In this July 2018 tweet by Cdr. Scott Kelley, with the north face of Mt. Everest over his shoulder, he mentions his "Emergency" watch.
I've searched and then found the Hodinkee.com article Hands-On: The Breitling Emergency, Or The Safest We’ve Ever Been With A Watch.
Last week, we welcomed astronaut Scott Kelly back to Planet Earth following his 342-day mission in space by taking a look at some of his watches – both in flight and on Earth. And many of you expressed some interest in what appeared to be a larger-than-life 51 mm wristwatch. It is in fact very real – developed specifically for the most perilous missions. Today, we go hands-on with the Breitling Emergency (not Kelly’s mind you).
That link in the block quote is also worth checking out.
Reading further in that article, it says
A follow up to the 1995 Emergency, this version (originally introduced as the Emergency II, although the "II" has been dropped) the watch presents an improved personal locator beacon (PLB) capable of transmitting its coordinates on two separate frequencies. The decision to create a dual-frequency version of the original Emergency is a result of the decision, by emergency beacon monitoring agency Cospas-Sarsat, to phase out its satellite monitoring at the long-standard 121.5 MHz aviation emergency frequency – which was also used by the first Breitling Emergency watch. (Cospas-Sarsat is a satellite-based search and rescue system, which has been around since 1982; Cospas is an acronym for "Space System for the Search of Vessels in Distress" in Russian, and Sarsat stands for "Search and Rescue Satellite-Aided Tracking.")
The latest beacon now carries a digital signal on the 406 MHz frequency via Cospas-Sarsat's worldwide satellite system, on top of an analog signal on the 121.5 MHz frequency, used by search and rescue (SAR) teams to home in on victims at land, air or sea. Cospas-Sarsat originally monitored both frequencies. The FAA in the United States began requiring 121.5 MHz emergency beacons on civilian aircraft in 1973, and though the frequency is still monitored (which is why the Emergency transmits on both) 406 MHz is both longer range and gives better accuracy.
To pick up signals from a watch, I would suspect that you'd need to be in LEO, and fairly close-by. The problem there is that it takes quite a large constellation of satellites to provide continuous coverage of all points on Earth.
Question: So I'd like to ask which satellites can hear emergency signals from Scott Kelley's watch? How many of them are there, what's the altitude, and how quickly will they detect a signal from any random spot on Earth and then route the message somewhere where action can be taken?