I owe my life to an EPIRB (Emergency position indicating Radio Beacon ) I'm not even sure if the name is still used for the Marine version of the devices. Technology has moved on so far since 2007 when I triggered the Epirb which brought about my rescue years ago. Just out of interest, the Marine versions are monitored by HM Coastguard Station at Falmouth in Cornwall. It's the Maritime Rescue Coordination Centre (MRCC) up at Pendennis Point.
Any Epirb triggered world wide would transmit a distress signal peculiar to that device only, which enabled the device to be identified. On purchase one registered the device with HM Coastguard with personal details and your vessels description and Port of registration etc:
Orbiting Satellites receive the distress signal and a location fix is obtained by triangulation, the position transmitted to Falmouth who then contact the nearest Rescue Centre to the distress signal location. In my case the Epirb was triggered by me on a reef in the Bahamas and 40 minutes later a US Coastguard Helicopter was airbourne from San Juan. Puerto Rico. Within 2 hours I was battered and half drowned but safe in the Turks and Caicos Islands which was the nearest British Protectorate and also a refuelling point for the big Helicopter, which was at the limit of it's operating range under those weather conditions. That is roughly how the maritime system used to work, anyway...I suspect the land system could be similar but may involve the land telephone network stations??
I was 1 year plus into a 2 year voyage which started in England took me to the far Eastern borders of Greece, back to Portugal then the Canary Islands and across the Atlantic to the Caribbean where the involuntary swim took place. In my case the system worked as designed. I had registered the device properly with mine and descriptive details of my boat. 6.5 tons, Cutter rig, blue hull, length, beam, bright red wind steering vane etc: etc: AND a brief itinerary of my voyage. April depart from Falmouth, May on Portugese Algarve, July Balaeric Islands area, winter in Kalamata in Greece and so on...When the Epirb triggered and Falmouth looked on their computer screen and saw my Epirb and my submitted itinerary they knew it was not one of literally thousands of false alarms and were able to react quickly without investigating if this was yet another idiot ....."I wonder what happens if I press this red button ?"
Enough waffle of days gone by......my opinion on buying one is if a realistic chance of needing a location device can be justified, then get the best available and be prepared to give enough personal information ( many people will not or fail to do so) to assist the Emergency Services in any rescue. Keep the information updated, if you are in one area almost permanently it doesn't matter, still update periodically, but if moving a significant distance...ie: living in England but going trekking in Iceland, update the information. Rescue operations will be started much quicker if the information on the computers tallies with the location of the distress signal origin.