Being a backcountry Luddite can be a wonderful thing. Steering clear of anything even remotely smacking of digital while plodding through the forest or the desert is a noble cause indeed. I wish I could do it, but I’ve fallen in with the GPS crowd. But besides GPS, there’s another piece of tech I’ll never embark on a distant multi-day backcountry trip without: my Personal Locator Beacon (PLB). It’s one of the rare bits of tech that you never actually get to use, hopefully, but I think it’s easily worth the expense (mine, the ACR ResQLink, sells for around $300).

Get into life-threatening trouble in the backcountry—break a leg miles and miles from help; get lost beyond all hope of reasonable rescue; fall into some kind of unrecoverable crevasse—you push the button and the helicopters come to get you. It’s as simple as that.

Simple idea, but an awesomely complicated process involving outer space and coordinated rescue efforts. Here are five things you probably didn’t know about PLBs:

1. PLBs are not the same thing as satellite messenger devices like Spot or DeLorme’s InReach. Satellite messengers allow you to send short digital notes back home: “I’m safe,” or “Put some beer in the fridge, I’m on the way home,” or “I got lost but I’m fine now, will be back a day later than I thought.” PLBs, on the other hand, have one function. Screaming “HELP” about as loud as a little piece of plastic and microchips can. They greatly outperform satellite messengers in terms of their ability to make the user heard, because…

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2. They use an intergovernmental satellite system called COSPAS-SARSAT. This is a military-run satellite network, not the commercial satellites used by Spot and InReach. Some serious space tech here. When you send for help, the PLB sends out a distress call at 406 MHz, a specialized frequency for just this purpose. Low Earth Orbit (LEO) satellites pick up your signal, use the Doppler Shift to figure out where you are, and relay your location to Search and Rescue. But LEO satellites are on a polar orbit, and there can be gaps in coverage. Enter the High Earth Orbit (GEO) satellites. These babies can see everything on earth, all the time. But they can’t use Doppler because they aren’t moving in relation to the earth. They will however pick up the GPS coordinates your PLB should also broadcast. Some PLBs don’t offer GPS—make sure you get one that does.

3. COSPAS-SARSAT started back in 1972 as an agreement between the U.S., the USSR, France, and Canada. COSPAS is a Russian language acronym for Space System for the Search of Vessels in Distress; SARSAT is an English acronym for Search and Rescue Satellite—not nearly as poetic. The whole thing began as a maritime distress signal program, was then expanded to airplanes, and back in 2003, was extended to help land-based people bumbling around in the wilderness.

4. If you press the help button on a PLB, the actual military is alerted. This brings great comfort to me. In the continental U.S., Langley Air Force Base in Virginia hears your cry for help. They then get the closest search and rescue personnel on the horn and they come get you. For free. Oh, but you don’t have to be in the U.S. Want to backpack the Alps? Siberia? Patagonia? The satellites see you ANYWHERE, and the COSPAS-SARSAT system will scramble the proper local authorities.

5. Since they came on board in the early 1970s, PLBs have saved more than 30,000 lives. Or at least came to the aid of people who were very seriously injured and for whom not making it out was a very likely possibility.

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