An emergency locator beacon (ELB hereafter) is essentially a transmitter that does a particular job at a particular time. As its name suggests, it is a transmitter that only operates during emergency conditions, when external assistance is needed, in order to allow others (typically, providers of said assistance) to locate the user of the beacon.
Types
There are many makes and models. Most generally, they can be divided into three types:
- Vehicle mounted
- Component mounted
- Personal
A vehicle mounted ELB is just that - an ELB that is permanently attached to or part of a vehicle such as an aircraft, ship, or automobile. In the event the vehicle becomes disabled or otherwise finds itself in danger, the beacon can be activated (either automatically or manually) and the signal from the beacon will (hopefully) allow those receiving it to home in on its position and locate the vehicle. Typically, these are radio beacons, designed to broadcast anything from a simple continuous tone for RDF techniques to locate to a signed/stamped data packet with information about the vehicle's position and condition encoded in the signal.
There are two general types of vehicular ELBs. EPIRB (Emergency Position Indicating Radio Beacon) units are intended for maritime use. As such, they are designed to be watertight and buoyant. If a ship or boat sinks, the EPIRB is designed to float free of its bracket and begin to transmit from the surface. If the vessel is in distress but not sinking, the EPIRB can be manually activated and broadcast from the ship or boat.
The first ELBs made were for use in aviation, and are known as ELTs (Emergency Locator Transmitters). They are used on airplanes to this day, although the technology has changed somewhat. They can be activated automatically, for example via motion sensors set to register the strong shock of an impact, or via a more simple method where being dislodged from their bracket turns them on. Some will activate when immersed, and will float in case the airplane is forced down over water.
A component mounted ELB does much the same job. Instead of being part of a vehicle, however, this type of ELB is strongly attached to a particular component or object of importance rather than a vehicle which, in normal function, can take care of itself. The assumption is that if the component or object ever finds itself separated from its parent vehicle or system, that itself is an indication of an emergency situation, one where external actors can be assumed to be trying to locate the component. The most prevalent example of this type of beacon is typically a Flight Data Recorder, otherwise known as an aircraft 'black box.' In the event an aircraft black box finds itself separated from its aircraft, the ELB attached to it is designed to begin operating to allow searchers to locate it. This is not the same as helping them locate the aircraft or vehicle itself - it is assumed that the black box, for example, has separated from the parent and needs to be found in its own right.
Finally, a personal ELB is an ELB which is small enough to be carried by a person and activated if that person requires assistance. Hikers, divers, skydivers, small-craft boaters, pilots, military personnel - anyone and everyone who in the course of their activity expects to spend time beyond the reach of 'everyday' communications or other people can benefit from a personal ELB.
Although the prior examples have used radio, there are other types of ELB. Aircraft black boxes, for example, typically contain two different types - one a radio frequency beacon in case the black box ends up on dry land, and one a sonar (acoustic) beacon or 'pinger' which emits sound waves underwater for relatively long distances. These latter types are generally activated by immersing them in seawater, and are sometimes called ULBs (Underwater Locator Beacons).
Technology
There are two general ways a radio ELB operates. Early models simply generated a continuous tone on the international 'emergency frequency' of 121.5 MHz. Since all traffic on that frequency is assumed to be of an emergency nature, the detection of a continuous tone or beacon on that frequency would cause emergency services to attempt to locate the transmitter using RDF. Initially, in addition to the normal radio listening points for 121.5, there were satellite receivers as well which could detect and do very rough direction finding on 121.5 MHz signals. However, these have been phased out in favor of a new system.
There are problems with 121.5 MHz continuous-tone operation. For one thing, such transmissions offer little or not information about the broadcaster. For another, they are very power-intensive, as the signal itself must be continuously broadcast to allow direction-finding to occur, and that signal must be audible at the longest range possible. Since ELBs must perforce operate on battery power, it is difficult to maintain a high-power 121.5 MHz continuous signal for very long. In addition, if multiple beacons are operating in the same locality, their transmissions can interfere with each other and make location difficult.
The solution (at present) is for most beacons to operate in two 'modes.' Most will still broadcast a low-power tone over 121.5 MHz, but one that is only intended to be useful within a range of a few (2-5) kilometers so that teams already searching for the beacon, and close by, can home in on it. For their initial mode, however, these beacons broadcast a digital data packet on 406 MHz.
On July 1, 1988, four countries (the U.S., The U.S.S.R., France, and Canada) signed the Cospas-Sarsat International Programme Agreement, formalizing a system that had been first operational in 1982. Those four states agreed to provide space hardware to set up an international satellite detection and locating system for emergency beacons. The system makes use of instrumentation aboard a number of different satellite constellations. In low earth orbit (LEOSAT) the system utilizes U.S. NOAA satellites containing French and Canadian SAR instruments, and Russian Cospas satellites. At the Geosynchronous level, U.S. GOES satellites, Indian INSAT and the European EUMETSAT MSG constellation all carry SAR instrumentation.
Some of these satellites originally could detect and roughly track a 121.5 MHz continuous tone beacon. However, nowadays, most are dedicated to 406 MHz data reception. Moden 406 MHz ELBs can be 'registered' with Cospas-Sarsat - that is, their identification numbers (which are in their broadcast) can be associated with a particular person, aircraft or vessel. That way, when their signal is received, the searchers can quickly identify the vehicle or person in trouble, as well as verify that the signal is not a false alarm via registered contact information.
The Cospas-Sarsat program consists of satellites, Mission Control Centers and ground stations in various nations, as well as the cooperation of numerous civil and military agencies who will respond to Cospas-Sarsat alerts that the system and organization have verified are not false alarms. Once those agencies have responded, if the position information available from the Cospas-Sarsat transmission is not precise enough to immediately locate the transmitter they will begin listening in the approximate location for 121.5 MHz beacon signals from the 'low power' mode.
The Cospas-Sarsat system data document from late 2008 tells us that the system now consists of over 600,000 registered 406 MHz beacons, an estimated 400,000 of the older 121.5 MHz beacons, 29 Mission Control Centers, 40 total participants (states, organizations, etc.) and ten-plus active satellites. In 2007 alone, 2,386 persons were rescued during 562 'events' handled by the system.