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[70.113.79.122]) by mx.google.com with ESMTPS id 34sm726090anu.6.2012.05.01.20.07.21 (version=TLSv1/SSLv3 cipher=OTHER); Tue, 01 May 2012 20:07:22 -0700 (PDT) References: <20120501151435.4a6a5373.gem@rellim.com> In-Reply-To: Mime-Version: 1.0 (1.0) Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 Message-Id: <0F96256E-B326-415A-ADD7-F5D86AEE75CA@netgate.com> X-Mailer: iPad Mail (9B176) From: Jim Thompson Date: Tue, 1 May 2012 22:07:20 -0500 To: Dave Taht X-Gm-Message-State: ALoCoQl/0Jr73EMqtaY9xkK+bd+92RYqNjdwmhYMJ8f3hOUQA41c9TkAiVsvLvRDkgclQnd7KKOc X-Mailman-Approved-At: Wed, 02 May 2012 01:06:06 -0700 Cc: "thumbgps-devel@lists.bufferbloat.net" , "gpsd-dev@nongnu.org" Subject: Re: [Thumbgps-devel] [gpsd-dev] PPS over USB X-BeenThere: thumbgps-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 02 May 2012 03:07:25 -0000 Hi Dave, There is a third option in-play, but I've not spoken about it in public. =20= I have an ARM (xscale) board with OpenWRT (amd gpsd) running that has provis= ions for a Trimble Copernicus-based GPS module with the PPS output tied to a= GPIO pin on the Xscale, and the UART on the GPS module connected to the Xsc= ale at TTL signal levels.=20 The module is in-transit, so I've not completed the integration or started t= esting. I should be able to use the fast-path IRQ stuff on the xscale to si= gnificantly reduce the latency & jitter over a USB-based solution. I know it= 's fast enough to be able to implement a distributed CSMA scheme among multi= ple 802.11 radios, with timing sufficient to occur in the same timespan as t= wo short OFDM symbols (800ns each, so 1.6 microsec.)=20 Outdoor enclosures, POE, etc. are all sourced, so the whole ball of wax coul= d be put outside, with either Ethernet or Wi-Fi connectivity. Alternatively= , an antenna with DC power could be used, (it's available). When they're ready, of course you can have one to play with. I'm doing it t= o support your efforts wrt bufferbloat.=20 Jim On May 1, 2012, at 8:40 PM, Dave Taht wrote: > I'm a little confused. >=20 > There are two threads going by today. One is about a sirfIII and the > other a ublox 6 >=20 > I have been assuming they are one and the same, but perhaps I'm confused. >=20 > 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.... >=20 > Thirdly, wow.... when can I get one of these puppies to play with? >=20 > :me looks out his bay window wistfully: >=20 >=20 > On Tue, May 1, 2012 at 3:14 PM, Gary E. Miller wrote: >> Yo All! >>=20 >> 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. >>=20 >> 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. >>=20 >> A preliminary result from a dual core laptop. >>=20 >> # ntpq -p >> remote refid st t when poll reach delay offset >> jitter >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D >> -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 >>=20 >>=20 >> 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. >>=20 >> 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. >>=20 >> SHM(0) is NMEA time over USB. SHM(1) is PPS time over USB. >>=20 >> 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. >>=20 >> 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. >>=20 >> The important part of the ntp.conf: >>=20 >> # for gpsd >> server 127.127.28.0 minpoll 4 maxpoll 4 >> fudge 127.127.28.0 time1 0.142 refid GPS >>=20 >> # for PPS and gpsd >> server 127.127.28.1 prefer minpoll 4 maxpoll 4 >> fudge 127.127.28.1 time1 0.001500 refid GPS1 >>=20 >> I start the daemons this way: >>=20 >> ntpd -p /var/run/ntpd.pid -N -u ntp:ntp -g >> gpsd -n /dev/ttyUSB0 >>=20 >> Any questions or suggestions? >>=20 >> RGDS >> GARY >> -------------------------------------------------------------------------= -- >> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701 >> gem@rellim.com Tel:+1(541)382-8588 >=20 >=20 >=20 > --=20 > Dave T=C3=A4ht > SKYPE: davetaht > US Tel: 1-239-829-5608 > http://www.bufferbloat.net >=20