406MHz & 121.5/GPS EPIRB Accuracy
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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.
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.
Last edited by ARINC661; 6th Mar 2008 at 22:29.
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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.
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.
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
50 secs - 30 secs it's still a databurst transmission you can't home to
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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/news...re/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.
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/news...re/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.
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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
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.
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
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.
Last edited by melipona; 8th Mar 2008 at 12:34.
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.
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.
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.
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.
Last edited by Fareastdriver; 9th Mar 2008 at 13:14.
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.
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406 GPS vs Dop
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.....
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.....
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121.5
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).
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).
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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.
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.
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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.
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.
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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.
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.
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Response to NoSig
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.
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.
