What happens when an EPIRB is activated

When I was a kid, sailing a dinghy around Perth’s Swan River, the idea of being able to press a button on a device that would transmit my exact position via satellites to the nearest Rescue Coordination Centre was in the realms of fantasy.

Not long after, in 1982, the 60ft trimaran Gonzo capsized 300nm off Boston, USA. It was carrying a first-generation Emergency Position Indicating Radio Beacon, or EPIRB, which was activated by the crew. The signal was initially picked up by international passenger planes then pinpointed by overhead satellite passes. The US Coastguard was dispatched, and the crew became the very first satellite-aided marine rescue.

Today, there are three main types of emergency beacons that use satellite constellations to pinpoint their distress transmissions on the internationally recognised distress frequency of 406MHz. 

1. EPIRBs are designed primarily for marine use, although they will also work on land or in the air provided there is a clear line of sight to satellites.
   
   In all Australian states and territories, it is a legal requirement to carry an EPIRB if you’re travelling further than 2nm (3.7km) from shore (or 3nm/5.5km in WA).
   
   
2. EPIRBs are designed to work best when floating upright in the water, with the antenna using the water’s surface as a reflector. They should be tethered to the vessel or life raft when deployed and will transmit for a minimum of 48 hours continuously.
   

Marine-specific PLBs are fast becoming an essential personal safety device when at sea. These beacons are small enough to be carried in a pocket or attached to your belt, harness or lifejacket, and can be held or attached to your person or life jacket above the water. 

PLBs transmit for a minimum of 24 hours continuously and should not be deployed into the water when activated. In some instances, PLBs do not meet the carriage requirements for vessels travelling more than 2nm (3.7km) from shore.

What these beacons all have in common is the way they work using the International Cospas-Sarsat Programme. This was initially conceived by Canada, the USA, France and the former Soviet Union, but is now made up of a cooperative of 45 nations.

‘Cospas’ is an acronym for the Russian words Cosmicheskaya Sistema Poiska Avariynyh Sudov, which translates to ‘space system for the search of vessels in distress’. ‘Sarsat’ is an acronym for Search and Rescue Satellite-Aided Tracking. The system comprises a space segment and a ground segment.

Three kinds of satellites make up the space segment: 

LEOSAR (Low-altitude Earth Orbit Search and Rescue). There are five of these in polar orbit with a small footprint, but as Earth rotates below them, they cover the entire surface of the globe, with each orbit taking about 100 minutes to complete. 

GEOSAR (Geostationary Earth Orbit Search and Rescue). Twelve of these orbit the Earth’s equator at a speed that keeps them stationary above a geographical position, providing coverage of the Earth’s surface between around 70 degrees north and south of the equator.

MEOSAR (Medium-altitude Earth Orbit Search and Rescue). These are the latest addition to the space segment, with 48 satellites in various orbits providing what is now regarded as the dominant capability of Cospas-Sarsat.

LEOSAR and GEOSAR constellations were put into space for meteorological purposes, and MEOSAR satellites belong to the GNSS (Global Navigation Satellite Systems).

The sensors that are fitted to these satellites can receive alerts from EPIRBs, PLBs or ELTs on the 406MHz frequency and are secondary payloads. Most recently, China fitted Cospas-Sarsat sensors aboard six of the navigation satellites belonging to its BeiDou (BDS) constellation as late as 2019.

Several more satellites of all three orbital kinds are in preparation for use within the Cospas-Sarsat Programme.

The ground segment is made up by the distress beacons themselves, as this is where the transmission on 406MHz begins. The transmission is then received by the space segment, and the information is beamed down to a Land User Terminal (LUT)  – an array of antennas that track the space segment.

There are four LUTs in our region: in Bundaberg (Qld), Albany (WA), Wellington (NZ), and a new MEOSAR LUT in Mingenew (WA).

The data is then transmitted to a Mission Control Centre (MCC) – in our region, the Joint Rescue Coordination Centre (JRCC) in Canberra. 

If the distress transmission originates with a locally registered beacon, the JRCC takes charge of the search and rescue operation. If not, the alert details are sent to another JRCC in the worldwide network closer to where the beacon was activated or is registered. Keeping your beacon’s registration details up-to-date are therefore crucial.

GNSS-enabled EPIRBs and PLBs are now widely available and far more affordable than when they first appeared on the market, and there are good reasons to upgrade if your beacon doesn’t have this capability.

GEOSAR satellites can detect your non-GNSS beacon pretty much instantly, but, because they’re stationary, they can’t calculate a beacon’s position. For this to occur, a LEOSAR or a MEOSAR needs to appear on the scene to provide some relative motion.

Even then, the position of the beacon is only accurate to around 5km, saddling rescue crews with the task of old-school radio direction finding (RDF) on the weaker 121MHz analogue frequency that all beacons also transmit. If the beacon’s battery runs flat before rescue, this RDF ability is gone.

A GNSS-enabled beacon will transmit your position digitally with an accuracy of 100m or less, which means the position can be recorded and saved by the MCC and JRCC. Not only that, but there’s no need to wait for a LEOSAR or MEOSAR satellite.

A RLS-enabled beacon that has been activated will receive an automatic message when its distress alert is received by a Mission Control Centre. The RLS signal is transmitted via a MEOSAR satellite and is called a Return Link Message (RLM).

An RLM only confirms that the beacon’s distress signal has been received, but not that a rescue has been launched yet. which would surely still provide some reassurance to a distressed crew.

An AIS communication device automatically transmits a ship’s position and identification using VHF to other AIS-enabled vessels to avoid collisions. The alert will appear as a red circle with a red ‘X’ within it, displayed on the AIS receiver and, if interfaced, on the electronic chart too. Depending on the hardware, an audio alert may also occur.

Some newer-model EPIRBs and PLBs can now also transmit an AIS user ID to all vessels in VHF range of the distress alert, but this is only useful if there are vessels nearby. However, the detection range is limited to the height of the transmitting and receiving antennas – given that the EPIRB’s transmitter will almost certainly be at sea level, a range beyond 10nm is unlikely.

If two-way communication isn’t available and you feel you’re in grave and imminent danger, activate your emergency beacon. But if the situation allows, then digital selective calling (DSC) – which transmits a distress alert in a predefined digital message using VHF radio – or a MAYDAY voice transmission via radio are the most effective actions you can undertake initially. 

Both let you communicate a lot of useful information to the responders, and you can also provide updates as your situation unfolds. This would also help emergency services determine their response according to your situation.