I'm not affiliated with this; I'm just a happy customer.
A guy on another forum I frequent put together a group buy on the SPOT Satellite Messenger. The units are normally $150 at REI, etc but he's selling them for $15 less than that which INCLUDES SHIPPING. He's also worked out a deal on the tracking option to get it for 50% off for the first year.
These run on satellite technology rather than cellphone technology so I've been told that it increases the likelihood that messages get through. They have three types of messages that can be sent;
1. 911 This will get an emergency response from police, ambulances, etc.
2. Help This sends a predetermined message to your designated person. (call RV Roadhelp!!, etc)
3. OK This just sends a message showing where you are.
There is an optional service (the discounted one) where you are tracked every 10 minutes during a ride.
Enough of my rambling... www.mylivetracks.com/
is his website.