[ntp:questions] Pi 4 and Ultimate hat weirdness

Gary E. Miller gem at rellim.com
Mon Jan 25 18:11:03 UTC 2021

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?

From your distro of from source?

Did xgps give any warnings on the command line when started from the
command line?

> 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?

What version of ntpd?  From where?

How are you running ntpd?  Root?  With what command line?

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.

> Third and biggest weird thing; I had fudge 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.

> 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.

> 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.

> 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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