The 911 messages are sent out every 5 minutes, over and over until the batteries go dead, which should be days. If one 911 message doesn't get out, it's likely another will. Since the receiving satellites are moving all the time, even heavy tree coverage will have a gap at some point. So, unless you're in a cave, the SPOT will get the message out. If you turn tracking on, even if you can't press the button you have a chance of being rescued because there's a "bread crumb" trail leading directly to you. As long as someone back home checks in on your tracking once in a while, and knows when you should be home, you may have a better chance of being rescued with a SPOT that a PLB. What good is a PLB if you can't press the button?