Satellite Broadcasting

Satellite broadcasting would be one the most effective method of ensuring that the alert can be sent to all of the devices, to matter where they are located. Satellite broadcasting would allow us to transmit the alert to all of our devices in a specific region, which we can predetermine based on the danger area and danger zone of each volcano.

It works by transmitting signals from ground based stations to a satellite orbiting earth and the satellite can retransmit the signals back to the designated devices on the ground, useful for areas with poor or no communication infrastructure.

Although in theory satellite broadcasting could be done and would work well but it would require an antenna, so the design of the pendant would need to account for the small antenna and still be durable for this to work

References:

https://www.techopedia.com/definition/30438/satellite-broadcasting

https://www.sciencedirect.com/topics/social-sciences/satellite-broadcasting

https://www.sciencelearn.org.nz/resources/270-satellite-communications

https://www.rantecantennas.com/blog/everything-you-need-to-know-about-antennas-satellite-communications/#:~:text=Antennas%2C%20along%20with%20transponders%2C%20are,from%20various%20sources%20on%20Earth.