I was out in the middle of the desert this weekend and thought of an idea for Search and Rescue in remote areas without cell service.
A small mobile cell phone site emulator that could be used in a helicopter, plane or drone flown over a search area. It doesn't need to connect to be cell phone network. It just needs to emulate a cell site for the person or people that they're trying to locate. If the lost party connects to the site, communication is established, the search team running the emulator will know it and the technical possibilities are endless from there. Text, phone, gps, etc.
It could be simple, cheap and small. It doesn't need to be a real cell site.