Re: Recommend a USB gps for Rpi

[TarotApprentice]

I have the following in ntp.conf for the the gps:

# GPS Serial data reference (NTP0)
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 0.14334 refid GPS

# GPS PPS reference (NTP1)
server 127.127.28.1 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.1 refid PPS

Cgps says "seen 30/used 18" which if I am not mistaken means it can see 30 
satellites and is using 18 for its 3D fix. 

If I do ntpq -p with a 16 second loop I get the following, that is the offset 
column is 24.526 at one point, drops to 15.025 and then back to 26 and 27. The 
average is 24.63. Its too small a sample set to use for tuning (the how-to 
guide recommends averaging over 4 hours) and admittedly I haven't fine tuned 
the time1 parameter above. Is it normal to see it see it drop and then come 
back like this?

xSHM(0)          .GPS.            0 l   14   16  377    0.000   24.526   1.837
*SHM(1)          .PPS.            0 l   13   16  377    0.000    0.073   0.135

xSHM(0)          .GPS.            0 l   14   16  377    0.000   25.904   1.870
*SHM(1)          .PPS.            0 l   13   16  377    0.000    0.162   0.215

xSHM(0)          .GPS.            0 l   14   16  377    0.000   25.473   1.623
*SHM(1)          .PPS.            0 l   13   16  377    0.000    0.161   0.208

-SHM(0)          .GPS.            0 l   15   16  377    0.000   15.025   9.884
*SHM(1)          .PPS.            0 l   14   16  377    0.000    0.081   0.132

xSHM(0)          .GPS.            0 l   15   16  377    0.000   24.432   3.865
*SHM(1)          .PPS.            0 l   14   16  377    0.000   -0.047   0.136

xSHM(0)          .GPS.            0 l   15   16  377    0.000   27.447   5.586
*SHM(1)          .PPS.            0 l   14   16  377    0.000   -0.071   0.150

xSHM(0)          .GPS.            0 l   15   16  377    0.000   26.551   4.551
*SHM(1)          .PPS.            0 l   14   16  377    0.000   -0.104   0.173

xSHM(0)          .GPS.            0 l   15   16  377    0.000   27.792   5.241
*SHM(1)          .PPS.            0 l   14   16  377    0.000   -0.070   0.115

MarkJ



On Sunday, 11 October 2020, 3:15:20 am AEDT, Gary E. Miller <gem@rellim.com> 
wrote: 





Yo David!

On Sat, 10 Oct 2020 07:55:11 +0100
"David J Taylor" <gm8arv@yahoo.co.uk> wrote:

> Can anyone on the list recommend a USB gps for the Raspberry Pi?

Any u-blox 8 should be fine, if the antenna is in a window, or better.

The newer u-blox 9 work with less signal than the 8's.

Sparkfun has  a NEO-M9N for $65:

https://www.sparkfun.com/products/15712


> My
> use case is for it to provide time services to a cluster so it
> doesn’t have to be super accurate. I currently have a u-blox 8 which
> works with gpsd but the time from it jumps all over the place.


What is you definition of "all over the place"?  Anything over USB
will never be better than 1 ms.  Much worse than that indicates a
problem.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
    gem@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