Also known as PLBs1), these devices communicate with satellites to give emergency-only communication anywhere in the world.2). They can typically be tracked from a website, which is nice for letting friends and family keep track of where you are, and have an “SOS” function that will alert a central monitoring center who will contact local authorities. More expensive models have the ability to send and receive 160-charecter text messages.
These require monthly subscriptions which can get very expensive, and due to the cost are primarily useful only for people spending extensive time in extremely remote locations.
They can roughly be broken down into One-Way or Two-Way messaging.
Common Models - SPOT Gen4, SPOT Trace
These are generally simplified “Come help me!” beacons. Activating them will send a signal out to the company who operates them, who will contact local authorities. More advanced models have the ability to send pre-programmed messages to pre-programmed phone numbers or e-mail addresses, useful for sending out “Camped here for the night” GPS-tagged messages to friends or family.
A major disadvantage of these is the lack of ability for authorities to verify if the emergency beacon being sent out is actually an emergency. The S&R industry estimates that that between 80-99% of activations of these devices are false alarms, either because the owners didn't understand how to use them or because they had a ...different... idea of what constituted an emergency.
While in a perfect world all emergency requests are taken as seriously as any other . . . the reality is that emergency services get “alert fatigue” when they get yet another call from the PLB company that will (probably) end up in a bunch of wasted effort.
Common Models - Garmin InReach
The major advantage of these is that they allow emergency centers to send messages back and forth to verify that it's an actual emergency. They also allow the sender to be more detailed about what their emergency is, which can greatly aid in rescue efforts. This makes it more likely that you'll get the appropriate help you need.
Additionally these devices allow for 160-charecter messaging (typically when paired via bluetooth to a smart phone) to and from any phone number capable of texting. While it's minimal, some communication is better than nothing when you're in extremely remote areas.