? Root? With what command line?
sudo systemctl start ntp
ps -eo euser,egroup,args | grep gps
gpsd dialout /usr/sbin/gpsd -n -r /dev/ttyAMA0 /dev/pps0
ps -eo euser,egroup,args | grep ntp
ntp ntp /usr/sbin/ntpd -p /var/run/ntpd.pid -g -u 115:124
> Then please copy the errors here.
> > I changed the mode to 29 to deselect $GPGGA; no errors from ntpq. So
> > maybe it was a satellite thing, but whatever it was, no more errors.
> We can only guess without seeing what you see.
Not a big deal to me if ignoring $GPGGA makes the errors go away.
> > Third and biggest weird thing; I had fudge 127.127.20.0 flag1 1 time1
> > 0.001 in ntp.conf. After everything settled down, the clock was
> > showing an offset of -1043. OK, So I changed time1 to 1.044 even
> > though it looked too big. ntp wouldn't lie to me...
> It can takes days for ntpd to calibrate your system. How long did you
> wait? It your serial port rate from your GNSS is slow, then you can
> easily get off one second. Or maybe the receiver does not have the
> current leap second yet.
The GPS has been powered up since I stuck in the battery for the
on board RTC 5+ days ago.
How long does it typically take a receiver to have the current leap
BTW, the GPS has an external antenna with a good view of the sky
and cgps alway shows 11+ satellites in view and always 8+ in use.
> > After everything settled down this time the offset was showing an
> > average of -738. For grins and giggles I set time1 to 0.738 and now
> > the average offset is -584.
> > time1 offset
> > 0.001 -1043
> > 1.044 -738
> > 0.738 -584
> > It seems there is no way to get time1 to correct the offset.
> I hope you waited an hour or more between changes? ntpd is a
> phase locked loop, it takes time to settle.
> > Also the jitter times look high. At this moment ntpq shows:
> That is to be expected early on. Don't bother to look at the
> jitter until the offset is stable for hours.
Right now ntp has been running for about 18 hours.
Here is what ntpq is showing now:
*127.127.20.0 .GPS. 0 l 7 16 11 0.000 -886.78 27.361
x127.127.28.0 .SHM. 0 l 5 16 377 0.000 -100.40 36.248
x127.127.22.0 .PPS. 0 l 9 16 377 0.000 -4.458 0.720
And this is from peerstats:
Std dev: 0.257913
Avg Skew: 0.237029
Std dev: 0.149433
Avg offset: 0.00498037
Std dev: 0.0291266
Avg Skew: 0.00530061
Std dev: 0.0110748
Avg offset: 0.00237325
Std dev: 0.080877
Avg Skew: 0.0593658
Std dev: 0.0643203
> > For comparison, a GlobalSat BU-353-S4 USB GPS on a PC ubuntu system
> > shows: *SHM(0) .SHM. 0 l 3 16 377 0.000
> > -0.728 1.356
> Which I assume had been running for months.
Yes and no; the system was rebooted 4 days before.
> > Anyone got any suggestions other than to trash the Ulitmate hat and
> > get another GlobalSat USB?
> The Ultimate HAT is a much better GNSS than the GlobalSat. The SiRF 4
> is not very sensitive and uses fewer sats and constellations than the
> 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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