How to Switch Your Sirf-based GPS from Sirf to NMEA Protocol

This post applies only to the Juno ST, and ONLY TO THE ONES I AM WORKING WITH!  I can’t make any guarantees about it’s effectiveness for anyone else, but the steps outlined have resolved serious GPS communication bugs, for the units I am working with.

I’ve been having a hell of a time getting ArcPad to communicate properly with my Trimble Juno ST units since version 8.  I think I have finally figured out what the problem is, though.  It seems that, when you run Trimble’s GPSCorrect software on a Trimble Juno ST (and I suspect any Trimble unit with a Sirf chip), the software sets a switch on the Sirf chip to output Sirf Protocol, only.

My problems started when I did a clean reset of Windows Mobile to update from ArcPad 7 to ArcPad 8.  At that point, our version of GPSCorrect was not supported by ArcPad 8, so I didn’t reinstall it, thinking I would just use the NMEA Output for GPS communication.  This is where the little hardware switch issue I noted above started spitting Gremlins into the Junos.  It seems the GPSCorrect software left the Sirf Chip outputing Sirf Protocol data streams.

Now, you might think “no problem, I’ll just set ArcPad to use Sirf,” right?  Well, that’s what I thought, too.  WIth the Sirf chip outputting Sirf Protocol data, I had problems with GPS communication in ArcPad whether it was set to NMEA or Sirf.  These problems included indefinite “No Fix”; obtaining a fix, then “freezing” on the initial location and never updating; or just flat out crashing ArcPad.

So here is what I did to resolve the issue:

  1. Download and place SirfTech.exe on the Juno Units []
  2. Use File Explorer to browse to and launch SirfTech.exe
  3. Open the COM Menu and set the COM Port to 7
  4. Click on the Find Baud button and wait for the program to find the Baud rate that the Sirf Chip is passing data at. (On the Juno ST Units I have the problems with, this is always 38400)
  5. Once it finds the Baud Rate, you should see the Messages updating for the SiRF Protocol, but not NMEA. Click OK.
  6. On the Menu go to Sirf>Switch to NMEA Protocol.
  7. Set the Baud Rate to 4800 and click on the Set button.  Click OK.
  8. Return to the COM page and Close the COM Port.
  9. Change the Baud Rate to 4800 and click Open to reopen the COM Port using NMEA.  You should see the Messages updating for the NMEA Protocol, now.
  10. Finally, Close the port, click OK and Exit the SirfTech program

You should now be able toSet the GPS Preferences in ArcMap to NMEA 4800 and successfully Fix and Update your GPS Position.