406MHz & 121.5/GPS EPIRB Accuracy

[no sig]

406 MHz/GPS EPIRB Accuracy

I am wondering if the GPS equipped EPIRB’s really offer such a great advantage, in SAR terms, over the standard 406MHz/121.5 MHz EPIRB.

On the face of it, it seems obvious if your position can be fixed to within meters rather than kilometres that one should go for the GPS 406 MHz EPIRB. However, in practice I wonder if it makes that much of a difference; working on the assumption the last leg of a recovery is done homing to the 121.5 signal and visual sighting. Also, is there any significant time lag between satellite passes and updated GPS fixes? I’d appreciate the views of any SAR crews on how it works in real life. Many thanks.

[Could be the last?]

You need to speak with the ICE IPT, RWOETU or the RCC.

There is a big change happening next year and although terminal homing won't be affected the original 'ping' will need the 406 and GPS to get the system going. Like I said speak with those above and they will give you the lowdown.:ok:

[henry crun]

Satellite monitoring of 121.5 will stop in Feb 2009, which will, in effect, make that frequency obsolete as far as EPRBs are concerned.

[Lima Juliet]

From the COSPAS/SARSAT website:

The digital 406 MHz beacons offer many advantages over analog 121.5/243 MHz beacons. With a 406 MHz beacon, the position of the distress can be relayed to rescue services more quickly, more reliably and with greater accuracy.

406 MHz Beacon Position Accuracy Within 5 km (Doppler), 100m if GNSS (GPS) position is encoded in message

121.5/243 MHz Beacon Within 20 km (Doppler only)


With a 121.5/243 MHz beacon, only one alert out of every 50 alerts is a genuine distress situation. This has a significant effect on the resources of search and rescue (SAR) services. With 406 MHz beacons, false alerts have been considerably reduced (about one alert in 17 is genuine) and when properly registered can normally be resolved with a telephone call to the beacon owner using the encoded beacon identification. Consequently, real alerts can receive the attention they deserve.

For further see:

http://www.cospas-sarsat.org/FirstPage/121.5PhaseOut.htm

[Brian Abraham]

Going back some years we had a 406 fire off (encoded with GPS and aircraft ident). Hard to recall the exact time interval but seemed like the search centre was onto ops within a couple of minutes of it going off. Was darn quick in any event.

[no sig]

As I understand it, the 121.50 homing frequency in EPIRB/ELT's will be retained even though the 121.50 SAT fixing service is terminated next year. The advantages of 406MHz is clear; the question I ask is whether 406MHz with 121,5 homing is good enough on its own, or if in real life SAR the EPIRB's with the GPS offers a real and significant advantage to the RCC/SAR crew?

[rjtjrt]

I stand to be corrected bit the biggest advantage is the speed with which SAR can be dispatched.
406 non GPS - they receive an immediate indication of beacon and registration on Geostationary satellite but poor location info until next low orbit satellite passes so can pinpoint location using Doppler to if I recall 5km.
With 406GPS the location to 10metersis received by Geostationary sat so no need to await low orbit sat/doppler pass.
John

[artyhug]

Rjtjrt that is my understanding as an operator in extremis of said beacons too. You'd hope having only had a refresher on the systems a couple of weeks ago I could remember the details, my only excuse was that the lecture was in Dutch...

406 gives immediate notification of a signal due to satellite geosynchrosity, without the GPS signal as you stated you wait a finite amount of time, up to an hour I think, for subsequent refinement from lower earth orbit satellites operating on 121.5 (and 243 ?).

[no sig]

Thanks for that. I suppose it boils down to the benefit of having, at first activation, a goood fix on you rather than the ARCC/MRCC waiting for a second pass for the non-GPS equipped EPIRB. That could make a big differnece in cold water I'd say.

[Vage Rot]

If he were still alive, i'd say ask the Cherokee pilot that ditched the other week! We got 1 406 hit with a fairly large datum - found nothing. A GPS hit would have given us a nice tight datum - and we all like tight datums!!:):}

[no sig]

Vage Rot

Yes, well that sort of makes the case for GPS then. So, in the case you mention, the first basic 406 position can be quite inaccurate and it presumably gets refinded with subsequent passes. Did you have 121.5 to home to?

[[email protected]]

The problem with 406 beacons, and the reason they have a 121.5 facility as well, is that unlike PLBs or SARBEs the signal is not continuous, it is a digital databurst transmission approximately every 30 seconds making conventional homing more difficult. The embedded GPS option is the desired one so every satellite hit gives an accurate position without any of the doppler errors we see from 121.5 or 243 on the cospas/ sarsat system.

One problem that is yet to be resolved is that the link from the LUT - land user terminal -(where the signal is downlinked), and the ARCCK is by BT phone line which doesn't have the bandwidth to cope with the information so some inaccuracies and data processing errors are inevitable.

Apparently the beacons should have been 486 (a harmonic of 121.5 and 243) but a printing error got too far along the production line for it to be changed - not unlike EHI - 01 becoming EH101:)

[tucumseh]

“Apparently the beacons should have been 486 (a harmonic of 121.5 and 243)”


Careful Crab, the In Car Entertainment (ICE) IPT will topple at such techie talk.:)

[canard68]

486mhz would be in the UHF tv band.

[melipona]

“Apparently the beacons should have been 486 (a harmonic of 121.5 and 243)”

Urban myth, 486 is now an allocated GSM frequency.

[Donna K Babbs]

GPS integrated 406 MHz beacons have two distinct advantage. Firstly they are generally more accurate than a doppler derived Leosat position, and secondly the speed of detection is almost instantaneous (in practice around 15 seconds).

With any traditional beacon, be it 121.5, 243.0 or 406Mhz, the position is gained following 2 Leosat (Low Earth Orbitting Satellite) passes. On the first pass two positions are produced, equidistant from and tangental to the satellite's track. At this stage the satellite cannot establish which is the transmission position and which is the mirror image. When the second pass happens two positions are also generated but one will match the position of on from the first pass, therefore resolving the position.

The second pass may be 20 minutes later, or up to 3 hours later, depending on the time of day. The 105 minutes quoted as the time between passes is an average and is actually the time taken for a satellite to orbit the earth. Positional accuracy of a resolved non-GPS 406 position are generally very accurate and, although a range of 2.7nm is generally quoted, most are within half a mile.

The 406 non-GPS beacons, being digitally coded, can sometimes be resolved (the correct position determined) from the beacon coding. If a UK ELT (aircraft) beacon gives one position near Heathrow and other in Poland there is a likelihood that the first position would be correct but this cannot be assumed. Registration of the 406 beacons helps in a similar way to rule out one position through detective work by the UKMCC operator.

The GEOSAT (GEOstationary SATellite) system remains in a fixed orbit relative the earth and covers a much wider area. Detections on this system are near instantaneous but do not give positional information, only the beacon serial number. 406 GPS beacons are detected on this system and do no need to rely on a LEOSAT pass to gain a position and action can be taken immediately.

406 beacons only transmit every 50 seconds in a data burst and UK Sea Kings cannot home on this frequency, which is where the 121.5/243 terminal homing comes in.

Sorry for the technogeek speak but there are lots of misconceptions, even in the SAR world, as to how the system actually works.

[no sig]

Donna K

Thanks for the detailed explanation, that clears up a number of questions; it seems that the bottom line, is that the GPS position data buys you precious time and removes much doubt in the RCC. But one question which remains in my mind is; during the run in to the beacon position, are the SAR crews now more relient on the SAT position ARCC/MRCC give them or on homing onto the 121.5 signal? I guess I'm asking if the positions you get are now sufficiently accurate that the 121.5 homing has become secondary to the sat. fix?

[SkyCamMK]

Don't forget the UK lifeboats also use the homing on 121.5 We part time sailors have heard of impressive results from the GPS PLB's from around the world. It seems without GPS sharks and hypothermia can have a field day before the SAR/CG turn up!

[no sig]

Good point SkyCam. I'm hoping VHF/HF DSC with position will save the day should me and my boat ever be in need, but my next EPIRB is GPS equipped for sure.

[Donna K Babbs]

There are still surprisingly few 406 GPS beacons. The position passed by UKMCC in nearly all cases is still based on the doppler position. In either case, the satellite position is merely a datum whereas the terminal homing signal on 121.5 and/or 243 indicates where the beacon is, rather than where it was.

Another misnomer is that only the military use 243. There are a significant number of civilian EPIRB and ELT beacons that are tri-band.

[ARINC661]

Modern Direction Finders (Chelton is one manufacturer) fitted to some SAR helicopters can decode the GPS position transmitted on the 406 frequency. This means you can plot the position into you nav system and fly directly to it without having to home down the bearing.

In theory you could start an transition into the hover as you approach, arrive at the most recently transmitted position and expect to find the Life raft or survivor very close to you.

[no sig]

AIRINC661

That clinches it! the case for 406 GPS EPIRB's is made.

[melipona]

The GPS position encoded into the 406 beacon signal is only accurate to around 100m, this is a technical limitation due to the way that the information is encoded. (The beacon knows exactly where it is but cant send out the last few metres).
Whilst this may be adequate in good visibility, imagine a dark and stormy sea rescue, then the GPS will get the rescue craft to a relatively small search area, (it can be directed there and so does not need to be able to receive the 406 transmission) and the homing signal will get the craft onto the beacon.

The new system allows a more rapid and appropriate response to an alert.

[[email protected]]

ARINC - I think you'll find most SAR crews would still go for a positive on-top to identify the position of the beacon using 121.5 and then letdown using overfly or similar rather than searching around in the hover taxy at 50' in the dark.

50 secs - 30 secs it's still a databurst transmission you can't home to:)

[ARINC661]

melipona

I was not aware that the encoded resolution was limited to 100m, I accept that in very bad conditions you will need to home the last few hundred metres. There are systems that will home on 406, they store the bearing between received signals. If the sea state was very severe then an older 121/246 receiver could struggle due to the intermittent signal caused by wave obscuration and wash over.

There is an interesting article here: http://www.uscg.mil/acquisition/newsroom/feature/paradox.asp about a USCG C-130 with a 406 receiver that found a capsized yacht after other A/C failed.

My father sometimes races single handed, he wears a GPS 406 PLB attached to his life jacket. I hope that if he were to go over the side then the accuracy of the GPS PLB would enable him to be rescued quicker.

[melipona]

The beacon protocol allows the GPS encoded signal to be accurate to 4sec, this equates to about 400ft at the equator and reduces in size as you go towards the poles.

The disadvantages you mention apply to all antenna systems, and the gps signal the beacon is searching for is particularly vulnerable.

More modern homing systems can use alternative signal discrimination systems to improve their reliability and accuracy for weak, intermittent and multiple received signals. However you can't choose who rescues you and most of the world will still use simple phase/amplitude homing systems.

Another advantage of 406 is that it is at 5W, whilst 121 & 243 transmit at around 100mW, so if a suitable homer is used then 406 may be easier to lock on to despite its pulsed nature. Mindful that beacons, particularly for aviators, needs to be effective in all terrains then the 5W will provide a greater "punch through" in for example heavily forested areas where the tree canopy absorbs the transmitted signal.

Here's hoping that your dad never has to actually use his beacon:ok: and that he, and all other 406 owners, have properly registered their beacon with the appropriate authority, the information that is held on the data base about the beacon, its owner and associated craft will be useful to the rescue organisations and for quickly closing down false alarms should they occur.

[[email protected]]

ARINC - if you read that article it says that the DF equipment on that USCG C130 switches to 121.5 automatically at 15 nm from the target position - therefore 406 was used as an area weapon but the precise location was still from 121.5.

A satellite hit with encoded GPS position would have provided at least as accurate an area to start a 121.5 homing in.

52 seconds is a long time in a fixed wing doing 300kts and, whilst the bearing might be retained by the DF kit, you will have travelled nearly 5 miles before the next transmission is received - unless you are inordinately lucky you will not find the beacon with 406 alone.

[Fareastdriver]

There still seems to be a miscomprehension that GPS satellites are fixed in space or in geosynchronous orbit. They’re not. There is about thirty or so and they go around the world in different orbits arranged so that normally there are half a dozen in view anywhere in the world. Your GPS set will tell you the azimuth and elevation of each satellite in turn if you know where to look. Four will give you a positive fix in metres varied by the distance that the aircraft travel between fixes. On the ground it is normally within the undercarriage track.
Five satellites will give you RAIM. (Receiver Autonomous Integrity Monitoring) and this is what you use in Australia and the USA for independent GPS approaches. The harmony between the receiver and the satellites enable the GPS receiver to predict that RAIM will be continuously available for the duration of the approach. Plug in the autopilot and it will fly it for you.
On my operation the first sixty miles is a common route. Should one be outbound and another inbound at the same height on autopilots they would literally slam into each other cockpit to cockpit.
Some time ago I had to pull a Chinese oil worker off a platform in the middle of the night with severe crush injuries. I was required to take him to hospital in Guangzhou which had been recced and photographed in case it was neccessary. The LZ was an undeveloped building site just outside the hospital perimeter. Using the GPS in hold function I made up an approach that brought me to 200ft at one mile. I knew that within that radius there were no buildings higher than the standard seven stories of old Chinese apartments. The visibility was about 0.75 Kms in haze (pollution). At 0,02 on the GPS the building site was picked by the landing light. Flare, indentify an object to land beside, Arctic technique for those that don't know, and land in a cloud of dust. No heroics, it was easy. He lived, twenty minutes later he would have died.
In my view should you end up in the sea, or anywhere for that matter, there is no contest, GPS has it every time.

[steamchicken]

One problem that is yet to be resolved is that the link from the LUT - land user terminal -(where the signal is downlinked), and the ARCCK is by BT phone line which doesn't have the bandwidth to cope with the information so some inaccuracies and data processing errors are inevitable.


Bandwidth? What is it actually sending back? I find this difficult to believe; a serial no, timestamp and position can't be more than a few hundred bytes.

[RS15]

As we all like back-ups... so if your 121.5 fails, homer on aircraft poor ( no, never...)

406 accuracies approx 5 km dop and 100m GPS.

Equivalent to 2.7 and 0.054 nautical miles repectively.

ATP10 / IAMSAR (SAR 'bibles') give sweep width (uncorrected for various other factors) for a person in water of 0.1 nm for a helicopter search. Therefore a single pass would theoretically find the target from a GPS 406 hit; a 406 alone would require 27 passes (area assumed to be squared-off, rather than a circle for ease of description).

now equate that to time at 60kts gnd spd (probably too fast for single person in water but makes maths easier). GPS = practically INSTANT, 406 alone = (2.7 * 27) /60 = 1.2 hours!! and this doesnt take into account turns etc.

HOWEVER... this all assumes that the person hasnt moved from the inital datum (tides etc) so the final 121.5 homing is still valid if available.

bored now.....

[no sig]

RS15

Your last post begs the questions, how close can you get with 121.5 homing alone?

[RS15]

no sig

121.5 = as close as you need. ie close enough to touch.

but... assuming the beacon works, aircraft homing instrument is accurate, the antenna is not being shielded by terrain / waves, no secondary reflections from terrain etc etc.

Me (do lots of nautical stuff for fun), given the choice, would use GPS 406 - extra redundancy. My body is worth more than a few quid for the GPS extra (although some may disagree).

[no sig]

RS15

I'm sure you're right. This has been interesting however; there are many variables which seem to come into play when tracking down a EPIRB. As you point out, should you ever need to use one in earnest you do want everything going for you. Thanks for the insight.

[Eweturn]

Hello, I don't know if this string is dead and buried, but noted a few bits on it that might be added to, so here goes (new at this, so go easy). It will be longer than your usual response, but you can always ignore it if it's totally boring.
All of the PLB functions are very good - if the conditions are right and it all works as advertised. Homing is fine, but performance is variable. If the beacon is activated on land, it depends on what obstructions are in the way, if the antenna is earthing, its attitude etc. Over and above this (pardon the punning) the performance of the helicopters' homing equipment varies from aircraft to aircraft, so there is no definite standard there either. If the beacon is in the water, it will depend on how rough the sea is, how low the antenna is, whether it is partly in the water or out of it, etc, etc.
On to the GPS bit. An increasing percentage of beacons have GPS receivers. Personally, I wouldn't consider buying a beacon that didn't have an integrated GPS receiver. The first generation of GPS beacons were slow to acquire and often didn't produce an embedded (within the data-burst) GPS position at all. They also got a bad name because, for various reasons, completely erroneous positions were produced that were outside the satellites' footprints and therefore definitely wrong. Even within the Cospas-Sarsat community, the positions provided were then viewed with considerable suspiciion. However, that has changed. The GPS receiver modules on modern beacons are now much improved and they 'acquire' (in reasonable conditions) very quickly. One recent UK-manufactured beacon tested provided a burst with a refined GPS position that was bang on, and got to the Cospas-Sarsat geostationary satellite in only 49 seconds from beacon activation. It takes about 3 or so minutes for the number-crunching to be done and for the distress-alert to arrive on the screen at the UKMCC. The approximately half-second data-bursts transmitted by the beacons (roughly 52 seconds apart, slightly randomised), contain separate data-fields. The first field has information like the unique identity; registration nationality; beacon type (whether EPIRB, ELT or PLB); and the coarse GPS position - if available. This coarse and refined position bit then:- There are two 'protected' fields in the databurst and the first check-summed field usually gets through. But it only has enough digits to provide a rough location. The second protected field contains the more precise. refined detail. There are a number of possible protocols that the beacon can be coded with, but the National Location Protocol provides the coarse position to 2 minutes and the refined position to 4 seconds (the Standard Location Protocol provides 15 minutes coarse in the first field and again, 4 seconds in the second protected field). There is a clear advantage in using National Location Protocol because that narrows down the coarse position to 2 minutes right away. The second field often takes a while to get through (if ever, if something bad is happening to the beacon like sinking or burning). 4 seconds equates to about 124 metres which is good enough for government work and will usually, except possibly in jungle, forest or vertical ground, be good enough to quickly find the casualty(ies).
Mentioned the variability of homing earlier on. Well, whether the beacon gets through to the geostationary satellites is also a bit variable and depends on whether or not it has clear 'sight' of the satellite. If the antenna is lying on the ground or obstructed, then it may or may not get through. Quite often, the beacon ID will get through, but the GPS, embedded location will not, always assuming of course that the beacon has managed to acquire the GPS location.
If the beacon has managed to acquire, but can't reach the geostationary satellite (about 36,000 miles above the equator), it has a much better chance getting through to the lower (700-1000km altitude) Low Earth Orbiting - pole-to-pole satellites - the trouble is here that instead of getting a near-instant GPS-derived location, there may be a wait until a LEO satellite that can 'see' the beacon comes round. Even if that satellite can't in turn see the UK tracking station, called Local User Terminals (LUTs - who thinks these meaningless acronyms up?), it will download the beacon's information to the first overseas LUT that it sees and its linked MCC will relay the data to the UKMCC to find out the associated registration details - and the MCC whose service area the beacon is within. That MCC will then pass it to the responsible RCC, whether that be an ARCC or a MRCC, or both. The MCC Operators give value-added advice and keep the RCCs apprised of refining Doppler locations and any change to the GPS position. Hope that's explained some of the bits and pieces. Sorry it's so long.

[no sig]

Eweturn, please don't apologise- that was a very interesting post, thanks.

But in light of the varibles you speak of in terms of obstructions and perhaps wave heights obscuring signal. I'm trying to understand why the recommendation is that the EPRIB beacon is best in the water, rather than being in the liferaft where a clearer view of the sky might be had; perhaps the saltwater offers a better ground plane for the signal? or maybe people holding a beacons themselves might obstruct the signal? It's clear from what you say that an unobstructed view of the sky has a marked efect on performance.

[Eweturn]

Hello NoSig. EPIRBs are specifically designed to be at their best floating in the water - their antenna characteristics are optimised for this. PLBs are a jacks-of-all-trades compromise, but still provide a respectable performance so long as the antenna is out of the water. Both transmitters deliver a 5-watt burst. All beacons are now tested in a variety of positions/situations with different ground-planes, etc, before they get their Cospas-Sarsat type-approval certification, but EPIRBs still have to deliver the goods floating in the water, whereas PLBs don't. In theory, PLBs should give a good performance with the antenna as high as possible on the outer liferaft canopy. This will give a little extra height and if the canopy is wet with salt water, that may improve rather than diminish the performance. Being tossed around in the waves doesn't seem to help with the GPS acquisition and if the waves are big, this will obscure the signal at times from the geostationary Cospas-Sarsat satellites.

Worst of all is to have beacons on the floor inside liferafts with people crowded around them. Get them high. If EPIRBs (bear in mind that aircrew and aircraft should have ELTs and PLBs rather than EPIRBs, but if the liferaft comes from a boat, then this will be relevant) are tethered to liferafts, there is a danger that the occupants get hit on the head through the canopy when things are rough, and of course the bit of cord can break. If this happens, it will be most unusual if the EPIRB and liferaft float at the same speed, so the EPIRB could be many miles away when the helicopter homes to it.

By the way, and despite what I've put above, I'd reckon (personal view only) that a high EPIRB and antenna (above about 5 feet) would probably outperform one in the water, particularly if it was rough. Have used VHF radios for much of my working life and reckon antenna height usually wins over things like ground planes, etc Cheers.