EPIRBs and PLBs are about to get a lot smarter, making faster rescues possible
The global network that receives distress signals from EPIRBs and PLBs is being upgraded. James Turner finds out about MEOSAR and its benefits for sailors
Did you know that the way your EPIRB's distress signal is received is evolving? Soon it will be possible for the search and rescue authorities to find out where you are within minutes and let you know they are on their way. New EPIRBs are being developed to take advantage of this new technology, but your existing 406MHz EPIB will still work with the new system.
The 406MHz rescue system was originally implemented in the 1980s, and is run by an organization called COSPAS/ SARSAT, a not-for-profit collaboration of 40 countries. In a nutshell, your emergency transmitter sends a signal that is picked up by a satellite and relayed to a ground station. It is not necessary to have dedicated satellites for this task, as the radio repeaters used to re-transmit the emergency signals to earth stations are quite small, so they share space on other spacecraft. The original repeaters
were on Russian and American weather satellites in low polar orbit, and the more recent geostationary repeaters (geosynchronous, if you’re being fussy) are on satellites belonging to METEOSAT (Europe), GOES (USA), INSAT (India) and Electro (Russia). These later satellites all sit high above the equator, so they are not brilliant for users in high latitudes as the satellites are seen to be low in the sky and a long way off.
To understand what the future holds with the upcoming MEOSAR system, it is first important to understand the workings of the previous/existing system.
When you activate your 406MHz beacon (PLB or EPIRB), the signal is received either by one of the geostationary satellites (GEOSAR), which record the identity of the beacon and its lat/long from the beacon's in-built GPS, or by one of the polar-orbiting satellites (LEOSAR), early ones of which were not equipped to pass on GPS data.
Doppler shift
The polar orbiting LEOSAR satellites, which move very fast across the earth’s surface, didn’t originally handle GPS, as they predated its invention. Instead, a position line is computed using Doppler shift. Beacons have to transmit a very precise frequency, which is one of the reasons they cost so much money.
As the satellite approaches the beacon, the frequency it receives is higher than the frequency transmitted by the beacon. Once it passes the beacon and is on its way to the pole, the frequency received is lower than the frequency transmitted by the beacon. At the point when the frequency is exactly correct, the satellite is at its nearest point to the beacon, so a position line is established.
Each successive satellite pass over the beacon produces a different position line so after two passes a rough position is known (a simple cross), and with each pass – typically an hour apart – the fix accuracy improves (a triangulated fix). Over time a more accurate fix can be established this way, but the process is slow. This system is still used today, though the more recent LEO satellites do store and pass on GPS positions to the ground stations, so the Doppler shift process isn’t always required.
The higher your latitude, the lower the chance of a geostationary satellite picking up your signal (with GPS) because of the increasing distance to the equator, combined with the higher altitude of the satellite. In high latitudes or on land, mountainous terrain will also restrict line-of-sight to the satellites over the equator. In practice, this means that with the present system, you may be lucky and rescue services may know of your plight in 10 or 20 minutes, but if your GPS position hasn’t been forwarded, or if your beacon does not have a built-in GPS, it could be quite a few hours before your precise position is known.
Whichever satellite picks up the distress signal, it is forwarded to a ground station in a global network, which in turn is connected to professional search and rescue services.
One limitation of the present system is that the user who has activated their emergency beacon has no feedback, no confirmation that their signal has been received and that help is on the way. It’s a given that it has been received, nevertheless it would be a great comfort to actually get a message back to say so.
How effective has COSPAS/SARSAT been?
To date, the COSPAS/SARSAT system has helped with the rescue of around 42,000 people in over 12,000 SAR events. I myself was rescued in 1986 after setting off a 121.5MHz COSPAS/SARSAT beacon (the predecessor to the 406MHz system) when my boat was in dire trouble.
It is doubtful that I would have survived to write this article, 31 years on, had I not used that beacon.
What is MEOSAR?
MEOSAR stands for Medium Earth Orbit Search And Rescue. MEOSAR's stated purpose is ‘to improve on time delays in the current system and to provide instant pinpoint positioning without having to rely on GPS’. MEOSAR satellites move over the earth’s surface (unlike GEOSAR) and are at a lower altitude.
Now don’t go throwing away your EPIRB or PLB just yet. It will work perfectly well with the MEOSAR system. MEOSAR is already being deployed, but has not yet been declared fully operational, though it is already starting to take the place of the LEO polar-orbiting satellites.
MEOSAR space segments – the radio repeaters and transponders – are being carried by new satellites in a number of different networks: Galileo, the European navigation satellites; Glonass, the Russian navigation network; and GPS DASS from the USA.
How does MEOSAR do the two things it’s meant to do, to find you quickly, and to do so without the need for GPS? The MEOSAR constellation is much bigger than
the LEO and GEO systems, and in the same way that a GPS receiver views a number of GPS satellites to compute its position, so a beacon in the MEOSAR system can be located by the MEOSAR satellites with the same level of accuracy. Lots of satellites see it at once, measure its transmission, and – in simplistic terms – triangulate the location.
A recent rescue in Australia illustrated how good MEOSAR is. A non-GPSequipped EPIRB was activated and the MEO satellites, working in conjunction with the ground station, were able to calculate the exact position within 10 minutes. Compared with 2-4 hours for the LEO system, that's truly remarkable.
What do I need to take advantage of MEOSAR?
There is no update necessary to existing PLBs and EPIRBs, so just ensure you are 406MHz-equipped. Remember, there are far fewer people looking out for you these days, even for coastal sailing round Britain’s shores. To go to sea without an EPIRB for the boat and PLBs for the crew, you are taking a risk you need not take.
One feature that will require a new beacon is the planned ability to offer a return link transmission, so the user receives confirmation that the distress message has been received and help is on its way.
The beacon manufacturers are being a bit cagy about when this feature will be available, and to date there aren’t any on the market that offer the return link, but it’s something to look out for in future products, a few years down the line. One thing’s for sure, such beacons will be a lot more expensive. Current products have only a transmitter. Beacons with a return link capability will have to incorporate a receiver as well, which means larger, more complex circuitry.
When does MEOSAR come into operation?
It’s happening already, quite seamlessly. The constellation should be complete by 2019. In the meantime, the LEO and GEO systems continue to fill any gaps in the MEOSAR network. Once MEOSAR is fully functional, the LEO system will be switched off, though the GEO system will stay to back up the MEO system. From a user point of view, the change to the MEOSAR system is seamless. Already, we are seeing response times come down, and it’s only going to get better. The return link is looking a bit like a pipedream for now, but rest assured Yachting Monthly will keep you informed of developments as they happen.