Emergency beacons transmitting an SOS signal along with GPS coordinates became available in 1998. Initially, these emergency beacons transmitted on two frequencies: the more powerful 406 MHz signal, detectable anywhere in the world, and at a less powerful 121.5 MHz. The 121.5 MHz signal still offers some usefulness because rescue aircraft are often equipped with 121.5 MHz receivers, allowing them to home in on this signal once near the rescue location. Some devices today continue to use both frequencies, but a 121.5 MHz signal alone won’t be acted on by search and rescue unless it’s corroborated by two independent non satellite sources. Devices transmitted on 121.5 MHz only are forbidden to be sold or used in the United States.