Campers: just ran across this thread and thought I'd add to it. It's something I've done quite a bit of research on over the last couple of years and like to think I might be helpful.
First understand that there's some critical differences between a PLB and what SAR nerds are beginning to call SENDs (Satellite Emergency Notification Device -- of course!). The latter include InReach, GeoPro, ShoutNano. PLBs include ELTs (Emergency Locator Transmitter for aircraft; PLB is, of course, Personal Locator Beacon, designed to be carried by a person; and EPERBs Emergency Position Indicating Radio Beacon which are carried on boats).
PLBs do not continuously track you. You activate the emergency button and it sends out a signal to any of a number of orbiting satellites. The signal is then passed to an earth station, the data recorded and sent, in the US, to the Air Force Rescue Coordination Center (AFRCC -- are you guys getting these acronyms down? There's a test later). The AFRCC then analyzes the data they get. They do this by analyzing the doppler shift of passing satellites to the stationary signal from the PLB. From that, they come up with several "solutions" and highlight the most likely based on magic algorhythms. Newer PLBs have a GPS built into them and the coordinates are sent with the signal. AFRCC passes this information on to, first, the state office of emergency services responsible for the area the signal is thought to have been activated in, THEN to the agency responsible for responding -- County Sheriff or NPS. This takes a number of hours.
The bad news is they can be seriously inaccurate. A plot of solutions to each ping can be scattered over miles of terrain. As noted, AFRCC provides a set of coordinates of the likeliest location but they usually don't pass on how that's derived. Most SAR responders aren't aware of the errors inherent in the solutions and don't know to ask questions that could help make a better guess as to the actual location. For various reasons, even if a GPS is part of the data stream, it might not be helpful, in part because the total data that can be sent is limited by the decades old architecture inherent in the system. Finally, many PLBs emit a signal at 121.5 megahertz. The idea is responding SAR can use an RF receiver to home in on that signal as they get close. Unfortunately, many SAR teams don't have that capability.
PLBs can work but because of serious errors inherent in the system, I don't really recommend them.
SENDs, though, are pretty accurate and depend on one of two satellite systems. They can continuously track you but ONLY if you set them up to do so. With permission (password access) the InReach, NanoShout and GeoPro can be activated by an Administrator but, of course, only if the unit is on. I'm not sure even the manufacturer is able to do that independently.
SPOT uses Globalstar satellites; the others use Iridium network. The latter may be a little more reliable and are capable of two way communication (e.g text) where the former (Globalstar) only seems to be able to handle outbound from the signalling device. When a location for any of these gizmos is actually acquired and sent, I'm not aware of any errors in finding the person. The problem is often in being able to both acquire a GPS fix then send when there's a satellite passing within the arc of sky available for the device. GPS and Iridium are two different satellite systems. I've seen no formal tests -- just what users have reported. From that anecdotal information, it appears the Iridium network (high earth orbit) seems more reliable for sending and receiving.
For emergency activation, SPOT goes only to SPOT International Emergency Response Coordination Center (yep, IERC). The other devices can all be configured to go to a call out list of friends or whoever. InReach can be configured to to to IERC for a small fee. Of course, you can also give friends limited web access for tracking or sending messages or location.
Finally, although all these gizmos have gotten a bad rap (rep??) in the press, my experience has been that they've been mostly used responsibly -- a few annoying exceptions but when you're in trouble, it's really great that responding SAR can find you right away. Fewer resources, less danger to responders.
There is a problem where you tell Mom you'll send a "OK" signal when you get into camp at the end of each day. Often, especially with SPOT, that signal doesn't connect and Mom immediately assumes you've been eaten by a borg and calls the NPS who, wrongly I think, might send someone that way. The key is telling mom -- or whoever -- that it's technology and you might go off radar for a few days. Don't worry... .
OK. Blah, blah .... . All things considered -- cost, reliability, two way messaging -- I'd recommend one of the InReach models but SPOT is pretty widely used and also OK. The other SENDs are designed for agency use. And, as above, I'd be really cautious about using a PLB.