SPOT
A satellite transceiver that allows family and friends to keep track of your position via internet mapping programs, assuming you transmit occasionally.
Also transmits an emergency signal for search/rescue. Again, assuming you (can) push the transmit button.
I believe there is a subscription fee for the service.
Don't hit the wrong button. The Coast Guard and other rescue services charge mightily for responding to false rescue requests.
Again the spot ou can find info here www.findmespot.com, I have one and have not used it yet but it gives me peace of mind. You can use it to let you friends follow your travels on a web site, not just for emergencies. If you buy one make sure you sign up for the insurance, It is about $8 a year and if you need to call for help everything is paid for, helicopters manpower everything. hope this helps.