[time-nuts] Updating Garmin GPS 25 LP after GPS week rollover

Dave Martindale dave.martindale at gmail.com
Tue Jan 3 20:29:31 EST 2017

Recently, I was invited to an event to celebrate the addition of a leap
I thought I would provide the entertainment by bringing a suitable GPS
plus a laptop running Lady Heather.  I had done this before, so I thought it
would be easy, but a whole collection of things went wrong, and I never did
everything working in time for the leap second.  (A story for another day).

Since Lady Heather 5.00 was out, I thought I would try using the new
Since LH now supports NMEA GPS receivers, I decided to dig out my old GPS 25
LP.  Though not a timing receiver (no fixed location mode), it does have a 1
PPS output and understands leap seconds, so it should be good enough for the
intended demonstration.

After editing heather.cfg for the serial port, bit rate, and receiver type
didn't try using autodetect), LH started up and began displaying data.  One
the things that was obviously wrong was the displayed date: sometime in May
2027.  A quick look at the raw NMEA data (any terminal emulator will do, one
nice thing about NMEA data) showed that the GPS 25 itself thought the date
in May 1997, exactly 1024 weeks in the past.  The NMEA output has only a
2-character field for the year, which was thus "97".  And Lady Heather
interprets "97" as 2097, not 1997.  That's why the date was wrong by 100
minus 1024 weeks (about 80.4 years).

So yesterday I looked for information about how to update a GPS 25 LP after
GPS week rollover.  I didn't find anything on Garmin's web site, but I found
this site instead:

Apparently BlackBoxCamera builds video overlay displays, and once upon a
they included a GPS 35 as part of their product.  (The 35 is basically a 25
a "puck" package).  Users reported seeing wrong dates in 2014, and they
obtained the update procedure from Garmin.  It depends on a Garmin
configuration utility program which is still on their website at
http://www8.garmin.com/support/download_details.jsp?id=925.  So you will
that (and a PC to run it on)

The update procedure is still on the blackboxcamera web page for now, but
knows how long any web page will remain accessible.  So I have posted the
directions here, to get them archived as part of time-nuts.

   1. Run SNSRCFG (or SNSRXCFG), choose the appropriate model of GPS
   receiver and click OK
   2. Use Comm/Setup, choose the correct com port, choose Auto for baud
   rate and click OK
   3. Use Config/Switch to NMEA mode and click through the pop-ups
   4. Use Comm/Connect (must be successful in order to continue, but a
   connection at a baud rate of 38400 or higher indicates a wiring error)
   5. Use Config/Get Configuration from GPS ( This step is critical. If
   missed the receiver may no longer send the correct data to the video
   overlay )
   6. Use Config/Sensor Configuration, make sure that Phase Output Data is
   not checked (this is called Garmin Binary Output when running SNSRXCFG),
   make any other desired changes and click OK
   7. Use Config/NMEA Sentence Selections, select the desired sentences and
   click OK ( Skip this step if only the date is being reset )
   8. Use Config/Send Configuration to GPS
   9. Use Config/Sensor Configuration and click the Reset NonVol or Erase
   NonVol button, whichever of these is present on the version of
   configuration software you are running
   10. Cycle power to the GPS receiver, re-perform steps 1 through 4 and
   then proceed with step 11
   11. Use View/NMEA Transmitted Sentences and see what date is coming from
   the GPS receiver. If it is still 1024 weeks behind, then follow steps 12
   through 16. (Otherwise, you're done.)
   12. Set your computer's clock ahead 9 years
   13. Use Config/Get Configuration from GPS
   14. Use Config/Send Configuration to GPS
   15. Repeat steps 12 through 14 two more times
   16. Cycle power to the GPS receiver, set your computer's clock to the
   correct date and time, re-perform steps 1 through 4 and proceed with step 17
   17. Use Config/Get Configuration from GP
   18. Use Config/Send Configuration to GPS

My own experience with these instructions is that at step 11 the date had
been updated, so I performed steps 12-15 a total of 3 times as described.
the date was correct.  The update process likely flushes the almanac,
it took longer than normal to acquire a first fix after the update (but
within 5 minutes or so).

The SNSRCFG software seems to be more recent than the GPS 25 LP.  The 25 LP
manual says nothing about SNSRCFG, but some later Garmin board manuals do
describe the config software and how to use it. (e.g. the Garmin 18X
The software seems to support the Garmin 10, 10x, 15/15U/15H/15L, 16/17,
17N, 18PC/LVC, 18(5Hz), and 25/36/36.  So the software and procedures above
be able to correct week rollover for any of these GPS receivers.

But it definitely did work for my GPS 25 LP, and now LH displays the
correct date.

- Dave

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