EPIRB stands for Emergency Position Indicating Radio Beacon: a maritime distress beacon used to alert search and rescue services. It forms part of the emergency equipment of a ship or boat. A 406 MHz EPIRB sends a digital distress alert with a beacon identification code through Cospas-Sarsat; suitably equipped models also transmit their own satellite-derived position.
A distress beacon for the vessel and its survivors
An EPIRB alerts rescue authorities to an emergency at sea and helps them locate the beacon. Its identification code can be linked through registration to a vessel and its contacts. This distinguishes it from an ordinary navigation device that only determines its own position, and from a radio used for ongoing voice communication.
In a shipwreck, when abandoning a vessel or if a group becomes separated, the beacon's actual location matters. It is the transmitter that is located; its position does not automatically show where every survivor is. This also applies to military survival at sea: receiving a distress alert does not amount to a commitment to conduct a tactical recovery mission. The responsible rescue coordination centre establishes the circumstances of the emergency and decides whether to deploy available forces.
Release and activation are separate processes
An EPIRB can be removed from its bracket by hand and switched on using the designated controls. An automatic float-free model may also provide this manual option.
In a float-free system, a Hydrostatic Release Unit, or HRU, releases the bracket at the water pressure for which it is designed. This allows the buoyant beacon to separate from the sinking vessel. This mechanical release is distinct from the electrical activation of the distress transmitter.
In a suitably designed model, a water switch activates the released beacon once it is exposed to water. The buoyant body keeps the beacon at the surface with its antenna pointing upwards. The electronics can switch on before the beacon reaches the surface.
| Function | Purpose |
|---|---|
| Bracket and HRU | Secure the beacon and release it under the intended pressure conditions. |
| Manual controls or water switch | Electrically activate the distress transmitter according to its design. |
| Buoyant body and antenna | Enable the intended transmission from the water's surface. |
The terms Category I and Category II, used in the United States, illustrate this distinction. Category I provides automatic release. A Category II beacon must be removed from its bracket by hand; depending on the model, it may still be activated manually or by immersion.
Why mounting and design matter
A released beacon needs a clear path out of its bracket and to the surface. Superstructure or other obstacles can trap it. An unsuitable attachment to the sinking vessel can also prevent it from floating free as intended. At the same time, the mounting must allow manual removal and protect the device from damage.
The beacon, bracket and release mechanism must be designed to work together. Placing an arbitrary manually removable EPIRB in an automatic bracket does not turn it into a functioning float-free system. Nor can an electronic self-test alone confirm that the bracket will release or that the path out is clear.
The intended method of operation differs between designs. Some EPIRBs need to float in water for their antennas to perform as intended; others have instructions for use with a life raft. The particular device's instructions and intended position determine how it is used.
Position, satellite alerting and homing
A built-in GNSS receiver can determine the beacon's position and make it available for the distress alert. Cospas-Sarsat routes the 406 MHz message through satellites, ground stations and alert processing to the responsible rescue authorities. The rescue coordination centre establishes the circumstances of the emergency and coordinates the response.
An additional signal on 121.5 MHz can help suitably equipped search forces home in on the beacon. This signal does not replace the digital satellite alert. The specific design determines which additional functions are available and how the antenna is used.
Registration and reachable contacts
The beacon identification code is linked to registration details; not all of those details are transmitted in the radio signal. For example, the UK beacon register run by the Maritime and Coastguard Agency, or MCA, links the EPIRBs registered with it to vessel and device details, owner information and emergency contacts ashore. These details help identify the vessel, estimate how many people may be affected and determine which rescue resources are needed.
Changes to owner or contact details must be recorded in the UK beacon register. If the beacon is transferred to another vessel, the registered vessel details also need updating. Incorrect information can direct rescue authorities to the wrong vessel or lead their investigation of an alert in the wrong direction.
How EPIRBs differ from PLBs and ELTs
An EPIRB is intended for maritime emergencies. A PLB is a portable personal distress beacon; an ELT forms part of an aircraft's emergency equipment. Sharing an alert frequency does not make these devices freely interchangeable. Intended application, bracket, activation, buoyancy and antenna must suit the operating environment.