[ntp:questions] Fwd: Long term SiRF data / NMEA Wandering
elliott.ch at verizon.net
Sat Mar 17 19:45:22 UTC 2012
I program the BU-353 with regular C++. I have a copy of the "SiRF NMEA
Reference Manual" (P/N: 1050-0042, Rev 2.2, 11/08) that I received from
SiRF Technology, Inc.
217 Devcon Drive
San Jose, CA 95112
PH: +1(408) 467-0410
support at SiRF.com.
It contains descriptions of all the NMEA messages SiRF supports (both input
and output) and many of the SiRF proprietary messages.
If SiRF is not forthcoming with another copy, I could try scanning it and
posting it on Windows Live. It is fairly long, so please let me know if you
> -----Original Message-----
> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
> Behalf Of Ron Frazier (NTP)
> Sent: Saturday, March 17, 2012 2:49 PM
> To: questions at lists.ntp.org
> Subject: [ntp:questions] Fwd: Long term SiRF data / NMEA Wandering
> Hi all,
> This is a cross post from the [Thumbgps-devel] list. It relates to
> some testing on some SIRF GPS's the original poster is doing (at the
> The part on top is my reply to him. I thought those here might like
> to see it, considering recent GPS related discussions we've been
> Hi all,
> I've been seeing similar wandering of the NMEA output on my BU-353.
> This graph shows what looks like the internet servers (colored lines)
> wandering off while my pc is locked to gps time (dark jaggy baseline).
> I suppose it's actually the GPS wandering off.
> Can someone please tell me, if known, why this happens? I've been
> discussing this a good bit on the NTP questions list. On my particular
> home network / internet connection, my offsets to internet servers with
> NTP running run about + / - 50 ms. I've decided to use the BU-353 GPS
> anyway, since in the short term, my offsets are + / - 6 ms or so, even
> if over days, my time varies + / - 70 ms from UTC. At least the
> variations are not every 15 minutes like they would be if I was polling
> the internet. I hope to shortly have a Sure Electronics GPS board and
> will be testing that. David Taylor, on the NTP questions list says the
> same NMEA wandering has been observed on the Garmin 18 ??. I'm not
> sure which model that was.
> Here's how to reprogram th BU-353. Lots of the support stuff is here:
> However, the program we need is not. To program the unit, you need
> First check out the FAQ here:
> And you can find a link to SirfDemo here:
> Unzip and install SirfDemo. Do the following to reprogram the BU-353.
> I assume other SirfIII units are similar. SirfDemo gives you access to
> a HUGE number of internal GPS functions, probably enough to really
> screw up the device if you're not careful. You can also do factory
> restarts, etc., from the menus.
> a) Shut down NTP, GPSD, or any other thing attached to the GPS virtual
> com port.
> b) Start SirfDemo.
> c) A Data Source window will pop up. Select the com port and data rate
> that the GPS is currently set to. If the baud rate is unknown, try
> 4800 then try to connect, then 9600, etc. If the com port is unknown,
> look in the Windows control panel, system, device manager under ports
> com and lpt to determine which com port is active.
> d) Under the view menu, turn on the Signal, Radar, Map, Messages
> Response, Messages Error, and Messages Debug windows if not on already.
> e) Click the 5th toolbar button, which is connect to data source.
> f) If the unit is outputting NMEA data, that should appear in the debug
> window. If it is outputting satellite data, you'll get that in the
> signal and radar windows.
> g) Under the Action menu, select switch to SIRF protocol. The NMEA
> data will stop and the response window will start outputting data.
> h) Under the Action menu, select switch to NMEA protocol.
> i) A parameter selection window will pop up. This allows the sentence
> output to be customized. Using the drop down boxes, put a 1 in every
> sentence you want to occur once per second. Put a 2 for once every 2
> seconds, etc. Put a 0 if you don't want the sentence to appear at all.
> You can click in the first number field, type a number, and tab to
> the rest if you like. I leave checksums turned on. Select your baud
> rate, then click send.
> j) The response view windows should stop updating and the debug view
> should start up again with NMEA sentences.
> k) Click the 5th toolbar button again which will disconnect you from
> the GPS.
> l) Close SirfDemo.
> m) You are now ready to resume using the GPS with NTP as normal.
> There are many many other options you can choose from the menu options
> of SirfDemo, including a factory reset, should you need it.
> Hope this helps.
> > I can't remember if I had shared this already.
> > ---------- Forwarded message ----------
> > From: Hal Murray<hmurray at megapathdsl.net>
> > Date: Tue, Oct 25, 2011 at 2:58 AM
> > Subject: Long term SiRF data
> > To: Eric Raymond<esr at thyrsus.com>
> > Cc: Hal Murray<hmurray at megapathdsl.net>, Dave Taht
> > <dave.taht at gmail.com>, Jim Getty<jg at freedesktop.org>, Gary Miller
> > <gem at rellim.com>
> > I've been collecting data from 2 SiRF units. I'm up to about 12 days
> > Quick summary: both suck.
> > Both are located inside my house, poor conditions.
> > The first is a Holux GR-213. It's setup to only send GPRMC
> > That's what I would use with ntpd.
> > Here is the startup:
> > http://www.megapathdsl.net/~hmurray/bb/gps/Holux-1.png
> > The green marks are "good" sentences. The Y offset is the difference
> > between the actual arrival time and the time stamp in the sentence.
> > The blue marks are the fraction part of the time stamp in the
> > sentence. The red marks are invalid sentences.
> > At about -2.94 (hours) the reported time jumped by 1 second. My
> > is that it learned about the latest leap second or something like
> > At about -2.82 hours, the fractional part of the report switched to
> > I have no idea what caused that. It doesn't really matter much. It
> > wasn't useful anyway.
> > Here is the big picture:
> > http://www.megapathdsl.net/~hmurray/bb/gps/Holux-2.png
> > There is a mode shift every 1-3 days. What's the right term?
> > For reference, here is an old graph with the mode shift every 8-12
> > http://www.megapathdsl.net/~hmurray/bb/gps/SiRF-GPRMC-4800.png
> > This is the previous 2 pictures glued together:
> > http://www.megapathdsl.net/~hmurray/bb/gps/Holux-3.png
> > Here is one day:
> > http://www.megapathdsl.net/~hmurray/bb/gps/Holux-4.png
> > -------------
> > The second unit is a Global Sat BU-353. It ignored my attempts to
> > change the configuration, so I let it run in the default setup.
> > Normally it sends GPGGA, GPGSA, and GPRMC. Every 5 seconds it
> > includes 3 GPGSV sentences before the GPRMC. I think that fits in 1
> > second at 4800 baud, but the GPRMC gets pushed over to the next
> > Here is the graph for the GPGGA sentences:
> > http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gpgga.png
> > The long term cycle time is 8-10 days.
> > Here is the graph for the GPRMC sentences:
> > http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gprmc.png
> > The top band of green is the data that gets pushed over to the next
> > The blue and purple are the number of satellites. (They are scaled
> > by
> > 100.) I don't see any pattern.
> > This unit doesn't always return 000 for the fraction part of the time
> > Sometimes it's 998 or 999 with the previous second.
> > http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gpgga-off.png
> > --
> > These are my opinions, not necessarily my employer's. I hate spam.
> (PS - If you email me and don't get a quick response, don't be
> I get about 300 emails per day from alternate energy mailing lists and
> such. I don't always see new messages very quickly. If you need a
> reply and have not heard from me in 1 - 2 weeks, send your message
> Ron Frazier
> timekeepingdude AT c3energy.com
> questions mailing list
> questions at lists.ntp.org
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