ELT

ELT stands for Emergency Locator Transmitter and denotes a distress beacon for aircraft. It supports the search for a downed aircraft and its occupants through automatically or manually activated radio signals. A 406 MHz ELT sends a digital alert through Cospas-Sarsat with a beacon identification code and, when suitably equipped, position data. For aircrew isolated during military operations, it provides a starting point for locating them; whether that point leads to the wreck or the survivors depends on the beacon design, whether it is taken along and subsequent movement.

From the transmitter to the search

After activation, satellites receive the 406 MHz message and relay it to ground stations. The data is processed there and sent to a Mission Control Centre. This centre combines associated messages and distributes the alert to the responsible rescue coordination centre or SAR point of contact. The beacon identification code links the alert to the registered aircraft and contact details. Those details are held in the registry; the beacon does not transmit the entire registry entry.

A suitably equipped ELT adds a GNSS position to its message. This can come from its own receiver or from the aircraft's navigation system. A position inserted only once can already be out of date if the aircraft or the removed beacon subsequently moves. The satellite system can also locate the transmitter without an included GNSS position. What matters to the search is therefore when the position was determined and how it was obtained.

An additional homing signal, usually on 121.5 MHz, allows suitably equipped search forces to home in on the beacon within the search area. The digital satellite alert and this final homing phase perform different tasks. Older ELTs operating solely as analogue beacons on 121.5 or 243 MHz are not detected by Cospas-Sarsat satellites; a ground or airborne receiver within radio range must detect their signal.

Which beacon stays with the wreck?

The term ELT covers several designs. For aircrew, the decisive questions are what can still be carried after the accident and whose location the beacon then marks.

For an ELT(AP), “portable” means removal after the accident as intended by its design. Tearing a fixed ELT(AF) from its mounting does not turn it into a usable personal beacon. With a deployed beacon, search forces must also distinguish between the beacon location, crash site and survivors' position. This is particularly important after ditching if the life raft and beacon drift separately.

Activation, carriage and failure

An automatic ELT detects the accident loads for which it was designed, often through an acceleration sensor. It must be in the correct armed state for this to work. A forced landing or other accident can fall outside the detected activation conditions. Depending on the device, a cockpit control or a switch on the transmitter permits manual activation. Even an activated beacon needs a working battery, antenna and connection between the transmitter and antenna. A severed cable can prevent a usable transmission despite successful activation.

Before a flight, the crew therefore needs to establish which beacon is present, what its operating indication looks like, whether and how it can be removed and which antenna will be needed. After a parachute descent or emergency escape, the aircraft ELT may remain on the aircraft. The survivors then have only the equipment worn on their bodies or actually retrieved for their own distress alert. A personal beacon or survival radio is an additional option only if it was issued, survived the escape and remained with the person.

After an accident, leaving an immediately dangerous location takes priority over retrieving the transmitter. The equipment that remains can then be checked: is the beacon accessible, is it in the intended operating state, does it show activation, and is the necessary antenna present and connected? Where the situation permits the planned beacon operation, device-specific manual activation replaces an automatic trigger that has failed to operate. A removable ELT uses its designated antenna and is secured against loss. Returning to a burning wreck or one threatened by an adversary solely to retrieve a beacon creates another danger.

An activation indication does not establish that a satellite or rescue coordination centre has received the message. The device can be switched on yet fail to deliver a usable alert because of damage. Function checks and self-tests depend on the model; a real 406 MHz distress alert is not an ordinary function test.

Wreck location and survivor position in military isolation

Fire, dangerous terrain or an adversary's search can force aircrew to leave the crash site. If the ELT remains at the wreck, its signal continues to lead there. Even an accurate wreck coordinate then says little about the survivors' current location. A prepared Evasion Plan of Action helps recovery forces interpret intended movement and actions. It records the actions planned before the mission, which may differ from the route actually taken afterwards.

When a connection is actually available, isolated personnel therefore report their current position, its time reference and any intended further movement using the designated communications means. An ordinary ELT does not provide a conversation in which the number of survivors, injuries or enemy activity can be explained. Without a reachable counterpart, a beacon transmitting at the wreck and the known movement intention remain only starting information for the search. An alert is no reason to wait for forces at a dangerous location.

Enemy threat also creates a conflict: the signal is intended to lead friendly search forces to the transmitter and can disclose its location to an adversary. An ordinary ELT is not a protected tactical reporting channel. Mission instructions must therefore establish the conditions for using international distress signals and the communications means available. Switching off interrupts further alerts and removes the homing signal for suitably equipped search forces; leaving the beacon on continues to produce radio emissions.

A beacon identification code identifies the beacon or its registry entry. It does not prove who is currently beside it. The prescribed identity check, where applicable using ISOPREP information, therefore remains necessary even with an accurate position. Whether forces can reach that position depends on factors including threat, terrain, weather and the assets actually available.

National alert paths and personal beacons

In the United States, NOAA maintains the registry for US 406 MHz beacons. The US Mission Control Center passes alert data to appropriate Air Force or Coast Guard rescue coordination centres according to the distress situation and location. Registered aircraft and contact details help associate the alert with the aircraft and identify an accidental activation. This national SAR path is not equivalent to directly alerting the crew's own military recovery forces in an overseas operational area.

In the British civil system, an aircraft address allocated for a Mode S transponder can also be used as the ELT identifier. Beacon registration with the Maritime and Coastguard Agency is nevertheless separate. A known aircraft address therefore does not replace the beacon and contact details needed for the alert. In military missions, national equipment issue, registration and the planned information path to the recovery organisation must fit together. Within the NATO framework, equipment for personnel at risk of isolation is a national responsibility; the same abbreviation therefore does not denote identical personal equipment for every aircrew.

A PLB is a personal, manually activated distress beacon. If it stays on the body, it can signal the person's location after they leave the wreck. This is its practical advantage over an ELT that remains on the aircraft. An EPIRB belongs to the maritime beacon category. Buoyancy or attachment to a life raft does not turn an ELT into an EPIRB.