EPIRBS & PLBS - Some Practical Considerations

Posted on , Last Updated

In February of 2009, the 121.5 MHz distress beacon system was finally switched off. The only technology relevant to recreational boating safety today involves those beacons that operate at 406 MHz (albeit that 406 beacons also incorporate a very low power (0.25 watt) 121.5 MHz homing signal for search aircraft). The new technology however is not just limited to a change in operating frequency. It also involves very significant changes to how the system works – changes that have produced a very much more robust and efficient means by which boaters can alert authorities to a distress situation. But because this is a considerably more sophisticated technology, it also provides for a number of choices in regard to the features of the various beacons available on the market – choices that need to take into consideration specific purpose, the various Australian state regulatory compliance regimes and what might ultimately provide rescue authorities with the best chance of finding persons in distress. In order to assist in putting all this into some sort of perspective, the following may be of interest.

Some Background on How 406 MHz Beacons Operate: The 406 MHz distress beacon system involves the COSPAS-SARSAT global distress warning system. It consists of the following components:
406 MHz beacons – emergency position indicating radio beacon (EPIRB) carried by vessels and personal locator beacon (PLB) carried by persons,
four geostationary search and rescue (GEOSAR) high altitude (about 36,000 km) satellites,
four polar orbiting low altitude (about 1000 km) low earth orbit search and rescue (LEOSAR) satellites.
local user terminals (LUTs) that download the distress data from the satellites
a constellation of twenty four global positioning system (GPS) medium altitude (about 20,000 km) satellites and
rescue coordination centres (RCCs) that coordinate the resources that effect a rescue.

When a 406 MHz beacon is activated, it immediately switches on two radio transmitters comprising:

A 5 watt 406 MHz unit that every 50 seconds sends out a burst of data including the beacon’s Unique Identifier Number (UIN) and, if a GPS receiver is fitted, its precise location.
A 0.25 watt 121.5 MHz unit that continuously transmits a homing signal used by search and rescue (SAR) aircraft to obtain a visual fix.
The importance of the UIN is that it contains information of vital importance to SAR authorities. Comprising a 15 character hexadecimal code, the UIN identifies the country in which the beacon is registered and the beacon’s serial number. Provided the beacon is appropriately registered with the relevant country’s SAR authority (for Australia that being the Australian Maritime Safety Authority or AMSA), its transmission of the UIN will enable authorities receiving it to link the alert to important information required to validate and manage a distress situation. That information includes the name and description of the vessel, the vessels (radio) call sign, the vessel’s (DSC) MMSI number, the name, address and contact details of the vessel’s owner and up to three emergency telephone contacts. To the extent that 95% of all beacon alerts downloaded by RCC Australia are proven to be false, the need to validate that a beacon alert represents an actual distress situation is of absolute importance. In the event that an alert cannot be validated because a beacon is not registered, considerable delays in the commencement of SAR activity may result. In most Australian states, where it is a requirement for a vessel to carry an EPIRB, it is also a requirement that the EPIRB be registered with AMSA.

Perched about 36,000 km above the equator, a GEOSAR satellite will receive and instantaneously download the burst of data transmitted by a beacon to a Local User Terminals (LUT). That information is then processed and immediately past on to the relevant RCC for validating and actioning. All this, the system can deliver within 5 minutes of beacon activation. But for search and rescue to be effective, it is also necessary to know where to look. If the beacon is equipped with a GPS receiver, included in the half second bursts of data transmitted every 50 seconds will be information that will tell the RCC exactly where the beacon is – within a search radius of about 0.5 nautical miles. And for the reason that most GPS equipped beacons will update position data every 20 minutes, it will also allow the RCC to calculate set and drift. If, however, the beacon is not GPS equipped then its position will be determined by one of the low altitude polar orbiting LEOSAR satellites. This is achieved by the satellite’s ability to measure the Doppler shift of the beacon’s signal (as the satellite hurtles by at 7 km per second) and process that information to give two (mirror) possible positions. But to obtain a fix can take some time because (a) the time for the satellite to make the initial pass to give the two possible positions, (b) the time for the satellite to make a second pass to eliminate one of two initial possibilities and (c) the time it takes for the satellites and the LUT, to which it sends the raw data, to process the Doppler shift information and calculate the beacon’s actual position. The nominal time it takes for a non GPS equipped beacon to obtain a fix by this method is 90 minutes but can be as long as 5 hours and the position is only accurate to within a search radius of about 2.3 nautical miles. Putting that into perspective, the difference here is a search area of ¾ of a square mile determined in a number of minutes for a GPS equipped beacon and a search area of nearly 17 square miles determined only after a number of hours for a non GPS equipped beacon. In poor visibility and deteriorating weather conditions, a GPS equipped beacon significantly enhances the chances of rescue and, not considered by many boaters, the time rescuers need to spend risking their own lives searching. As AMSA likes to advise those willing to listen, “do whatever can be done to take the search out of search and rescue.”

Choosing a Beacon: Subject to the overriding requirement that they comply with current Australian standards (AS/NZS 4280.1 and AS/NZS 4280.2 for EPIRBs and PLBs respectively) and carry a specific approval to that effect, there are a wide variety of 406 MHz beacons available for Australian boaters to choose from. But before purchasing a beacon it is important to take into consideration a number of factors – the purpose for which the beacon is being purchased, regulations concerning the type of beacon to be carried, the technical capability of the beacon to not only alert but also locate and finally, as it concerns the particular application, the method of deployment and activation.

One important warning that should be raised at the outset concerns the temptation to purchase one of these devices from overseas from a foreign supplier at a considerable discount to the price from a local retailer. There are two important points to be raised in this regard. The first is that a beacon purchased from a foreign supplier may not comply with Australian and New Zealand standards. Also, a beacon purchased from an overseas supplier is unlikely to have a UIN (Hex) code that carries an Australian country code. And this becomes a problem because, unless that code is changed, it cannot be registered with AMSA. An EPIRB not currently registered with AMSA is not a compliant EPIRB.

The second matter to be considered is whether to purchase an EPIRB or a PLB. PLBs are typically cheaper and smaller and in any event, from an electronic perspective, exactly the same. Both transmit the same burst of half second data at exactly the same power output at exactly the same time intervals, both transmit a constant 0.25 watt homing signal and both are required to be waterproof. But that is where the similarity ends. Important differences between an EPIRB and a PLB concern two particular differences in the standard to which each must comply. The AS/NZS 4280.1 standard, governing the obligatory characteristics of an EPIRB, includes the requirements that (a) it float the right way up when in the water and (b) it has an operating battery life of at least 48 hours. The AS/NZS 4280.2 standard, governing the obligatory characteristics of a PLB, on the other hand, does not require that it float the right way up when in the water and, furthermore, only requires that it have a battery life of at least 24 hours.

The difference between EPIRBs and a PLBs is important to understand. And this is so because all Australian state maritime authorities have regulations in place concerning the carrying of EPIRBs. And all state maritime authorities make it very clear that an EPIRB is a device that complies with AS/NZS 4280.1. It is a device that when activated will float upright and will have a minimum battery life of 48 hours. In terms of state regulations governing the carrying of EPIRBs therefore, a PLB is not a compliant device!

Where PLBs have a particularly applicable fit (in addition to the carrying of an EPIRB), is the covering off of circumstances where a crew member may become parted from their vessel – falling over the side. Lone tinnie fisherman and the crews of racing yachts are particularly more vulnerable. And if the EPIRB is safely stowed on the vessel the crew member just fell off, the EPIRB is not likely to be of much help. Notwithstanding certain technical limitations of a PLB, it’s a whole lot better than nothing at all. Yachting Australia recognises this and requires that all crew members competing in Category 1 and 2 offshore events wear a PLB when on deck (Racing Rules of Sailing 2009 to 2012 – Rule 5.05.1).

Whether purchasing an EPIRB or a PLB, a decision has to be made whether the device should be GPS equipped or not GPS equipped. The difference in price is probably in the range of $100 to $200. But when this difference translates into the time it takes to be rescued, it might represent no difference at all! As already discussed, a GPS equipped device is capable of indicating its own position to search and rescue authorities within a matter of minutes. And that position will be accurate to within a radius of 0.5 nautical miles. A non GPS equipped device, on the other hand, has no ability to indicate its own position. It has to be found – by low orbit LEOSAR satellites involving a process that could take up to several hours and resulting in a derived position accurate to a considerably greater radius of 2.3 nautical miles. As it concerns the purchase of EPIRBs for yachts racing in offshore events, attention should be drawn to Yachting Australia’s Racing Rules of Sailing 2013 to 2016, which, it is understood, will require that after 2015, all EPIRBs be GPS equipped.

The only other features concerning the purchase of EPIRBs, in particular, concern methods of activation and deployment. Most EPIRBs are supplied with a cradle into which they are housed. The cradle, in turn, is attached to a bulkhead or other vertical surface. Most EPIRBs are activated when (a) they are removed from their cradle and, by such removal, armed and (b) come into contact with water. Both have to have occurred for the beacon to activate. Typically, such units also provide for manual activation by way of a lever switch. But there are other EPIRBs available that rely only on manual activation. Which of these to choose from clearly depends on the particular application. Also available for use with water activated units are float free devices. These are essentially capsules that house the EPIRB and are attached to the vessel somewhere on deck. The capsule automatically (hydrostatic release mechanism) releases the EPIRB once the vessel sinks – it floats free and automatically activates by contact with the water.

Conclusion: 406 MHz distress beacons are highly sophisticated pieces of electronic engineering designed to save lives. Developing some sort of understanding as to how they and the global safety system works is not only interesting but might also assist in determining what sort of devices would best suit particular boating activity. In this regard, it is also recommended that boaters fully familiarise themselves with regulations governing the carrying of EPIRBs relevant to their particular area of boating activity and the steps required to be taken to register beacons with the relevant search and rescue authority (if in Australia, AMSA).