APRS Automatic Packet Reporting System. Must have a HAM license to broadcast the GPS signal along with your identifier (call sign). All of us who wheel in the back country should have that anyways. If you don't have a HAM license they are easy to get. Classes are often offered by amature radio clubs for free. Here is the link to the micro tracker that I bought.
http://www.byonics.com/mt-rtgOnce you have the equipment there are no service charges and signal reception is likely better than cell phone coverage. We are going to start using this in search and rescue so we can see where all our units are.
