+1 on SPOT. Other models are available as well, search for "Personal Locator Beacon" GC.
While I will most definitely agree with T-Bone on the 2 levels of communication on cell phones, everything about them relies on communication with tower(s), there's no satellite involved. Even if you can communicate, the ability to track you through cell phone is based on triangulation of your signal on multiple towers. The fewer towers you reach, the less apt the odds of them being able to pinpoint your location accurately enough to find you.
If you're hitting a single tower, they can narrow that to a general direction (+/-10 degrees or so) and guess at your range based on signal strength. Signal strength can however vary greatly dependent upon your phone itself, battery strength, overhead foilage, in/out of vehicle/structure, weather conditions, any number of atmospheric anomolies, all sorts of things... A 1 tower search is pretty much hopeless from what they've suggested on the topic up here in ND, in talks on lost motorists searches in blizzards. They really need 3 to pinpoint you, and even then, dependent upon conditions, it can leave a fairly large search area.