The primary objective of this system would be to allow authorities or officials to broadcast a text message to the public, even in the absence of electricity, radio transmitters, or cell phone towers. It's not designed for a member of the public to send a message to call for help, although I have some ideas about how you could implement that in a future phase.Authentication: you would probably want to use some form of public key cryptography, so that handsets could verify that the text message package was officially authenticated, and that eavesdroppers couldn't learn enough to spoof an official message. My limited understanding of PKC says that this is possible.Here's how a message would be sent. In an emergency, an official would use a specially-programmed handset (or one into which an authentication code could be entered) and would author and send a special emergency message. The message would be broadcast on the special "listen" frequency that cell phones use to receive messages from a cell network tower. The broadcast would be repeated something like every minute for 10 minutes (network engineers can figure out what a good rate would be).Ordinary handsets would be listening in to the tower frequency even if they aren't in range of a working tower. (I'm not sure how exactly handsets operate when towers are down or out of range, but presumably they are least hunting at regular intervals. Perhaps they would have to be programmed to be listening constantly for a special message, which might cause more battery drain; I'm not sure.) When the handset receives and authenticates the special text message, it displays it and then rebroadcasts it every minute for 10 minutes.You would probably use message IDs and a rate limiter to avoid overwhelming transmission or unnecessarily repeating a message.The net result is that if your handset is within a few miles of another handset, it should receive the emergency message within minutes, even if there is no power, no cell phone towers, etc.
frank smith on 18 Mar 10someone of course is discussing this