"SPOT" locators

I have one, use it on hunting and multiday river trips. Ive never used it for an emergency, but its nice to send "all good" messages to loved ones, inform shuttle services of delays, and your friends can track your progress from your spot website.

I dont think they run on cell service, as that is the whole point, pretty sure its a service similar to sat phones.

There are a few other similar devices, but most require a costly service plan.

SPOT locaters run off of satellites

never had to call for search and rescue with one, but they get service way deep in the wilderness
I think that the spot is not GPS capable which means you can be tracked to within a little over a mile of your exact location. pricier beacons like this one

http://www.rei.com/product/843146/acr-electronics-resqlink-406-gps-personal-locator-beacon

can track you to about 100 meters