AIS MOB, AIS-SART, EPIRB and PLB: which beacon alerts whom

Every distress beacon answers one question: who needs to know, and how fast? The devices on a boat split cleanly into two systems that answer it differently. Mixing them up is the most common mistake people make when buying safety equipment, and the reason many crews end up carrying both.

This page is reference, not safety advice. Carriage rules, beacon coding and registration vary by country. Check the rules where your vessel is registered and where you sail, and register any beacon you buy.

Two systems: nearby and global

Local alerting uses VHF radio — AIS and DSC. It reaches vessels within radio range, typically a few nautical miles from a beacon at water level. It is fast: a plotter or radio on a boat nearby sounds an alarm within seconds. It reaches the people best placed to help — your own boat, if you have gone over the side. It does not, on its own, reach a rescue centre.

Global alerting uses 406 MHz beacons and the Cospas-Sarsat satellite system. The alert goes to a rescue coordination centre wherever you are. It reaches the professional rescue chain, but that chain may be hours from you, and it then has to find you. Most 406 MHz beacons also transmit a weak 121.5 MHz homing signal, and many newer ones add an AIS signal so that vessels nearby can find them too.

The four devices

Device	Alerts	System	Registered to	AIS identity
AIS MOB beacon	Vessels within VHF range, including your own	AIS (often DSC too)	Worn by a person	972xxyyyy
AIS-SART	Vessels and aircraft searching nearby	AIS	Carried in survival craft	970xxyyyy
EPIRB	Rescue coordination centre	406 MHz satellite, 121.5 MHz homing, sometimes AIS	The vessel	974xxyyyy if it has AIS
PLB	Rescue coordination centre	406 MHz satellite, 121.5 MHz homing, sometimes AIS	A person	—

AIS man-overboard beacons

A small unit fitted to a lifejacket, usually activated automatically when the jacket inflates. It transmits position reports with a 972 identity, which plotters and AIS units display as a distinct man-overboard target, typically with an alarm. Many also send a DSC alert, and some can be programmed with your own boat’s MMSI so that your radio alarms first.

Its strength is speed and proximity: the crew left on board see the alarm and the position at once. Its limit is range. An antenna bobbing at water level reaches a few nautical miles at best, and if no one nearby has AIS, no one hears it.

AIS-SART

An AIS search-and-rescue transmitter is carried in survival craft and switched on after abandoning ship. It broadcasts with a 970 identity so that searching ships and aircraft can home in on the life-raft. Under SOLAS, ships carry a search-and-rescue locating device for their survival craft — either an AIS-SART or the older radar SART. Active AIS-SART, MOB and EPIRB-AIS devices report navigational status 14, “AIS-SART, MOB-AIS or EPIRB-AIS active”.

EPIRB

An emergency position-indicating radio beacon is registered to a vessel and alerts the rescue system through the 406 MHz satellites. Category I EPIRBs sit in a bracket with a hydrostatic release: if the boat sinks they float free and switch on by themselves. Category II units are manually released and activated. Some newer EPIRBs also transmit on AIS with a 974 identity, so ships nearby can see the beacon directly on their plotters.

PLB

A personal locator beacon is the wearable equivalent of an EPIRB: registered to a person, small enough to carry in a lifejacket pocket, and activated by hand. It is the device that tells the rescue services someone is in the water, wherever that water is. It does nothing to alert your own boat unless it also carries an AIS signal — some newer models do.

Which one for which risk

• Falling overboard from a crewed boat in coastal water: the boat you fell from is the fastest rescue. An AIS MOB beacon (and an AIS-capable plotter or transponder on board to hear it) addresses that directly.
• Sailing single-handed or short-handed: there may be no one on board to hear a MOB alarm. A PLB — or a PLB with AIS — reaches help that is not on the boat.
• The vessel itself sinking or being abandoned: an EPIRB, ideally Category I, plus a SART for the life-raft.
• Far offshore: local VHF alerts may reach no one. The 406 MHz system is the one that works everywhere.

Many offshore crews carry a MOB beacon and a PLB per person, plus a vessel EPIRB. They are not duplicates.

How beacons appear on AIS

Emergency devices use their own MMSI formats from ITU-R M.585: 970 for AIS-SARTs, 972 for man-overboard devices and 974 for EPIRB-AIS. These identities carry no country digits; the next two digits name the manufacturer and the last four are a serial number. That is why our vessel pages and the MMSI decoder never print a flag for them.

Devices in test mode identify themselves as tests — AIS-SARTs, for example, send “SART TEST” rather than “SART ACTIVE”. Most beacon identities a shore receiver picks up are tests, demonstrations or accidental activations on the quayside.

Never act on a distress identity seen on a public website, including this one. Positions here can be minutes old, a shore receiver may have heard only some of the messages, and rescue is co-ordinated by the authorities on official channels. If you believe someone is in danger, contact the coastguard.

Rules that are changing

AIS MOB beacons sit in a category the ITU now calls autonomous maritime radio devices (ITU-R M.2135). Countries are revising how such devices may be used, coded and certified, and a MOB beacon that is legal in one country may not be in another. Check current national rules before you buy, and check that the unit you buy is approved where you sail.

Registration and testing

406 MHz EPIRBs and PLBs must be registered with the national authority for the country they are coded to — in the United States, NOAA SARSAT. An unregistered beacon still raises an alert, but rescuers lose minutes finding out whose it is and whom to call. Update the registration when you sell the boat or change your contact details.

Test beacons only in the way the manufacturer describes. A real activation, even a few seconds long, starts a real search.