What to check on a EPIRB (406 MHz).
An EPIRB is not a general distress radio — it is a single-purpose beacon designed to work after the ship itself has stopped working. Where a DSC distress call or a satellite phone depends on the ship's power and antennas still being intact, an EPIRB carries its own battery, its own antenna, and its own float-free release, so it keeps transmitting after the vessel that carried it has gone down. The 406 MHz signal alone is not enough to guide rescuers to the exact spot; most modern units pair it…
What sets a 406 MHz EPIRB apart
An EPIRB is not a general distress radio — it is a single-purpose beacon designed to work after the ship itself has stopped working. Where a DSC distress call or a satellite phone depends on the ship's power and antennas still being intact, an EPIRB carries its own battery, its own antenna, and its own float-free release, so it keeps transmitting after the vessel that carried it has gone down. The 406 MHz signal alone is not enough to guide rescuers to the exact spot; most modern units pair it with an integral or linked GPS for position and a 121.5 MHz homing signal that search aircraft and lifeboats use for final approach.
Main components
Float-free hydrostatic release (HRU)
The bracket holds the EPIRB in its mount until water pressure at roughly 1.5-4 metres depth triggers the hydrostatic release, letting the beacon float free and activate automatically if the ship sinks with nobody able to launch it by hand.
406 MHz transmitter and battery
A sealed, non-rechargeable lithium battery pack powers the unit for a minimum 48 hours of continuous transmission once activated, and has its own separate expiry date from the unit's overall service life.
Integral or coupled GPS
Most current beacons include GPS so the first transmitted burst already carries a position accurate to around 100 metres, cutting search time from hours to minutes compared with Doppler-only positioning from the satellite pass.
121.5 MHz homing transmitter
A separate, lower-power signal used only for final homing by search units once they are in the area — it is not monitored by satellite and has no distress-alerting function of its own.
Selection and sizing
EPIRB choice on a commercial vessel is largely fixed by GMDSS carriage requirements rather than open selection, but purchasers still choose between models on:
- Battery and hydrostatic release expiry dates, which drive the replacement schedule independently of the unit's electronics
- GPS integration versus GPS-less units, increasingly rare and slower to get an accurate fix
- Bracket type matched to the mounting location and float-free clearance required
- Country code programmed into the beacon, which must match the vessel's actual flag registration
Regulations and class
Carriage is mandated under SOLAS Ch. IV (GMDSS) for all vessels engaged on international voyages, with the beacon registered to the ship in the relevant national or Cospas-Sarsat register — an unregistered or incorrectly registered beacon delays search and rescue even if the signal itself is received. Battery and HRU replacement intervals are set by the manufacturer's type approval, commonly around five years for the HRU and battery expiry printed on the unit; class surveys check the expiry dates directly rather than testing the transmitter, since a live test would trigger an actual satellite alert.
Typical faults
- Expired battery or HRU left in service past the printed date — the unit may not activate reliably when needed
- Registration details not updated after a change of ownership, flag or contact number — rescue coordination centres reach a dead line or the wrong operator
- Bracket corrosion jamming the HRU mechanism so it cannot release even after the pressure trigger fires
- Beacon test button pressed without following the manufacturer's self-test procedure, causing confusion over whether a real alert was sent
What to look for in a supplier
- Type approval current under the applicable GMDSS performance standard for the flag state
- Clear battery and HRU expiry dates supplied with the unit, not just the beacon's own manufacture date
- Registration support or clear guidance for updating the Cospas-Sarsat or national register entry
- Bracket and mounting hardware suited to the exposed deck location, in a marine-grade corrosion-resistant finish
A self-test pass is not a test of the HRU — schedule the physical hydrostatic release mechanism into the planned maintenance routine separately from the electronic self-test, since a jammed bracket fails silently until the day it matters.