[ntp:questions] Pi 4 and Ultimate hat weirdness

David Taylor davidtaylor at writeme.com
Mon Jan 25 11:03:46 UTC 2021


On 25/01/2021 02:06, Jim Pennino 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.
>
> First weird thing; xgps does not show a skyview but both xgps and cgps
> show at least 10 satellites in use.
>
> I don't care too much about this one as AFAIK xgps is broken on Pi.
>
> Second weird thing; I started the 20 driver in mode 0. ntpq showed lots
> of syntax errors on the $GPGGA message.
>
> 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.
>
> 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...
>
> 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.
>
> Also the jitter times look high. At this moment ntpq shows:
>
> xGPS_NMEA(0)     .GPS.            0 l    6   16  367    0.000  -849.36  25.536
> *SHM(0)          .SHM.            0 l    4   16  377    0.000  -106.55  72.688
> xPPS(0)          .PPS.            0 l    4   16  377    0.000   18.702  18.812
>
> 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
>
> Anyone got any suggestions other than to trash the Ulitmate hat and get
> another GlobalSat USB?

I've been playing with the RPi4 recently and had mostly success.  I've
used a couple of different devices including the Uputronics/ublox HATs.

GPSD has proved problematical, and the only satisfactory way is not to
use the one in the distribution, but to build from scratch.

   https://www.satsignal.eu/raspberry-pi/UpdatingGPSD.html

It's the PPS which matters, and for that I've found it most satisfactory
to use the type 22 driver.  For time of day I tend to use other local or
Internet servers.  I've been experimenting with an RTC, but
experimenting with its unstabilised drift first to understand how well
it might behave left for a day, week or month.

   https://www.satsignal.eu/raspberry-pi/Uputronics-RTC.html

Offsets near 1 second introduce an ambiguity and I would suggest
avoiding such devices.  However, I would expect the AdaFruit to be good
- have you set a sufficiently high baud rate?  Recent ublox are being
set to 115200 to capture all the data.  IIRC, $GPRMC alone is enough.

Just some random thoughts, not a definitive solution!

Cheers,
David
--
SatSignal Software - Quality software for you
Web: https://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk
Twitter: @gm8arv


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