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

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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.

Ex­ist­ing 406MHz EPIRBs will still work un­der the new sys­tem

Per­sonal lo­ca­tor bea­cons need to be held out of the wa­ter and the an­tenna must be ver­ti­cal

An EPIRB works best when float­ing – if ac­ti­vat­ing it on board, place it in bucket of wa­ter

A EPIRB or PLB could be your last hope of res­cue, should a catas­tro­phe hap­pen

Res­cue co­or­di­na­tion cen­tres re­ceive dis­tress calls and pass them on to lo­cal SAR ser­vices

A Galileo satel­lite form­ing part of the new MEOSAR sys­tem

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