Beacons of hope

An electronic MOB device might one day save your life. In a complex marketplace, Nigel Sharp explains the jargon and assesses what is on offer

- Nigel Sharp

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.”