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

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Sat Mar 17 18:49:28 UTC 2012

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.




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 
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 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.
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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