[Thumbgps-devel] Fwd: Long term SiRF data

Sat Mar 17 10:58:56 EDT 2012

PS to my prior message.  The following should reset the GPS's NMEA 
offset from UTC to it's initial point.  I don't know how varying 
satellite view affects this.  Shut down NTP, unplug the GPS for 30 
seconds, replug the GPS, wait 30 seconds, restart NTP.

Sincerely,

Ron

> Hi all,
>
> (I'd like to cross post this message, including the original message 
> below to the NTP questions list since we've been discussing this nmea 
> wandering effect.  Let me know if there are any objections.)
> (Please forward this to the original sender of the first message if 
> needed.)
>
> 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.
>
> http://dl.dropbox.com/u/9879631/drifting01-peerstats.20120312.jpg
>
> 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:
>
> http://www.usglobalsat.com/s-122-bu-353-support.aspx
>
> However, the program we need is not.  To program the unit, you need 
> SirfDemo.
>
> First check out the FAQ here:
>
> http://www.usglobalsat.com/store/gpsfacts/bu353_gps_facts.html
>
> And you can find a link to SirfDemo here:
>
> http://www.usglobalsat.com/downloads/setupSiRFDemoV387.zip
>
> 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.
>
> Sincerely,
>
> Ron
>
>
>> 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 
>> now.
>>
>> 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 sentences.
>> 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 
>> guess is
>> that it learned about the latest leap second or something like that.
>>
>> At about -2.82 hours, the fractional part of the report switched to 
>> 0.  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 
>> hours.
>>   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 second.
>>
>> 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 
>> second.
>> The blue and purple are the number of satellites.  (They are scaled 
>> up by
>> 100.)  I don't see any pattern.
>>
>> This unit doesn't always return 000 for the fraction part of the time 
>> stamp.
>> 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 concerned.
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Ron Frazier
timekeepingdude AT c3energy.com