Why?The HP48 is a good platform for GPS (Global Positioning System) related programs, I think. It's small, sturdy, quite versatile, fun to program, and has excellent battery life and built-in mathematics capabilities. Plus there's the thrill (yes, yes, geekish I'll admit :-) of connecting your GPS to a calculator.
What's been doneI've written GxPS, which exchanges waypoints, routes, tracks, and other data with Garmin GPSs, and NMEA48, which is a stand-alone version of GxPS's NMEA support for those who don't need the Garmin protocol features. Both of these programs provide commands to return position, altitude, and time, plus other utilities to facilitate working with the Garmin and/or NMEA protocols. Hopefully these commands will make writing GPS-enabled programs easier on the 48.
Also, I've collected a few related links and a little information about building cables and cases, all of which you will find on these pages.
What still needs to be doneHere are some ideas people have suggested for GPS related programs. If you are working on something like this or would like to, please let me know!