Lorsqu'une personne tombe à l'eau, comment éviter que tous les récepteurs dans un rayon de 5 km ne donnent l'alarme ?

This can be implemented by following the steps below:

Register a sender by scanning its QR code and sending the information to the web server.

During the process of registration, the user can associate a receiver or a group of receivers with the sender in the web application.

When the web server receives the registration for a new sender, the sender ID will be pushed out to the corresponding receiver(s).