[ntp:questions] Pi 4 and Ultimate hat weirdness

Jim Pennino 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 -V
gpsd: 3.17 (revision 3.17)

ntpd --version
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?

xgps
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.

On buster?

> 
> > 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?

As above.

> 
> 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
second.

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.

Of course.

> 
> > 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:

showpeerstats
127.127.20.0
Count:4133
Avg offset:-0.514542
Std dev:    0.257913
Avg Skew:   0.237029
Std dev:    0.149433

127.127.22.0
Count:4945
Avg offset: 0.00498037
Std dev:    0.0291266
Avg Skew:   0.00530061
Std dev:    0.0110748

127.127.28.0
Count:4948
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
> u-blox
> 
> 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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-- 
Jim Pennino


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