[ntp:questions] Pi 4 and Ultimate hat weirdness

Jim Pennino jimp at gonzo.specsol.net
Mon Jan 25 02:06:12 UTC 2021

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?

Jim Pennino

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