"If you're lost or injured in the wilderness, there's a good chance that there won't be cellular coverage where you are – that means you can't just phone for help. Your smartphone may still get you found, however, thanks to a new app designed at Spain's Universidad de Alicante.

When the app is activated by the user, it periodically emits a Wi-Fi signal that acts as a distress beacon which can be detected over a distance of several kilometers. The signal contains information such as the user's GPS coordinates, along with a text message such as 'I am injured' or 'I am disoriented.'

To pick up the signal, rescue crews will require a small antenna-equipped receptor device that connects to a smartphone of their own."

https://newatlas.com/smartphone-wifi-beacon-lost-wilderness/52765/