GPS fails over Shawbury

[jayemm]

Last friday I routed via Shawbury to get up to Blackpool.

In the Shawbury vicinity, my GPS III began to misread, at one point telling me I had a Ground Speed of about 4356 knots (unlikely in a PA28). I was using it as a secondary aid, so had no problems.

After powering off and reinitialising the GPS, it seemed to recover approaching the Wallasey VOR. I thought nothing of it, and had no further problems to Blackpool and then on to Scotland.

However, on the return journey this Monday, exactly the same thing happened again around Shawbury. The GPS started working again about 20 nm south east of SWB.

Did I miss a NOTAM? Is there something special about Shawbury? Can someone explain this?

[VP959]

Interesting.......... Had it been over Hereford I could understand it. Perhaps some of those chaps were playing up at Shawbury without telling anyone?

[Fly Stimulator]

I landed at Aalborg in northern Denmark a couple of weeks ago. It's a combined military and civilian field, and it has an impressively large radar rotating at ground level not far from the taxiway.

As I taxied past it my Garmin 196 was stunned into silence and had to be powered off and back on once away from the radar before it would come to life again.

I was also getting a distinct 'thump' sound in my noise-cancelling headsets every time the radar rotated past me.

Reminds me of the time I worked on computers in a port-side building. One day they started to go bonkers every few seconds. Took a little while to connect this with the Navy ship which had come to visit and tied up at the nearby wharf with the radar still operating.

[Big Hilly]

Hmmm . . Interesting. . .

I too was up in the Shawbury/Welshpool area a couple of weekends back and my GPS III froze, twice!! Now this is something that it has never done before and I hadn’t given it a second thought, but after reading this. . . . :suspect: :confused:

Best wishes,

BH

[Whirlybird]

Never happened to me, and I use my GPS to find Sleap - inside Shawbury's MATZ - in anything other than perfect vis. Yes, I know it's my home airfield, but it sits in the middle of the somewhat featureless Shropshire plain, and I still have trouble finding it sometimes. :(

I guess you have to be based in the area for your GPS to work there. :)

[Big Hilly]

I guess you have to be based in the area for your GPS to work there.
Nah Whirly, it's because your fling-wings repel the rays from the evil device that they're secretly testing on us. . . . :D ;)

BH

[Whirlybird]

Big Hilly,

It can't be that. I fly a C150 out of Sleap...and it had non-whirly wings the last time I looked. There are no helicopters based there unless you own one...I fly them from Hawarden. Nope, it's a special dispensation for Sleap based aircraft, that's what it is.:ok:

[Shaggy Sheep Driver]

And I too am a frequent visitor to the skies around Sleap, Shawbury, Sherlowe, and Shrewsbury. My Pilot 111 has never faltered, though it did so once over the Lake District from Morcambe up to the Solway. And the other pilot in the aeroplane also had a Pilot 111, and his did exactly the same (both units showed 'no signal' just for that leg).

SSD

[Megaton]

Did have GPS fail consistently whilst living in Texas but it was always in the same place....in the vicinity of a certain small town called Crawford! No coincidence, I think.

[maggioneato]

I am based at Sherlowe, just inside the Shawbury MATZ , have never experienced any problem as described with my Pilot111 in the three years I have been flying from there.

[Whipping Boy's SATCO]

I must admit, it was me. I was at home last Friday afternoon and was getting sick of just looking at yellow and black helicopters flying over the top of my house (Peplow airfield 5 nm E of Shawbury). I did a Blue Peter special with a coat hangar and a 12 volt battery and, before you know it, there was all sorts overhead. Didn't do it for long as the incessant noise woke the dog and he started to knaw on my ankle.

Seriously, 4 years flying in N.Shropshire and I've never had a GPS drop out. Malvern, yes; Shawbury, no.

[Reichman]

I flew into Shawbury on Sunday. I wasn't at all surprised to find that my map still had a line and heading drawn on it, my compass still showed the correct heading and my stopwatch still showed the correct elapsed time for that particular leg. Spooky.

Reichman

PS. They all worked when I left Shawbury as well. Amazing!

[IO540]

The only likely reason why a GPS would regularly fail in a particular spot is that the unit is affected by high power radio signals.

This is why the GPS aerial/antenna should always be roof mounted.

The handhelds with integral aerials (e.g. most Garmins) are probably hanging on by a thread most of the time but they do it pretty well so most people are not aware.

If I was using a handheld regularly I would connect it to a roof mounted GPS aerial. Every plane in which a GPS is being used should have one of those fitted, IMHO.

[VP959]

IO540 wrote: "If I was using a handheld regularly I would connect it to a roof mounted GPS aerial. Every plane in which a GPS is being used should have one of those fitted, IMHO."

This could be a bit of a problem if you fly something without a roof..........................:hmm:

[IO540]

VP959

Indeed, but if you don't have a roof, your GPS reception would be a lot better :O

[Fly Stimulator]

I wasn't at all surprised to find that my map still had a line and heading drawn on it, my compass still showed the correct heading and my stopwatch still showed the correct elapsed time for that particular leg. Spooky.
Reichman,
You may get away with that sort of complacency now, but if you intend on keeping up flying into old age you could find that the lead in your pencil lets you down. When that day comes, your views on the merits of GPS may change. ;)

[Big Hilly]

I've worked it out!

Have you noticed that it's only the 'regulars' in the area that claim they've never had a problem????

It's because Whirly and Whipping Boy's SATCO (he even admitted as much in the above post) have rigged up an evil device to give us visitors grief. . . I can hear them now, sat there in a field with a pair of binos, a flask of Borders Ale and a laptop with G-INFO installed on it:

"What's the reg"?

"G-XXXX, quick look him up, where's he from"?

"Ah, the South, right Whirly, get out the Pangalacticgargleblaster, let's play with his mind. . . .":D ;)

BH

[Whirlybird]

Damn, we've been rumbled..... :) :D

[Reichman]

Fly Stimulator,

How did we all navigate around the world before GPS?

I find your attitude to navigation incredible. Are you advocating that GPS navigation should take precedence over map reading skills? I do hope not.

Thanks for the compliment about flying to a ripe old age though. Maybe I already have?

As for running out of lead in my pencil - I carry a spare. ;)

Reichman

[david viewing]

The only likely reason why a GPS would regularly fail in a particular spot is that the unit is affected by high power radio signals.

Not necessarily. Low power signals from inside your own aircraft could also do the trick, perhaps related to the COM frequency or navaid in use at the time, especially if you have a sucker mounted antenna. This would give the appearance of a geographical base to the fault.

[Fly Stimulator]

Reichman,
I find your attitude to navigation incredible. Are you advocating that GPS navigation should take precedence over map reading skills? I do hope not. I wasn't actually expressing any particular attitude to navigation. The ;) symbol in my post indicates that it is not intended to be taken seriously - just like the one in yours!

[IO540]

DW

Fair point, but I did say "in a particular spot". There is a good theoretical basis for high harmonics of certain navaid frequencies interfering with GPS. However, I have never met anyone, and have never met anyone who has ever met anyone, who can just go up in their plane that is fitted with any current-model GPS and reproduce this.

In any event, all this is an excellent reason for having a roof mounted GPS aerial. It's all down to the signal to noise ratio; if a GPS with an integral aerial is only just managing, it won't take much to push it over the edge. The total cost of a proper rooftop aerial should be about £400 incl installation, coming out on a BNC socket.

Reichman

"How did we all navigate around the world before GPS?"

The answer is that, like today, often they didn't. Even post-WW2, airliners used to get lost through gross nav errors.

I have often asked your sort of question to various old hands in sailing and flying. How did the seafarers of centuries ago manage to find Easter Island, for example, and do so at will? If you assume the horizon from say FL100, it is certain that nobody, not even a robot on a windless day, could fly a heading for that distance and end up there. These people were extremely good with the sextant and their tables. The sailors had a table on which to spread their chart and al the time in the world. The early airliners also had a full time pilot or two, a full time engine/fuel manager, and a full time navigator. They also had NDBs and even an NDB is vastly better than dead reckoning.

The trick, as far as I can tell, is that the early navigators (and this includes post-WW2 airliners) had continuous guidance along their track. Even a sextant gives you a line of latitude and is vastly better than dead reckoning and post-WW2 there were NDBs everywhere of interest which are better still. A GPS, like VOR/DME, gives you continuous guidance along a track.

Flyers that did solo transatlantic flights would have tried to use a sextant but in any case would have aimed for an easier target, e.g. the east coast of the USA, or Ireland if coming this way. You've got to do a wind corrected heading to within 5-10 degrees and you will be fine.

The difficulty, which a GPS handles very well, is that today's PPLs get barely enough training to take off, fly an easy XC flight on a perfect day, and land, yet they are expected to fly precise tracks between bits of controlled airspace.

[Reichman]

Fly Stimulator,

Sorry, didn't see the ;) in your post. Apologies. I'll go and stand in the corner - If I can find it.

IO540,

It was a rhetorical question, but thanks for the interesting post. We were using a sextant in the VC10 to cross the Atlantic until the late 80s.

I'll go and sharpen my chinagraph.

[boomerangben]

So the locals are not affected by this? Could it be because of the various pronounciations for Sleap and a GPS drop out is punishment for those of us who have tried to pronounce it Sleep?? (I now know better)

Either that or it is a deliberate attempt to confuse those young pilots learning the finer art of rotary flight at the Queen's expense.

[FNG]

Hey, those guys are plenty confused already: my mate big D is their instructor.

[mad_jock]

After a few posts with IO540 about roof mounting and the rest.

I have asked one of a avionics techs andhe assures me that all our units have to be up to class 1 radio fit for BRNAV. which i presumes means arials in the best place checked every year etc etc.

Ours still go tits up on a regular baises. Usually only giving a RAIM error for 5-10 mins then clearing. GPS isn't as good as some make out.

MJ

[IO540]

MJ

What model of GPS is it, and how old?

Re RAIM, have you checked whether RAIM is indeed available at the location and time in question?

[Keef]

I thought RAIM was inbuilt in the receiver?

When I flew with a GPS3Pilot, it would relatively often lose signal for a brief period. That was with an extension antenna (not "arial" - that's a misspelled washing powder) on the windscreen.

Then we had a GNS430 fitted, with a proper antenna mounted on the top of the aircraft. We've never yet had a loss of signal message from that.

[IO540]

The capability to do RAIM checking is (or isn't) in the receiver, yes. Few handhelds have it in any form, and none I know of have the proper version which requires an encoding altimeter input, and manual setting of QNH.

Whether you can get RAIM at a given place and time depends on the satellite positions. This can be checked computationally; here is one URL I had saved from a while ago

http://augur.ecacnav.com/

However I don't think RAIM has much relevance to PPL-type en-route flying, and one can hardly use GPS for approaches, as there aren't any in the UK.

Incidentally I haven't seen a RAIM warning in 300 hours (2 years' flying), with a KLN94B. If somebody sees something a lot worse than this, they might consider getting a better avionics engineer :O

[mad_jock]

the units are.
http://www.bendixking.com/static/catalog/viewproductdetails.jsp?pid=120

and

http://www.trimble.com/cugr.html

And 300 hours is about 4 months flying for me. I would say on average the signal will drop out maybe 2-3 times a week in nice wx. And if you get stuck in or near heavy rain. ie viz getting down to less than 2k in RA or huge wet clouds for a couple of grand above. If your above the tops it very rarely drops out but does if you go near hot danger areas sometimes.

As for the avionics engineers. They will all be licensed to work on public transport aircraft because thats what I fly, so I presume they know what they are doing before the CAA will let them loose. :=

MJ

[reynoldsno1]

In all these cases, of course, GPS has not actually failed. It was still working fine. However, local interference is a factor, and that is almost certainly what occurred in these cases. There is a huge difference between an IFR certificated (TSO'd) unit, and a handheld unit. As the frequency spectrum becomes evermore crowded, particularly in Europe, things may get worse. Let's hope Galileo manages to squeeze in somehow, and relieve the situation - though you will have to pay for it, of course.

[mad_jock]

yes I agree there is a huge difference.

But alot of people champion GPS as a 100% reliable system which they can depend on in any wx all the time. When in my experence when it gets horrible and you really need the bloody thing, the atmo conditions are such that there is strong possibility of signal drop.

I was just trying to illustrate that even with the BRNAV IFR GPS which have all the bits and bobs to tell you when its giving you good data. They still loose it occasionally and in my experence its not a rare event. I will admit that the likely error that the units are experencing is far less than the error for those devils devices NDB's

Both those units are certified for GPS approaches in the US.

I am not saying that they arn't great usefull devices. And have done many a IFR leg to intersections using the GPS for track and loosely monitoring the nav with radio nav aids. I just worry that all the chat about them will lull pilots into thinking that the box is god and will never tell a lie. And for the average PPL pilot unless they are very professional in there attitude to flying will tend to go for the easy option and put blind trust in the box. Which could cause a pilot to push on in crap wx when they really should have diverted.

As said on other threads its all about educating the users in the limitations and use of the units. The kit that IO540 has is way out of the league of most UK ppl's. Who after skrimping and saving and watching ebay after xmas may get themselves a very nice moving map colour display. Great but no one explains to them the importance of keeping the database up to date, limitations on using the GPS altitude, the fact that when the wx goes poo thats when you have to watch for signal drop.

BTW out of the 2 units the none graphic trimble is by far the best in my opinion. But its not what people want for VFR flying. In fact both of them are great for what we use it for. But lets face it not much point showing all the controlled airspace or giving warnings that we are in it. We want to be in it as much as possible. So neither of them is a solution for VFR aircraft.

MJ

[IO540]

MJ

Interesting - are the KLN90 and the Trimble really installed in an airliner? Surely you must have triple inertial nav? I recall looking at a used SEP ~ 2yrs ago which had a KLN90B and I checked it out then; discovered the design was about 8 years old back then. That's a very long time in GPS receivers.

I don't particularly disagree with anything you say. However NOTHING is 100% reliable. Everybody should know that. It is widely assumed that one's brain and the "Mk1 eyeball" (to use the favourite expression of the anti-GPS crowd) are 100% reliable, but they aren't. GPS is a lot more reliable than any other means of en-route nav.

As you suggest, the "IFR approved" units (which were generally intended at the American PPL/IR market which is almost nonexistent in Europe) are not suitable for the average PPL flyer because their moving maps are too small or non-existent. That's why I think fitting a GNS430/KLNxx in a VFR plane is pointless. A much better solution is a GPS with a large MFD or, if you want to save a lot of money and don't have stormscope/radar data to show, a large moving-map GPS like the KMD150. This is a modern product which, with a rooftop aerial, will be highly reliable in reception. Garmin are not a player in that market, for some reason.

You say "Great but no one explains to them the importance of ..." Perhaps this is the real "GPS drawback". Officially, nobody likes it, so nobody is going to tell people about limitations.

[Fly Stimulator]

Did a similar debate go on after the introduction of radio nav aids?

I'm too young to remember (don't get to say that very often any more :ugh: ), but the ability of the ADF in particular to give some rather creative directional suggestions must have caused some people to question its value compared with the well-known infallibility of map, pencil and stopwatch.

[Keef]

Horses for courses. The GNS430 is a super piece of kit, and I love it. Used in conjunction with the VOR and ADF, it's ideal.

As backup for a "looking out of the window" VFR flight, it's brilliant. But you do need to keep looking out of the window!

[boomerangben]

I don't think MJ is flying airliners, and if he flys what I think he flies, he will be using his GPS far more intensively than an average ppl would (or even should - single pilot vs multi pilot).

We use an even older version of GPS receiver, never had too much of a problem with it and whilst I agree the Trimble is excellent, I reckon what we have is even better. Whilst electronics will have advanced over the last ten years, most of that advancement will have been in the user interface and miniturisation rather than a significant improvement in quality or reliability. Indeed, could it been that modern cheaper GPS units be of poorer quality to make them more available to the masses?

I agree GPS is probably the most relaible means of navigation for most pilots, but everyone needs to know the limits and still be able to navigate the old fashioned way. Problem is that other means of nav need practice and if you only ever use GPS, your old skills deminish and you are ill prepared for the fancy black box loosing its fix.

[englishal]

This could be a bit of a problem if you fly something without a roof..........................

Just stick it to your head with a bit of Velcro :D

[mad_jock]

H'mm I don't know if you would call it an airliner more of a multi crew regional turboprop on timetabled routes ( Jetstream 32). It does have a bog fitted though of sorts. We don't have TCAS in any or Auto pilot in most.

You will also find that the trimble is fitted to BA 146's DASH 8's old 737's in fact anything that has a steam driven instrument set that needs BRNAV for flying in controlled airspace would mind betting that Concorde might have had one on as its a BAe machine.


Yes i agree with you IO540 there are alot of political issues with GPS. Who owns it who has control of it etc etc. And as you say nothing is 100% which is why we always check the ident when using enroute VOR's (I might add if its identing test we just go with the GPS :D )

I agree GPS is probably the most relaible means of navigation for most pilots, but everyone needs to know the limits and still be able to navigate the old fashioned way. Problem is that other means of nav need practice and if you only ever use GPS, your old skills deminish and you are ill prepared for the fancy black box loosing its fix.

I couldn't agree more with the above statement.

I think we both agree on the major points about GPS IO540. Its just that it does have limitations which professionals know about and have been trained / slapped / sworn at / been called slack bastards into always backing up with traditional methods. If it goes tits up on our flights its a very minor annoyance. If your average PPL who flys max 20 hours a year 2 of which is IMC. Who thinks that because they have a GPS on board they can dump the tradional methods because they have set the pink string up. When it does go wrong in WX that they wern't expecting hadn't been in before they won't have the capacity to deal with it. And they won't be up to speed with the traditional methods to just slip striaght over into using them.

The reason why I can be bothered arguing the point is by just having these reruns of the GPS debate it might mean that new PPL's or new GPS users might think "right its a good bit of kit when used properly". As long as they have it in the back of there mind that there is a possibility of it cocking up and they need to plan for that eventuallity my time has not been wasted.

MJ

[airborne_artist]

A friend of mine has an interest in GPS in mobile phones - most new mobiles in the US now have a GPS chipset.

He sent me this:

"GPS is increasingly at risk from a rising noise floor caused by anything from electrical pylons, to WLAN, to faster chips in computers, and many more sources. It is a very weak signal - some 10,000,000 times weaker than GSM for example. It will mean GPS performance gets worse over time, as opposed to the normal trend of improvement."

Clearly an aircraft, when airborne, is some distance away from most of the low power sources, or passing through their influence fast enough not to be affected, but it's worth bearing in mind.

[IO540]

It is true that the GPS signal is below the noise floor, but it has special characteristics which enable it to be recovered as well as or better than a signal without those characteristics which is above the noise floor. In principle, a signal can be arbitrarily below the noise floor and still be recovered, if there is sufficient predictable redundancy in it.

I do think that intentional (non-military) jamming is GPS's biggest potential enemy, but again there are ways to provide resistance to that. A rooftop aerial is the starting point.

As MJ suggests, these things are worth discussing.

[mad_jock]

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

[IO540]

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 :O

"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.

[mad_jock]

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

[Fly Stimulator]

But when posters come back and promote that GPS is the saviour of navigation. Has anybody (on here at least) ever actually advocated relying on GPS to the exclusion of all else? I don't remember it, but perhaps somebody could post a link to the relevant thread if there is one.

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.

[VP959]

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.

[david viewing]

Ok i will list the things I can think off that could cause signal loss.

Sorry chaps, but I think you are missing the point. All the wonderful things listed could perhaps cause signal loss, but conflict with numerous anecdotal evidence of GPS reliability in bush regions from pilots who do rely on it (rightly or wrongly).

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

[IO540]

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 :O

[jayemm]

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. :uhoh:

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:cool: ), 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.;)

[david viewing]

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?