Not only that a PLB let you send a distress signal to low orbit network of satellite, it sends its 406MHz signal to three types of networks: low earth orbit (LEO) , geosynchronous equatorial orbit (GEO) and medium earth orbit (MEO), as well as 121.5MHz signal which is monitored by private aircrafts and can help to home SAR responders.
Garmin InReach on the other hand uses Iridum (LEO only as far as I know) which is used to report to IERCC (a private emergency response coordinator).
The reason you might want GEO and MEO are:
1. While LEO (unlike GEO) cover the poles as well due to their North-South orbit, they have 60-90 min orbit period, which means more latency, where as GEO covers 70% of earth at all time.
2. Even if there multiple overlapping LEO around you which shorten the orbit period and hence the max latency, you might have limited view of the sky (especially if you happen to be in a North-South canyon) in which case it's good to have an increased chance of delivering that distress signal.
LEO satellite strength, as far as I know is in their ability to receive signal even in severe weather.
The first downside of LEO applies only to COSPAS-SARSAT, but not e.g. Iridium.
Iridium has inter-satellite links and the satellites accordingly don‘t need to store a message until they are in view of a ground station again – message delivery is effectively instant, globally.
Unlike PLBs, Iridium devices require a paid subscription, though.
It is my understanding that inter-satellite links makes communication effectively instant, not ground to satellite delivery in challenging environment, such as deep canyons (especially north to south ones) with narrow view of the sky. GEO might fail under these conditions too, but when it suceeds, delivery is immediate.
Moreover, GPS can be flaky under such conditions too, in which case MEO satellites might be able to provide fast Doppler detection to approximate the device location to few miles area and responders can further reduce the search area.
On the other hand, satellite tracker (such as Garmin InReach) advantage would be that it can be configured to try and send its location every 10 minutes, therefore, even if at the current location there's no signal, your last tracked location might be not far away.
Yes they do. These are 406 MHz Emergency Distress Signal devices: "Distress alerts transmitted from ELTs, EPIRBs, SSAS, and PLBs are received and processed by the International Cospas-Sarsat Programme, the international satellite system for search and rescue (SAR). These beacons transmit a 406 MHz distress signal every 50 seconds, varying over a span of 2.5 seconds to avoid multiple beacons always transmitting at the same time." [0]
It doesn't let you send a message but it does communicate (one-way only) by satellite. They're the hiker version of the emergency locator transmitters on airplanes.
