[ntp:questions] Fwd: Long term SiRF data / NMEA Wandering

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Sat Mar 17 21:31:54 UTC 2012


Cross posting to the thumbgps-devel list.

Ron


On 3/17/2012 3:45 PM, Charles Elliott wrote:
> 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
> need it.
>
> Charles Elliott
>
>    
>> -----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
>> bottom).
>>    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
>> having.
>>
>> Sincerely,
>>
>> Ron
>>
>> -----------------
>>
>> 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.
>>
>> 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.
>>>
>>>
>>> questions mailing list
>>> questions at lists.ntp.org
>>> http://lists.ntp.org/listinfo/questions
>>>        

-- 

(PS - If you email me and don't get a quick response, don't be concerned.
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 again.)

Ron Frazier
timekeepingdude AT c3energy.com



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