Years ago, NASA put together an educational program for public television called the NASA SciFiles. It did not earn much recognition and only lasted a few seasons, but one episode in particular caught my eye...it's called The Case of the Technical Knockout. It is all about geocaching and how GPS works and it's pretty good. This could be a nice introduction to use with your students before you launch into GPS based lessons. You can follow the link to download an hour long video (which you can show in segments) and the great educator's guide shown below!