As satellite communications have developed, so too have the ever more ingenious devices that can save us if we fall overboard. It is a world of tech that is worth understanding for obvious reasons, but for the beginner, abbreviations proliferate. The term PLB (Personal Locator Beacon) is often incorrectly applied to any type of MOB electronic device. It should be used only when referring to the type that works on the same principle as an EPIRB (Emergency Position Indicating Radio Beacon), which will then distinguish it from two others, the AIS (Automatic Identification System) and OLAS (Overboard Location Alert System) types.
What, you might ask, is the difference?
Personal Locator Beacons
A PLB has always worked on the same principle as an EPIRB – that when it is activated it sends out two signals, one on the 406MHz frequency and one on 121.5MHz. The 406MHz transmission is picked up by the COSPAS-SARSAT satellite system and then relayed via a local user terminal (LUT) to the Rescue Coordination Centre nearest to the incident. To locate the casualty, the search and rescue services initially use the 406MHz signal, which guides them to within a few miles. The 121.5MHz transmission provides a homing signal that takes them to within about 100 metres, but nowadays most PLBs and EPIRBs take advantage of the increased number of satellites in the GNSS (Global Navigation Satellite System) and MEOSAR (Medium Earth Orbiting Search and Rescue) systems, which allow significantly faster and more accurate searches. “It is still a legal requirement for EPIRBs and PLBs to transmit a 121.5MHz signal,” said Sean McCrystal of Orolia, which manufactures McMurdo and Kannard products, “but with the European Galileo satellite system now joining the USA’s GPS system, GNSS accuracy is massively improved.”
It is also a legal requirement that PLBs should be able to transmit for a minimum of 24 hours, and most have a battery life of up to seven years. They also display some sort of bright light to help with the search.
As with EPIRBs, PLBs must be registered (with the MCA in the UK), but unlike EPIRBs, by law PLBs cannot be activated automatically. “A possible change to the law is frequently discussed, but it is unlikely to happen,” said Ocean Signal’s James Hewitt. “There is legitimate concern regarding the potential increase in false alarms. There are enough of those already with EPIRBs.”
An added function that will soon be available to PLBs is the Return Link Service (RLS). This is due to become operational this summer and will allow a message to be sent back to PLBs to confirm that its 406MHz signal has been received by a Local User Terminal (LUT). Most PLBs currently on the market are not designed to receive such messages, but two that are ready to do so are ACR’s ResQLink View and McMurdo’s FastFind ReturnLink. This reassurance signal is likely to be of priceless psychological help to a casualty who may otherwise have thought that all hope was lost.
Both ACR and Ocean Signal offer to replace any PLBs which have been used in anger in exchange for a “survivor story” that can be published on their websites. They currently have about 70 such stories between them.
AIS was originally designed as a collision avoidance and safety system for commercial vessels; it became mandatory in 2002. It is now also used extensively by pleasure boats for the same reasons. Since 2012, it has also been possible to use the system to help rescue casualties after they have fallen overboard – first by alerting the parent vessel (and other local vessels equipped with AIS-enabled instruments) to a man overboard incident and then by helping to locate the casualty. AIS-MOB devices are commonly attached to the oral tube of a lifejacket and most can be automatically activated. The activation can be prompted either by contact with water (WamBlee’s RescueMe 420-LP and W420 OneMan MOB AIS, for instance, or Weather Dock’s easy ONE and easy RESCUE-PRO) or when the life jacket itself inflates (such as the Kannard R10, specifically developed for the Volvo Ocean Race, in which, unsurprisingly, water-activated devices are not considered a good idea). The latter typically has a line wrapped around the deflated bladder and connected to the AIS MOB device’s activation switch. When the bladder inflates, the line is tensioned and activates the device.
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