GPS to GPS communication

My normal caveat #1: this may already exist. The idea involves taking a regular GPS unit but adding a 2-way radio that (among other things) allows the GPS to communicate with other units. This means your GPS can relay its route to other GPS units so that your friends can see exactly where you are and where you are going. For example, if you're backpacking and part of your group falls behind, they can still see where you went and follow your exact route.