GPS fails over Shawbury
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Ok i will list the things I can think off that could cause signal loss.
1. Military Jamming which can be done without NOTAM and will effect both roof and handheld.
2. Illegal Jamming some little baw bag has bought a jammer on holiday or has made one for the crack and has turned it on. Roof mounts should be better due to the airframe shielding them from it.
3. Terrian shielding. Not Much of a problem in the SE but wales and Highlands in the glens it is an issue. The roof mounts will be better but it will still cause problems as they will only be able to pick up the straight up contacts. Which as we know from the GPS sphere theory gives you the worst fix. You might say so what. But if you decide to go up the wrong glen your maybe looking at trying to turn in a climbing situation as the sides come in.
4. Wx. Anything which contains large amounts of water will block the signal.
5. EMC clutter as the previous poster mentioned (hadn't thought of this before but radius squared rule means that once your above 500ft I would be very doudtfull that it would effect either)
6. EMC hot spots as indicated on charts (bit obvious this one)
7. The yanks start mucking about with the SA on the civi signal.
8. Yanks go to war and decide to turn the civi off.
Numbers 7 and 8 are getting less and less likely. The troops are carrying more and more civi gps because the mil spec ones are very expensive. So if they started mucking about with that it would cause to much problems for allied troops.
9. Some internal problem with the unit.
10. Fault in the wiring to either the unit or the remote reciever.
Have seen this one once with a GA and many times in boats. For VFR club flyers at the weekend I think it will be the most likely.
The likely hood of any of these causes which are no way complete. Is completly dependent on where you fly. JMC starting soon up in the highlands i would suspect there will be lots of drops in signal due to the largest live fire exercise in europe blowing hell out of the north of scotland for a week. Around Leeds the MOD site to the north could be suspect (apart from the ****e wx). Bad wx is suspect everywhere. As for the highlands the hills arn't going to move so you still won't be able to get a signal in the usuall places.
MJ
1. Military Jamming which can be done without NOTAM and will effect both roof and handheld.
2. Illegal Jamming some little baw bag has bought a jammer on holiday or has made one for the crack and has turned it on. Roof mounts should be better due to the airframe shielding them from it.
3. Terrian shielding. Not Much of a problem in the SE but wales and Highlands in the glens it is an issue. The roof mounts will be better but it will still cause problems as they will only be able to pick up the straight up contacts. Which as we know from the GPS sphere theory gives you the worst fix. You might say so what. But if you decide to go up the wrong glen your maybe looking at trying to turn in a climbing situation as the sides come in.
4. Wx. Anything which contains large amounts of water will block the signal.
5. EMC clutter as the previous poster mentioned (hadn't thought of this before but radius squared rule means that once your above 500ft I would be very doudtfull that it would effect either)
6. EMC hot spots as indicated on charts (bit obvious this one)
7. The yanks start mucking about with the SA on the civi signal.
8. Yanks go to war and decide to turn the civi off.
Numbers 7 and 8 are getting less and less likely. The troops are carrying more and more civi gps because the mil spec ones are very expensive. So if they started mucking about with that it would cause to much problems for allied troops.
9. Some internal problem with the unit.
10. Fault in the wiring to either the unit or the remote reciever.
Have seen this one once with a GA and many times in boats. For VFR club flyers at the weekend I think it will be the most likely.
The likely hood of any of these causes which are no way complete. Is completly dependent on where you fly. JMC starting soon up in the highlands i would suspect there will be lots of drops in signal due to the largest live fire exercise in europe blowing hell out of the north of scotland for a week. Around Leeds the MOD site to the north could be suspect (apart from the ****e wx). Bad wx is suspect everywhere. As for the highlands the hills arn't going to move so you still won't be able to get a signal in the usuall places.
MJ
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MJ
"1. Military Jamming which can be done without NOTAM and will effect both roof and handheld."
Agreed
"2. Illegal Jamming some little baw bag has bought a jammer on holiday or has made one for the crack and has turned it on. Roof mounts should be better due to the airframe shielding them from it."
Agreed. Unless somebody floats it off on a baloon.
"3. Terrian shielding. Not Much of a problem in the SE but wales and Highlands in the glens it is an issue. The roof mounts will be better but it will still cause problems as they will only be able to pick up the straight up contacts. Which as we know from the GPS sphere theory gives you the worst fix. You might say so what. But if you decide to go up the wrong glen your maybe looking at trying to turn in a climbing situation as the sides come in."
This one featured in various stuff emanating recently from the CAA and has since been copied by some GA magazines. But it doesn't stand up. Look at how low you would need to be relative to nearby hills in order to create significant shielding. I haven't drawn a diagram but very roughly, you would need to be flying say 3000ft (half a mile!) away from (but parallel to) a 5000ft high rock face, while flying below 1000ft agl, and with another rockface half a mile on the other side, to create a cone with an included angle of 45 degrees. Put it another way, in the bottom of the Grand Canyon.
Many would say that somebody flying thus is an idiot, but certainly he ought to be navigating visually
"4. Wx. Anything which contains large amounts of water will block the signal."
Perhaps a massive downpour might but I have not seen any reception loss in heavy rain, or under/next to really massive CBs, a few miles in diameter (in N Spain). I think this is because, typically, one is receiving 6-8 satellites and they tend to be spread around in the sky at various azimuth angles. I've been looking at this on Oziexplorer which has a satellite display and it is clear enough.
"5. EMC clutter as the previous poster mentioned (hadn't thought of this before but radius squared rule means that once your above 500ft I would be very doudtfull that it would effect either)"
A rooftop aerial would help a lot.
"6. EMC hot spots as indicated on charts (bit obvious this one) "
Yes, a powerful signal could saturate the receiver and it might take a while (minutes) to recover. Again, a rooftop aerial would help a lot from ground based sources, but I have flown close enough to hilltop radars (a few miles away but at a similar level) to hear their RF in the headsets (with the headsets not plugged in!) and the GPS was not affected.
"7. The yanks start mucking about with the SA on the civi signal."
Re-introduction of SA would have no impact on en-route navigation. 200m or so is still good enough. It would muck up useful GPS altitude indication though.
"8. Yanks go to war and decide to turn the civi off."
Agreed, but their economy depends so much on GPS that if they turn it off, the situation is likely to be so dire that most likely no GA will be flying anywhere. They have never turned it off, and despite stories spread by some instructors it was never even degraded, beyond the original SA. The military have developed jamming and that, I expect, would be used to deal with specific threats. It can also be applied to the civilian signal alone.
"9. Some internal problem with the unit."
As likely as anything. Probably the most likely scenario, in a particular installation on a particular flight.
"10. Fault in the wiring to either the unit or the remote reciever.
Have seen this one once with a GA and many times in boats. For VFR club flyers at the weekend I think it will be the most likely."
Sure, as for 9.
The smart thing would be to embrace GPS officially but teach a technique whereby it is used together with checking the displayed features on the map. This implies a moving-map GPS. This could be done at PPL level. Obviously the proper way is to use GPS and ground navaids together but that isn't going to interest the sort of pilots some here think will just rely on GPS completely.
"1. Military Jamming which can be done without NOTAM and will effect both roof and handheld."
Agreed
"2. Illegal Jamming some little baw bag has bought a jammer on holiday or has made one for the crack and has turned it on. Roof mounts should be better due to the airframe shielding them from it."
Agreed. Unless somebody floats it off on a baloon.
"3. Terrian shielding. Not Much of a problem in the SE but wales and Highlands in the glens it is an issue. The roof mounts will be better but it will still cause problems as they will only be able to pick up the straight up contacts. Which as we know from the GPS sphere theory gives you the worst fix. You might say so what. But if you decide to go up the wrong glen your maybe looking at trying to turn in a climbing situation as the sides come in."
This one featured in various stuff emanating recently from the CAA and has since been copied by some GA magazines. But it doesn't stand up. Look at how low you would need to be relative to nearby hills in order to create significant shielding. I haven't drawn a diagram but very roughly, you would need to be flying say 3000ft (half a mile!) away from (but parallel to) a 5000ft high rock face, while flying below 1000ft agl, and with another rockface half a mile on the other side, to create a cone with an included angle of 45 degrees. Put it another way, in the bottom of the Grand Canyon.
Many would say that somebody flying thus is an idiot, but certainly he ought to be navigating visually
"4. Wx. Anything which contains large amounts of water will block the signal."
Perhaps a massive downpour might but I have not seen any reception loss in heavy rain, or under/next to really massive CBs, a few miles in diameter (in N Spain). I think this is because, typically, one is receiving 6-8 satellites and they tend to be spread around in the sky at various azimuth angles. I've been looking at this on Oziexplorer which has a satellite display and it is clear enough.
"5. EMC clutter as the previous poster mentioned (hadn't thought of this before but radius squared rule means that once your above 500ft I would be very doudtfull that it would effect either)"
A rooftop aerial would help a lot.
"6. EMC hot spots as indicated on charts (bit obvious this one) "
Yes, a powerful signal could saturate the receiver and it might take a while (minutes) to recover. Again, a rooftop aerial would help a lot from ground based sources, but I have flown close enough to hilltop radars (a few miles away but at a similar level) to hear their RF in the headsets (with the headsets not plugged in!) and the GPS was not affected.
"7. The yanks start mucking about with the SA on the civi signal."
Re-introduction of SA would have no impact on en-route navigation. 200m or so is still good enough. It would muck up useful GPS altitude indication though.
"8. Yanks go to war and decide to turn the civi off."
Agreed, but their economy depends so much on GPS that if they turn it off, the situation is likely to be so dire that most likely no GA will be flying anywhere. They have never turned it off, and despite stories spread by some instructors it was never even degraded, beyond the original SA. The military have developed jamming and that, I expect, would be used to deal with specific threats. It can also be applied to the civilian signal alone.
"9. Some internal problem with the unit."
As likely as anything. Probably the most likely scenario, in a particular installation on a particular flight.
"10. Fault in the wiring to either the unit or the remote reciever.
Have seen this one once with a GA and many times in boats. For VFR club flyers at the weekend I think it will be the most likely."
Sure, as for 9.
The smart thing would be to embrace GPS officially but teach a technique whereby it is used together with checking the displayed features on the map. This implies a moving-map GPS. This could be done at PPL level. Obviously the proper way is to use GPS and ground navaids together but that isn't going to interest the sort of pilots some here think will just rely on GPS completely.
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Yes we agree. Although I can give you a few routes around the highlands that 3. is a problem even flying at none daft levels. Glen next to Nevis comes to mind.
As for 4 I will admit that maybe most ppls don't have the pleasure of bouncing around at FL140 in the crap dodging CB's on the wx radar for 2 hours. Trust me when i say it happens, and i will admit that its not likely to affect a PPL VFR pilot but it is still a possibility if it all goes pair shaped.
And a small note on GPS altitude. The whole point of 1013 and QNH settings regional or otherwise is so that we are all talking from the same sheet. ATC can seperate us we can work quadrangal rules etc. If say someone gos up with a knackard Alt and decides that the GPS altitude will do, it could cause all sorts of problems. In fact if I had my way altitude would be blocked on all aviation GPS's unless it was fitted with an encoder from the static. BUt unfortunatly nearly all these units are barstadised off a basic unit which is also used for surveying etc where they do want the altitude.
The real debate here isn't the units themselves its the promotion of the airmanship involved in actually using one.
If posts were answered along the lines of yes gps is great but good airmanship dictates that we don't put all our eggs in the one basket. For VFR flying its best to have....with a good roof... Fine. And if someone comes on and starts saying you mad using a GPS i will come down whole heartly on the side of using GPS as a cross reference to other nav methods.
I am not having a go personally at you.
But when posters come back and promote that GPS is the saviour of navigation. And report that they have never had errors etc etc. All it does is allow a false sense of security to develop in GPS usage.
Now its just to get someone at the CAA to see sense and produce a good safety sense on how to use them properly and we can all work together to promote good airmanship gps usage.
MJ
As for 4 I will admit that maybe most ppls don't have the pleasure of bouncing around at FL140 in the crap dodging CB's on the wx radar for 2 hours. Trust me when i say it happens, and i will admit that its not likely to affect a PPL VFR pilot but it is still a possibility if it all goes pair shaped.
And a small note on GPS altitude. The whole point of 1013 and QNH settings regional or otherwise is so that we are all talking from the same sheet. ATC can seperate us we can work quadrangal rules etc. If say someone gos up with a knackard Alt and decides that the GPS altitude will do, it could cause all sorts of problems. In fact if I had my way altitude would be blocked on all aviation GPS's unless it was fitted with an encoder from the static. BUt unfortunatly nearly all these units are barstadised off a basic unit which is also used for surveying etc where they do want the altitude.
The real debate here isn't the units themselves its the promotion of the airmanship involved in actually using one.
If posts were answered along the lines of yes gps is great but good airmanship dictates that we don't put all our eggs in the one basket. For VFR flying its best to have....with a good roof... Fine. And if someone comes on and starts saying you mad using a GPS i will come down whole heartly on the side of using GPS as a cross reference to other nav methods.
I am not having a go personally at you.
But when posters come back and promote that GPS is the saviour of navigation. And report that they have never had errors etc etc. All it does is allow a false sense of security to develop in GPS usage.
Now its just to get someone at the CAA to see sense and produce a good safety sense on how to use them properly and we can all work together to promote good airmanship gps usage.
MJ
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But when posters come back and promote that GPS is the saviour of navigation.
Personally I use a combination of all the tools that Mr Cessna, Mr Piper or Mr Klapmeier has provided, in all cases backed up with a chart on my lap.
Out of the usual choice of GPS, VOR and ADF as nav aids then GPS (especially with a big screen moving map) seems to me to be by far the easiest tool to use from both the accuracy and workload-reduction points of view. It also maximises the time available for keeping a lookout (if you happen to be in VMC) compared with any other option.
This always seems to turn into a religious debate, but surely in practice most people use at least two navigational aids for non-trivial flights. Perhaps there are some GPS-only fundamentalists out there, but I don't think anybody here is actually advocating that position.
QUOTE: "8. Yanks go to war and decide to turn the civi off."
Can't actually be done easily. What we use is actually the Coarse Acquisition signal, which is needed for the secure military precise system to get a fix relatively quickly (i.e. better than a few tens of hours). The net result is that although Selective Availability (SA) jitter can be turned back on at will, or indeed changed from the default 100nS randomisation to another value, it is extremely unlikely that we will completely lose the signal.
Jamming is dead easy though, particularly over short ranges, as the sub-noise floor, spread spectrum, technique used for GPS makes it very susceptible to blocking by even modest jammer power levels.
Can't actually be done easily. What we use is actually the Coarse Acquisition signal, which is needed for the secure military precise system to get a fix relatively quickly (i.e. better than a few tens of hours). The net result is that although Selective Availability (SA) jitter can be turned back on at will, or indeed changed from the default 100nS randomisation to another value, it is extremely unlikely that we will completely lose the signal.
Jamming is dead easy though, particularly over short ranges, as the sub-noise floor, spread spectrum, technique used for GPS makes it very susceptible to blocking by even modest jammer power levels.
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Ok i will list the things I can think off that could cause signal loss.
The one factor that could fit all of the spurious behaviours described on this thread is interference from within the aircraft .
It's far more plausible than the idea of interference from the ground, which might come and go in seconds because of the square law of attenuation, or so-called jamming trials, which would have small boats beached all around our coastline if they are half as effective as some people seem to think.
The EMC (electromagnetic compatibility) issue is the reason that airlines ban electronic devices during all or part of the flight, rulings that are themselves based largely in superstition as no-one can know if a laptop with an unspecified electrical defect (permitted in flight) is a worse EMC hazard than a cell-phone (banned) that's working properly.
EMC problems would be hard or impossible for engineers to diagnose without installing specialised recording equipment, but persistent and unexplained GPS outages of the type related in this thread must have a tangible explanation that could be found without scrapping the equipment or indeed the airframe!
The 'rising floor of noise' is a point well made and definitely a problem for the future, although perhaps easily addressed in antenna designs that favour the sky where the satellites are supposed to be. But internal EMC problems won't be fixed until the technicians (and the pilots) figure out that an interference source 30cm away inside the plane is 10 to the power of 8 (don't know how to write that on Prune) or 100 million times more powerful than one 3 km away on the ground!
David
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David,
Anecdotal evidence of GPS failures is just that. GPS goes back over 10 years now, and the early units were frankly absolute c**p. I used to rent a PA28 with one of them and it barely worked. It was labelled "VFR only" which saved the flying school from ever having it checked by anybody, and it probably had thoroughly corroded electronics (parked outdoors all year, duff door seals, inside stinking of mould). This doesn't prove anything. I used to fly with a VOR which would ident but showed anything you wanted it to. I did my IMC Rating with a DME which did likewise. Doesn't prove anything, other than that avionics maintenance is expensive and doesn't get done unless it must.
I don't think MJ or myself are missing anything yet. Certainly internal interference is possible. But it ought to be repeatable if you reset the avionics to those frequencies - if it is bad enough.
If you have a decent rooftop aerial then you have much better immunity from all sources of interference (that are below the sky, which most are) than if you are using some handheld with an integral aerial which is just about hanging in there, getting just enough signal from what manages to seep in through the windows in an all-metal aircraft....
This is the problem. Probably the majority of pilots who use GPS today fly with old junk, or badly installed, or using inadequate aerials.
Even a brand new top-end Garmin handheld for £1000+ falls into that category as far as I am concerned, in an all-metal aircraft. You will be doing well to be getting half the possible satellites because most of the sky is covered up. It will still work well enough because you need only about 3 for a fix, and most of the time there are about 8 in view. Most of the time
Anecdotal evidence of GPS failures is just that. GPS goes back over 10 years now, and the early units were frankly absolute c**p. I used to rent a PA28 with one of them and it barely worked. It was labelled "VFR only" which saved the flying school from ever having it checked by anybody, and it probably had thoroughly corroded electronics (parked outdoors all year, duff door seals, inside stinking of mould). This doesn't prove anything. I used to fly with a VOR which would ident but showed anything you wanted it to. I did my IMC Rating with a DME which did likewise. Doesn't prove anything, other than that avionics maintenance is expensive and doesn't get done unless it must.
I don't think MJ or myself are missing anything yet. Certainly internal interference is possible. But it ought to be repeatable if you reset the avionics to those frequencies - if it is bad enough.
If you have a decent rooftop aerial then you have much better immunity from all sources of interference (that are below the sky, which most are) than if you are using some handheld with an integral aerial which is just about hanging in there, getting just enough signal from what manages to seep in through the windows in an all-metal aircraft....
This is the problem. Probably the majority of pilots who use GPS today fly with old junk, or badly installed, or using inadequate aerials.
Even a brand new top-end Garmin handheld for £1000+ falls into that category as far as I am concerned, in an all-metal aircraft. You will be doing well to be getting half the possible satellites because most of the sky is covered up. It will still work well enough because you need only about 3 for a fix, and most of the time there are about 8 in view. Most of the time
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It was something else
Back to my initial situation in which the GPS(s) failed.
It's unlikely to have had anything to do with "Within the aircraft", rain, the aerial and many of the other possibilities suggested, apart from maybe, aliens.
I had two GPSIII Pilots with me, one mounted within the aircraft and connected to an external aerial (but not loaded with my route), and my own GPSIII which was on the seat next to me, loaded with the route.
They both failed at the same time, in different conditions (going north was heavy showers, coming back was dry), on different days, but in the same location. They both recovered in approximately the same locations. The location seemed to be 15 - 20 miles north west and south east of Shawbury.
I use the GPSIII as a "third level" navigation aid, for several reasons, one being that I have had the "Poor GPS Coverage" message enough times not to want to depend on it.
But this was different; both GPSs didn't 'fail' immediately, but started to give incorrect readings for location and Ground Speed, (as stated earlier, over 4000kts
), followed by a message to "Switch off the GPS and re-initialise"). Initially, switching off and on did not work either. Eventually, when about 15 - 20 miles from Shawbury, they both started to work again.
If no-one has had any experience similar to mine, I guess a call through to the Martian invasion force at Shawbury might sort it.
It's unlikely to have had anything to do with "Within the aircraft", rain, the aerial and many of the other possibilities suggested, apart from maybe, aliens.
I had two GPSIII Pilots with me, one mounted within the aircraft and connected to an external aerial (but not loaded with my route), and my own GPSIII which was on the seat next to me, loaded with the route.
They both failed at the same time, in different conditions (going north was heavy showers, coming back was dry), on different days, but in the same location. They both recovered in approximately the same locations. The location seemed to be 15 - 20 miles north west and south east of Shawbury.
I use the GPSIII as a "third level" navigation aid, for several reasons, one being that I have had the "Poor GPS Coverage" message enough times not to want to depend on it.
But this was different; both GPSs didn't 'fail' immediately, but started to give incorrect readings for location and Ground Speed, (as stated earlier, over 4000kts
), followed by a message to "Switch off the GPS and re-initialise"). Initially, switching off and on did not work either. Eventually, when about 15 - 20 miles from Shawbury, they both started to work again.If no-one has had any experience similar to mine, I guess a call through to the Martian invasion force at Shawbury might sort it.
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The US Coast Guard have an interesting site on GPS outages at http://www.navcen.uscg.gov/gps/default.htm . Not much reference to foreign countries though. However, US jamming trials have a 'stop' option which I believe can be exercised by a pilot through flight service. Maybe the same is true here? "Shawbury, are you jamming GPS, if so, please desist" or something?
