Re: [Thumbgps-devel] [gpsd-dev] PPS over USB

[Dave Taht <dave.taht@gmail.com>]

I'm a little confused.

There are two threads going by today. One is about a sirfIII and the
other a ublox 6

I have been assuming they are one and the same, but perhaps I'm confused.

Secondly an item that has not been looked into much is the quality of
the antennas, or the quality of the clock source when only a single
sat is available....

Thirdly, wow.... when can I get one of these puppies to play with?

:me looks out his bay window wistfully:


On Tue, May 1, 2012 at 3:14 PM, Gary E. Miller <gem@rellim.com> wrote:
> 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
>        gem@rellim.com  Tel:+1(541)382-8588



-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://www.bufferbloat.net