An Emergency Beacon is a device that is capable of sending a distress signal over satellites. This signal includes a GPS signal that the device picked up and some kind of identifier. It's useful to have when you are out in the wilderness. Not to be confused with avalanche beacons, which are low range (up to 40m) and have neither GPS nor other satellite communication.
There is a confusing range of options. This page explains some of the different kinds of emergency beacons. If you are just looking to borrow an emergency beacon for a week, you can get a SPOT from the club.
SPOT and DeLorme are two companies that over a variety of emergency beacons. In addition to being able to call for help in an emergency, they feature tracking modes and communication (sending messages to pre-defined recipients). Some of them can even be used for navigation. The simpler of the devices simply feature a small number of buttons that send (in part customizable) messages, such as "I am OK", "I need some kind of help, but not search and rescue" and "call search and rescue". See SPOT for more details on how to use those.
On a technical side, SPOT and DeLorme use commercial satellites, which are not free to use. Even if you are just intending to use your SPOT as an emergency beacon (and not for your friends to track where you are), you will still have to pay monthly a subscription, which starts at about $10.
SPOT uses the Globalstar satellite constellation, which offers coverage in almost all of North America and most parts of the world, but most noticeably not at the poles . DeLorme uses the Iridium satellite constellation, which offers true global coverage. Iridium satellites, unlike Globalstar, have inter-satellite links and are able to forward messages from the satellite above your head to a satellite connected to an Iridium ground station. With Globalstar, you only have connectivity when the satellite above your head has direct connectivity to a Globalstar ground station. For this reason, depending on your location DeLorme may be more reliable in getting your message through quickly (or at all).
EPIRB / PLB
Besides the commercial infrastructure that SPOT and DeLorme use, there is a satellite network called Cospas-Sarsat, which is explicitly for Search and Rescue. It seems many products are targeted at aircrafts and ships, but some of them, the Personal Locator Beacon (PLB) come in a backcountry friendly size -- some weigh only 160g including batteries. They are not meant to let your loved one know that you are doing great, or that you'll be late for dinner, but to contact Search and Rescue in an emergency.
Since these devices don't use commercial satellites, you don't need a monthly subscription. You will need to send in the device after 5-7 years though, to replace the battery (at a charge). Why don't they just simply utilize AAA batteries? I am not sure. Probably because they were primarily designed for maritime users where you don't want to risk the seal to break and your device to fail.
There has been some discussion on the VOC message board which of the solutions is better -- a SPOT-like device or a PLB. The consensus seems to be that PLB can be a bit more reliable, but that the ability to send custom messages with a SPOT is useful in some cases (e.g. send a message "We are unable to exit on our own, but not in a life threatening situation.", such that SAR can wait for better weather to rescue you).
|Advantages SPOT-like device||Advantages PLB device|