Beacons are radio transmitters that can be activated in a life threatening emergency to send coded messages that allow rescuers to detect airplanes, boats and people in distress.
Signals are transmitted on the 406 MHz international distress frequency, which is the band monitored around the Earth by the Cospas-Sarsat satellite system. Some beacons also transmit a lower powered signal on 121.5 MHz as a reference for local search teams to home in on the signal once they arrive near the location calculated for the beacon.
In some countries, such a homing transmitter is a mandatory beacon feature. As an emergency may happen in different places, it is important to provide people with the right beacon according to the environment. For this reason, there are currently three types of beacons used to transmit distress signals, which differ by their technical features and mode of activation.
Personal Locator Beacons (PLB's) are generally carried by individuals who are away from normal emergency services, but they can also be found on ships and airplanes. They can be used everywhere, both on land and at sea by adding a simple floating case. They can only be activated manually by pressing a button and operate exclusively on 406 MHz. PLBs are small, not much larger than the size of a mobile phone, have a medium battery life and, unlike other beacon models, are registered to a person rather than a vessel.
Emergency Position Indicating Radio Beacons (EPIRB's) are installed on marine vessels. Some of them can also be activated after an impact, so they will send distress signals when in contact with water if the boat is sinking. For a marine EPIRB to begin transmitting a signal, it first needs to deploy out of its bracket (holder). Deployment can happen manually, where someone must physically remove it from its bracket, or automatically where water pressure will trigger a hydrostatic release unit to release the EPIRB from its bracket. If it does not come out of the bracket, it will not activate. There is a magnet in the bracket which operates a reed safety switch in the EPIRB. This is to prevent accidental activation when the unit gets wet from rain or spraying water from rough seas. A brief inadvertent signal could generate a false alert because the signal can be instantly detected by geostationary satellites like Galileo Search and Rescue or MEOSAR.
Emergency Locator Transmitters (ELT's) are designed to be installed on aircraft. ELTs were the first emergency beacons developed and were originally intended for use on the 121.5-MHz frequency to alert air traffic control. They can be activated automatically by detecting an unusual deceleration force, such as the one associated with a crash or a forced landing, or and by pilot request by pressing the "armed" button in the remote control to send an emergency declaration message. Different types of ELTs are currently in use, but 406 MHz ELTs dramatically reduce the false alert impact on search and rescue resources and have a higher accident survivability success rate.
A 406 MHz beacon, whether on a boat, plane, or in your pocket while hiking, does not transmit signals until it is activated in an emergency. This means it cannot be located by Cospas-Sarsat unless activated in one of the ways explained above.
When the beacon is purchased, it should be registered as soon as possible with the relevant national or international authority, since it carries crucial information such as:
Beacons can transmit a distress signal constantly for a minimum of 24 hours and will operate at temperatures down to -20°C.