Last July, two hikers were on a backpacking trip in California’s Shasta-Trinity National Forest. Just northeast of Granite Lake—a small body of water edged by deadfall and a rocky mountainside—one of them fell and was too badly hurt to continue.From their supplies, they pulled out a personal locator beacon. They extended the device’s antenna and pressed the button beneath. Immediately, a radio signal began beaming out at 406 megahertz, eventually hitting detectors on orbiting satellites. These instruments, part of the National Oceanic and Atmospheric Administration’s Search and Rescue Satellite-Aided Tracking program (Sarsat), picked up the signal and immediately pinged alerts to Earth.Someone’s in trouble near Covington Mill, California, the alerts told the Air Force Rescue Coordination Center, along with details about who owned the device and how to get in touch with them. Soon, a helicopter was en route to the distressed trekkers’ latitude and longitude. After hoisting both hikers, the aircraft flew them to the hospital.As far as wilderness distress calls go, that was not only a happy ending, but an easy one. (This incident, along with thousands of others, lives in the Sarsat program’s Incident History Database.) Locating the hikers required no scouring of trailhead sign-in sheets, nor deciphering notes taped to cars left at the starting spot. That’s by design: Sarsat’s catchphrase is “to take the ‘search’ out of search and rescue.” Sarsat is a little-known US program that aims to save lost or hurt hikers and climbers, overturned ATV and snowmobile drivers, sailors aboard sinking ships, and passengers in crashed planes. It is part of an international collaboration called Cospas-Sarsat, involving 45 countries and two independent organizations. The system relies on simple devices that have one job—send a location-revealing distress signal, anywhere, in any weather—and a system of satellites that listen for those calls. “If you really need your life saved, this is, in my view, going to be the one that is there for you,” says Sarsat ground systems engineer Jesse Reich.As of 2022, NOAA’s database has more than 723,000 registered rescue devices, mostly owned by those who hope they’ll never have to use them. There are, though, more than 50,000 people worldwide who have been rescued because they activated their 406 beacons, sending an SOS signal to space.SARSAT began after an incident that could have benefited from its technology: In 1972, two members of Congress, Hale Boggs and Nick Begich, were flying in a twin-engine Cessna 310 across Alaska. Their plane disappeared in a remote region in ill-tempered weather. A 325,000-square-mile search that took 39 days and 90 aircraft found nothing. The search was called off, and the politicians and their plane remain missing to this day.Afterward, Congress declared that aircraft had to carry emergency beacons that would automatically broadcast in the event of a crash. But the plan had a technological limitation: Another aircraft would have to be flying nearby to pick up the call. NASA, perhaps unsurprisingly, realized that satellites would have a much wider view and could also survey the vast swaths of the planet that are, in fact, ocean. A group of space agency scientists researched what was possible, and by 1979 the US, Canada, France, and the former Soviet Union had signed papers in Leningrad. The international collaboration, which would later be made more official as Cospas-Sarsat, launched its first satellite in June 1982.