Ocean Navigator Communications Newsletter #29

EPIRBs: The Modern Call for Help

In 1979 (ancient history when considered within the maritime satellite communications timeline), Canada, France, the USA, and the USSR signed a memorandum of understanding that established a satellite based system to reduce the time it takes to detect and locate distress situations around the world. Of primary importance were aircraft groundings and maritime casualties. The system was appropriately labelled the Search and Rescue Satellite-Aided Tracking (SARSAT) system, which when combined with its Russian equivalent acronym yielded the COSPAS-SARSAT system. For us sailing folk, this organization is most recognizable as the force supporting our Emergency Position Indicating Radio Beacons (EPIRBs).

Older EPIRB models emitted a signal on a frequency of 121.5 MHz. These models relied on satellite in a low-Earth orbit to relay the signals back to a nearby ground receiving station. As the satellites flew overhead, they acted in a sense as mirrors, reflecting the signal from the EPIRB to the rescue coordination center. However, this limited its usefulness to times when a satellite was in view of both the beacon and a ground receiving station. On the other hand, the quickly moving satellites were able to locate the beacon by making use of the Doppler principle. Think of it this way -- if you're standing near some train tracks that are obstructed from your view, you can still tell when the train passes by listening to the train whistle switch from a higher pitch to a lower one as it passes. In a similar way, the satellite can tell when it's passing over a transmitting beacon. After a few passes, the location can be pinpointed to within about 20 kilometers. That's an awfully big area of ocean to search.

Improvements were quickly implemented - and we now have use of beacons that operate on a new frequency of 406 MHz. The change in frequency alone doesn't solve much, but along with this change came new satellites and new methods. Newer 406 MHz EPIRBs (and their aviation equivalents - Emergency Locator Transmitters or ELTs) send their signal both to the old quickly moving low Earth orbit satellites and to geostationary satellites in high Earth orbit that hover over a single point on the Earth's surface. In addition, they are also able to transmit a digital message containing the identification of the vessel in distress. This message, which was not present on the older 121.5 MHz EPIRBs, significantly reduced the number of false alarms that clogged the COSPAS-SARSAT system at times. Since the geostationary satellites are always within view from any location on Earth (except for the poles), the distress signal relay is almost instantaneous, with a ground station receiving notice of a distress within seconds. Whereas waiting for a low Earth orbit satellite to pass into view entails an average wait of about 45 minutes, with some wait-times approaching two hours. Of course, even with the almost instant notification of the 406 MHz beacons from the geostationary satellites, the authorities still need to wait for a couple of low Earth orbit satellite passes in order to derive the location of the distress through the Doppler principle. The good news is that the newer beacons reduced the search area to about 5 kilometers.

This last hurdle of problems associated with Doppler location has been addressed by EPIRBs that include the ability to transmit a location fed by a GPS receiver either built-in or connected to the beacon. These GPS-enabled EPIRBs or GPIRBs are able to notify authorities within seconds of activation and typically result in a search area of 150 meters! This makes the GPIRB well worth the price premium over a conventional 406 MHz EPIRB. The older 121.5 MHz EPIRBs have been largely discontinued and will eventually be phased out by COSPAS-SARSAT (though a 121.5 MHz homing frequency is still used on the newer beacons to help "home in" once the rescuers are on the scene).

With just about one million beacons worldwide, and about 1,500 people rescued each year (the vast majority of which are maritime events), the COSPAS-SARSAT/EPIRB system is firmly established and is a valuable tool in your emergency planning. Just don't forget to register the beacon when you buy it! The registration is one of the few government forms that can be submitted without any payment. It's free, and it's a lifesaver.

Keep in touch.

- Dan Piltch
dpiltch@MarineComputer.com