Wednesday was the annual 406 Day, celebrating the life-saving importance of emergency beacons, named for the radio frequency they operate on, as well as the satellites that relay their signals - with Europe's own Galileo constellation prominent among them. While Galileo's main purpose is satellite navigation, the system also picks up distress messages from across the globe and relays them to regional search and rescue authorities.

Anytime you venture beyond mobile phone network coverage, such as hiking in wilderness, sailing out to sea or taking to the air, you are stepping into a riskier environment, out of easy contact with local emergency services. This is why 406 Day encourages you to take a flashlight-sized emergency beacon along. Run into trouble and the beacon transmits a 406 Mhz SOS signal that will be picked up automatically by participating satellites, then relayed to the international Cospas-Sarsat system to pass on to rescuers.

The only system that can independently locate a beacon anywhere on Earth, Cospas-Sarsat has saved thousands of people since it was founded in 1982. Originally its transponders were hosted aboard low-Earth orbit and geostationary satellites, with a significant extension to medium Earth orbit spearheaded by Galileo joining the system.

Because Galileo satellites have such a high orbital altitude, at 23 222 km up while still moving steadily through the sky, they provide wide views of Earth with the ability to quickly source a distress signal through a combination of time-based and Doppler ranging.

"Galileo is the biggest contributor to this satellite-based system, which assisted search and rescue teams to save more than 2200 people in 2020 alone," explains ESA Galileo Search and Rescue engineer Eric Bouton.

"Galileo has also introduced a new search and rescue function, the Return Link Service, informing beacon users that their SOS message has been received and help is on the way. In March 2021 the Service reached Full Operational Capability to enter routine use, becoming the first Galileo service to reach this milestone."

The performance of Galileo's Search and Rescue Service is monitored on an ongoing basis by the EU Agency for the Space Programme, EUSPA, Galileo's service provider. In December 2021 the service achieved its best ever 'quality of location service' - assessing the probability of success of finding any emergency location to an error better than 2 km and 10 minutes - to a figure of 98.12%.

The satellite transponders are supplemented by a trio of ground stations at the corners of Europe, known as Medium-Earth Orbit Local User Terminals (MEOLUTs), based in Norway's Spitsbergen Islands, Cyprus and Spain's Canary Islands and coordinated from a control centre in Toulouse, France, along with a fourth station on France's La Reunion Island in preparation, for Indian Ocean coverage.

"Last October EUSPA led a search and rescue exercise at the edge of European coverage area, in freezing waters off Svalbard in the Norwegian Arctic," adds Eric. "A couple of hundred people were rescued from a ship by Norwegian authorities. Upon beacon activation it took only 2 minutes 20 seconds for Galileo to track down the vessel to a precision of 730 m, guiding helicopters and a coast guard vessel to begin the practice evacuation."

Galileo's Search and Rescue service is operated by EUSPA, while ESA is responsible for its design, development and future evolution.

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