[gpsd-dev] PPS over USB

[Gary E. Miller]

Yo All!

As some of you may know, esr has been helping the bufferbloat project
with some gpsd issues.  Their goal is to get good time from a USB
connected GPS.

Esr negotiated with Navisys to special build three units of a ublox 6
and a pl2303 with PPS conencted to USB.  They call them a GR-601 and I
just received the samples.  The preliminary results are pretty good if a
clock stable to about 1 milliSec is your goal.

A preliminary result from a dual core laptop.

# ntpq -p
     remote           refid      st t when poll reach   delay   offset
jitter
==============================================================================
-backup          .SOC1.           1 u   45   64  377    0.233 -0.006   0.067 
+fuzzy           .GPS1.           1 u   47   64  377    0.241 -0.047   0.071 
+SHM(0)          .GPS.            0 l   10   16  377    0.000 -2.995   1.656 
*SHM(1)          .GPS1.           0 l    9   16  377    0.000  0.317   0.428


backup and fuzzy each have a PPS clock directly connected over serial.
The jitter on backup is about 2 microSec and fuzzy about 0.5 microSec.
And they tend to agree over ntp to about 20 microSec or better.

All the NTP servers are adjacent to each other and connected over GigE.
The jitter over the GigE seems to be about 100 microSec or better.

SHM(0) is NMEA time over USB.  SHM(1) is PPS time over USB.

As you can see the NMEA/USB short term jitter is about 2 milliSec and
the PPS/USB time is about 0.5 milliSec jitter.  Long term NMEA/USB drift
is quite a bit larger, maybe 100 milliSec or more.

So the jitter of PPS over USB is about 50x worse than PPS over GigE, but
I think for the purposes at hand pretty good.  Of course I expect the
results to be worse when run on a single core router.

The important part of the ntp.conf:

    # for gpsd
    server 127.127.28.0 minpoll 4 maxpoll 4
    fudge 127.127.28.0 time1 0.142  refid GPS

    # for PPS and gpsd
    server 127.127.28.1 prefer  minpoll 4 maxpoll 4
    fudge 127.127.28.1 time1 0.001500 refid GPS1

I start the daemons this way:

    ntpd -p /var/run/ntpd.pid -N -u ntp:ntp -g
    gpsd -n /dev/ttyUSB0

Any questions or suggestions?

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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