[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 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.


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.


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!

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Email: david-taylor at blueyonder.co.uk
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