EPIRBs and PLBs

How 406 MHz Beacons Work

When a 406 MHz beacon activates — whether manually or automatically — it transmits a coded digital signal on the 406.028 MHz frequency. This signal is detected by the Cospas-Sarsat satellite network, a joint US, Russian, European, and Canadian system operated since 1982 specifically to detect and locate distress beacons. The satellite network currently includes both low Earth orbit (LEO) and medium Earth orbit (MEO) satellites, as well as geostationary (GEO) satellites.

The 406 MHz signal is encoded with a unique identifier — either the vessel's MMSI (for EPIRBs linked to a vessel) or a registration code linked to the owner (for PLBs). This encoded ID is what enables rescue coordination centres to identify who is in distress before they make contact. The Cospas-Sarsat system processes the signal, calculates a position (from Doppler analysis or, if the beacon has a built-in GPS, from the encoded GPS coordinates), and forwards the alert to the responsible Maritime Rescue Coordination Centre (MRCC) or Aeronautical Rescue Coordination Centre (ARCC) for the geographic area.

Typical alert-to-MRCC notification time has improved dramatically with the introduction of MEO satellites. Modern GPS-equipped beacons transmitting to MEO satellites can have alerts processed within minutes of activation. Older non-GPS beacons relying on Doppler positioning with LEO satellites may take longer and produce a less precise position. A GPS-integrated beacon encodes the vessel's GPS position directly into the 406 MHz signal — providing rescue services with a near-exact location from the first transmission.

121.5 MHz was the original civilian distress frequency and was monitored by Cospas-Sarsat satellites until February 1, 2009. Satellite monitoring of 121.5 MHz beacons ceased on that date. A 121.5 MHz-only beacon is useless for alerting rescue services via satellite. Aircraft and some vessels still monitor 121.5 MHz locally, but it is not a reliable primary distress system. Every vessel going offshore should carry a 406 MHz beacon — no exceptions.

AIS EPIRB units combine a standard 406 MHz EPIRB with an AIS transmitter. When activated, they simultaneously alert the Cospas-Sarsat network and broadcast an AIS MOB alarm on AIS frequencies. Nearby vessels with AIS plotters will see the alert and position directly on their chartplotters — providing an additional local rescue notification layer that does not depend on satellite relay.

EPIRB vs PLB

EPIRB (Emergency Position Indicating Radio Beacon) is a distress beacon registered to a vessel. It is designed to be mounted on the boat, associated with the vessel's registration, and to alert rescue services to a vessel in distress. EPIRBs are generally larger than PLBs and carry a 48-hour battery at 0°C — providing sustained transmission through extended search and rescue operations.

Class 1 EPIRBs are the standard for offshore sailing: they are designed for automatic float-free deployment via an HRU, activating automatically when submerged (like a life raft HRU, approximately 4 metres depth). They also have a manual activation capability. Class 2 EPIRBs are manually activated only — no float-free capability. For offshore use, a Class 1 EPIRB in a deck bracket with an in-date HRU is the correct choice. It will deploy and activate even if the crew is incapacitated.

PLB (Personal Locator Beacon) is registered to a person, not a vessel. It is designed to be worn or carried on the body — clipped to a life jacket, worn on a belt, or stowed in a sailing suit pocket. PLBs are smaller than EPIRBs, carry a 24-hour battery minimum (some carry more), and do not have float-free capability — they must be manually activated. The PLB activates when the person activates it, not when the vessel sinks.

When to carry which: an offshore vessel should carry both. The EPIRB protects the vessel — if it sinks rapidly and the crew cannot retrieve the EPIRB from its bracket, the float-free HRU deploys it automatically. The PLB protects the individual — if a crew member goes overboard at night or is separated from the vessel, their PLB can alert rescue services to their personal location independent of the vessel.

Tether a PLB to the life jacket, not to a harness or pocket. In the water, an unconscious person cannot activate a PLB stowed in a pocket. A PLB attached to the life jacket's outer webbing is accessible to the victim or a rescuer and is positioned near the surface for optimal signal transmission. Some life jackets have purpose-made PLB pockets on the shoulder or chest — use them.

Registration

Registration is not optional. An unregistered 406 MHz beacon is a beacon that rescue services cannot quickly identify. When an unregistered beacon activates, the Cospas-Sarsat system receives a signal with an encoded ID — but the ID does not match any record in the registration database. Rescue services must treat the alert as potentially real while simultaneously attempting to trace the source. Response is slower, more uncertain, and may be deprioritised if the alert pattern is consistent with accidental activation (which is a common scenario with unregistered beacons).

In the United States, all EPIRBs and PLBs operating on 406 MHz must be registered with NOAA through the national beacon registration database at beaconregistration.noaa.gov. Registration is free, takes less than five minutes, and is permanent — you update the record as details change rather than reregistering.

The registration record must include: owner's name and contact information, vessel name, type, colour, length, and registration number (for EPIRBs), emergency contacts (people who will be called during an alert and who can confirm whether you are actually at sea), and the beacon's 15-hexadecimal-digit ID (found on the beacon label). The emergency contact is one of the most important fields — a rescue coordination centre will call that number to verify whether the activation is accidental before committing full SAR resources.

Keep your registration current. If you sell the vessel, transfer or cancel the EPIRB registration — a new owner who has not registered their EPIRB will have an alert correlate to the previous owner's record, wasting SAR time. If you change your phone number, email, or emergency contact, update the registration. If the beacon's battery is replaced and the unit reprogrammed, verify the ID is still correct. NOAA recommends reviewing your registration annually and sends an annual reminder to registered owners.

If you are sailing in international waters, your registration in the country of vessel registration is typically sufficient — the Cospas-Sarsat network links national databases. However, if you are spending extended time in another country's waters, check whether local registration is also required or recommended.

Activation and Maintenance

Manual activation: all 406 MHz EPIRBs and PLBs have a manual activation mechanism — typically a guarded switch or a pull-tab combined with pressing a button. The specific procedure varies by manufacturer and model. Know your beacon's activation procedure before you are standing in the water at night trying to read the label. Review it during every pre-departure check.

Automatic activation (EPIRB Class 1 only): in addition to manual activation, a Class 1 EPIRB activates automatically when it contacts water (via a water-sensitive switch) after float-free deployment from its HRU cradle. This is the scenario where the vessel sinks rapidly and the crew has no time to retrieve and activate the beacon. The automatic activation is a backup to manual — if you have time and the ability to manually activate the EPIRB before deployment, do so. This confirms activation and gives you control over timing.

Self-test: all beacons have a self-test function that confirms the unit's internal electronics are operational without transmitting a real distress signal. Perform a self-test every 30 days and before every offshore passage. The test is typically done by pressing and holding a test button (consult your manual for the exact sequence). A failed self-test means the unit must be serviced before use.

Battery expiry: 406 MHz beacon batteries have a stamped expiry date. Do not exceed it. Do not replace beacon batteries yourself — the process involves opening a sealed, pressure-rated housing, replacing a specialised battery, and re-certifying the unit. This must be done by an approved service centre. The beacon ID must be re-verified after battery replacement. Note the battery expiry date in your maintenance log and book service several months before it expires, not after.

Accidental activation: if your beacon activates accidentally — knocked in the boat, dropped in the bilge, accidentally triggered during a check — contact the USCG or your national maritime authority immediately on Channel 16 or by phone. Provide your beacon's 15-digit ID. Explain the accidental activation and confirm you are safe. If you do not cancel within minutes, a full SAR response may be initiated. The longer you wait to cancel, the more resources are committed. There is no embarrassment in an accidental activation — but there is significant cost and risk to SAR crews if you fail to cancel it.

Storage: keep EPIRBs and PLBs away from sources of water ingress when in storage. A PLB in a waterproof bag in a drawer is at risk of activation if the bag floods. EPIRBs stored in bilges or wet lockers can activate accidentally if the water-sensitive switch contacts bilge water. Store beacons in dry locations with the activation guard secured. When transporting beacons — by car, aircraft, or cargo ship — ensure the activation mechanism is physically guarded against accidental triggering.