CAA Report: GPS Integrity
 

GPS Integrity and Potential Impact on Aviation Safety

The CAA has published a report of a study of "the level of safety as measured by the integrity (i.e. trustworthiness), afforded by the Global Positioning System as a source of navigation data for civil aircraft."
The study presents "a number of augmentation methods which have been shown to have the potential to satisfy the required navigation performance requirements for all phases of flight."


It's a large file (2.9mb) so you may wish to right-click on the link, select 'Save target as...' and save the file to your computer - especially if you're not on broadband.



Link

Very interesting report.

Conclusion was again that it was unsuitable for primary navigation.

Even with such a well sourced report I am sure people will still say GPS is best.

But if they did a report looking at VFR navigation the old way, I wonder which would be least prone to failure?

MJ

A good read, containing little or nothing of relevance to GA.

The Volpe GPS Vulnerability report (online somewhere I am sure) is more practical, showing that a proper roof-mounted GPS aerial is pretty important to signal integrity. Everybody using a GPS should read it, IMHO.

Ultimately, one still needs to use paper charts for ground based flight planning, have a printed/handwritten plog so one can see where one is supposed to be going, and use TWO navigation methods, of which GPS being one is great.

Flying in IMC solely by VOR tracking and getting a VOR failure is no worse than flying in IMC solely by GPS tracking and getting a GPS failure. But you get a lot more bollocking for the latter.

And at the VFR level there will always be somebody who goes up without any planning and whose GPS batteries go flat, and he will be made an example of for years to come. But if he didn't carry a GPS, he would be just another fool. Not really relevant to GPS!

In the interests of saving bandwidth while we have this discussion again, I offer the following options which can be used in the same way as the numbered menu in a Chinese restaurant:

1. GPS users are either lazy, or incompetent navigators or poorly trained or all of these things.

2. It is acceptable to carry a GPS as an emergency aid, but it should only ever be switched on as a last-ditch alternative to calling D&D. Primary navigation should always be carried out with compass, stopwatch and lines on a chart. Radio aids are acceptable at a pinch, but proper pilots should always fly by dead reckoning alone.

3. All navigation aids are acceptable for primary navigation, except GPS which isn’t because you are almost certain to bust CAS or get lost when the batteries go flat, you get the co-ordinates wrong, somebody jams the signal, or all of these things happen.

4. GPS is acceptable as a supplement to a PLOG and lines on a chart and is no less suitable in this role than traditional radio navigation aids.

5. GPS is suitable for primary navigation on its own. In order to stay on the right side of the law, you might consider keeping a current chart somewhere in the aircraft.



Mine's a number 4 for what it's worth.

Well at least I learnt what a RIAM failure is or whatever its called.

Maybe I won't put any effort into working out how to turn it off.

Comes up on a pretty regular baises as well. Which gives me the unfortunate conclusion that the GPS isn't as accruate as people think quite alot of the time with out them knowing about it.

H'mm how about no lines on a chart (which is store behined the seat), plog and using radio aids and GPS for navigation ;)

MJ

Always someone who wants something that's not on the menu. I'll have to check with the chef.

Fly Stim...

I'll have a number 4. Can I get a fried rice and NVG on the side?

Interesting article in this months Practical Boat Owner that basically says sail with out manual nav at your peril. GPS satelites are getting older and their 3 levels of redundancy more or less depleted. Apparently at various times over the last year or so, signals in Scotland have on occasion been up to 40km out and a mile or so out on the south coast. At that was reported by a yacht in harbour!

MJ

In 300 hrs with a GPS (KLN94B) I've never seen a RAIM warning. If yours doesn't have a rooftop aerial, it will have marginal performance at best. What model is it and how old is it? If your aerial is on the roof, is it an active one or a passive one? Do you always load the QNH into the GPS at startup?

FS

I agree with #4, in the PPL context.

For IFR, GPS/VOR/DME/ADF concurrently, whatever is available, plus the plog and the chart. The GPS course on the map display is the primary reference and the others must confirm it. Shame PPLs aren't taught to fly that way too.

boomerangben

You can always find these stories from somebody. Was it an ancient GPS with an internal aerial, used below deck perhaps?? Or did they enter a user waypoint and got a couple of digits transposed?

Oh god I am losing the will to live. Another GPS debate!!! :p

Have to agree with IO540 here, several hundred hours of GPS usage and never seen an error. I always fly radio aid, VOR/DME/NDB in parralell to the GNS430 which is connected to my altitude encoder (amongst many things) and they have always given the same story, unless of course the steam gauges are wrong as well........

Of course I always draw a line on my charts and have a PLOG (I need somewhere to put the R/T frequencies and QNH etc.

And of course when flying IFR I just draw a track on a blank piece of paper...... :p

bose:

"several hundred hours of GPS usage and never seen an error" - your perception may well be that, based on a small sample of the overall GPS availability, every time you go flying GPS is available with minimal errors.

Aviation being the conservative pursuit that it is, I'd suggest your sample of GPS error rates is a little on the low side.

TNF,

You could well be right but I fly on average 350hrs a year which equates to around 30hrs a month which is not a bad exposure?

Unless others like IO540 are flying at the exact same time as me then they have further exposure and don't seem to be experiancing problems either.

The only time I have ever seen an error in GPS on my boat was when we had a faulty antenna connector which had corroded in the sea air. The loss of satelite coverage due to the unit going "blind" gave position errors that were still within about 300m.

I have also never recieved a RAIM warning on my GNS430 which is certified for BRNAV. I have certainly suffered far greater errors on an NDB do to the usual problems of atmospherics etc.

I am more of the view that people find a reason to fault GPS rather than embracing it as a usefull tool.

I do consider the person who trundles off with a handheld GPS and no map a fool, whereas the person who uses all the tools availble to them is very smart.

Gods knows m8, Its either a Trimble or Bendix and its an area GPS fitted to a pref A machine, certified to be used in controlled airspace.

THe GPS drops out a fair bit round here especially when GMC is running on the west coast.

FRA planes are jamming it off and on. Sometimes with a NOTAM sometimes without.

But the RIAM warnings havn't just been in the North of Scotland. Have had them in the North of England as well but again that might be FRA up to there usual tricks out of teeside.


MJ

And I am flying about 20hrs a week.

I like belt, braces and a piece of string to maintain my dignity. I also like to use all the kit I can lay my hands on while flying.

This weekend I was flying with PLOG and chart (with lines), one built in GPS one handheld GPS RMI HSI and VOR and because I get as much as possible set up before I leave the ground and there's an autopilot I can still find time to look out of the window!

What I use depends on where I am, so I might be using VOR and cross checking with GPS or I might be using GPS and cross checking with NDB. The fixed GPS doesn't have moving map, so the handheld is good for helping spatial awareness and if I can see out of the window I can cross check with the chart and lines!

Use the best available tools for the job in hand is my motto and never ever just rely on one!

MJ

Sadly, without more info, one cannot really take that info as being worth anything. Just because some installation was once certified doesn't mean it's any good. How do you know an aircraft is jamming it? Have you been out with the equipment and detected the jamming, or do you have some inside line to the military?

I've flown in enough planes in which the GPS was nearly falling out of the panel. Yes, I expect it worked just fine once upon a time. Doesn't prove anything.

TNF

That's the same old anti-GPS story you are spreading, without support.

I know it's a waste of bandwidth but people who spread these stories are doing a dis-service to GA.

I do about 150hrs/year, have never seen a RAIM failure, have never been uncertain of position, and this is UK, France, Spain.

Incidentally, and this might really get the anti-GPS crowd going, I have never seen the GPS altitude to be more than 50ft off, in any location where I knew the exact elevation (i.e. on the ground at an airfield). It's also spot on aloft, once one has adjusted the altimeter for the deviation from ISA. Not that I use it for that.

American military aircraft based in the UK use GPS as primary navigation, and how do those cruise missiles hit the target if gps is unreliable?

Not to mention that fact that most surveying these days is done with GPS as well!!!

Two potentially useful thoughts from a recent trip to the Antarctic on well-equipped Russian boat.

1. The Falklands are currently being surveyed using pairs of (interferential?) GPS's, accurate to a couple of centimetres.

2. The ship's crew used radar cross-schecked with the chart, not GPS, for going in and out of anchorages, day or night. They used only GPS for en route navigation and used the sextant once a month for practice.

I'm with FS: Option 4.

QDM

God knows the legalities and checks involved in a BRNav GPS fitted to a commercial airliner. If it lights up and the card is valid it gets used.

And the Mil do publish the times and areas which they are jamming via NOTAM.

To be honest this has already turned into those that swear nothing ever goes wrong and those that don't think they are quite as good as some make out.

Reports can be written at vast expense and people will still say "well i have never had a problem" so its bollocks or doesn't apply to me.

I am quite sure in 50 years time we will be having a similar debate about panel mounted INS systems in light aircraft.

And as the report points out the current set of hardware in orbit is rapidly reaching its sell by date. And who knows what the next system will bring. Considering the cost involved I wouldn't mind betting some where along the line users will be getting charged a liscense fee to use it. And the americans don't even have a viable launch system at the moment so any progam to undate the hardware is on hold.

MJ

Claiming that GPS is too unreliable for primary navigation but that the NDB/ADF combo is fine seems a bit daft. I've tried to use an NDB as a navaid a few times (IMC training and post-PPL nav) and have never been very satisfied with it - the list of errors is well known, and it's a bit much when you're two miles from BIA and the ADF needle is 90 degrees out. VOR is fine, but I was well into the IMC before I stopped making stupid mistakes with it. That system is OK, but the user (at least in my case) wasn't.

I would have thought that GPS is similar - errors and failures aren't that much of a problem compared to simple finger trouble and lack of understanding. I'd bet for every airspace bust due to some mystical RAIM failure (whatever that is) there have been a dozen or a hundred from people hitting GOTO and going in the wrong direction. IO540 and bose-x may just be unusual in that they understand the box and can work it correctly... :)

MJ

I was merely pointing out that someone saying (paraphrasing) that Joe Smith, 75, was sitting on his boat sometime last year and 6 months later reported, over not a few pints of Guinness at the Rose and Crown, that his trusty old GPS wasn't working for half an hour, doesn't amount to good evidence of GPS unreliability. Yet this is the sort of "evidence" we get all the time.

A few high-hour pilots on Prune saying they've never seen problems doesn't amount to categorical proof of GPS reliability, but it's a whole lot better supported than the above. From my own point of view, I know a good number of IFR pilots with modern "IFR GPS" gear (which, in the UK basically means KLN89/90/94, GNS430/530) and none of them has ever had the slightest problem.

There are reports quoting frequent losses of accuracy, say from 20m to 200m, and make a meal of that. But for en-route navigation, 200m is NOTHING, zero, zilch, utterly irrelevant. And one cannot use GPS for anything else other than en-route (yet). GPS accuracy could be degraded an order of magnitude and nobody in GA would be affected - although it would show you on the wrong taxiway :O

The U.S. economy is heavily dependent on GPS now. They won't let it fall apart. Who says the system is reaching it's sell by date? It works perfectly well.

The Europeans will have Galileo to give them theoretical independence from the Americans (always a good topic on which to get the Brussels crowd working on yet another device to immortalise themselves) but they will charge for the high accuracy signal. Not that that matters, because the GPS, even with its worst errors due to selective availability, was miles more accurate en-route than anything else. But your airliner IRS will sync from DME/DME anyway, whenever it can.

I sincerely hope that fibre optic gyro IRS will reach GA long before 50yrs from today! The JDAM package reportedly goes for about US$20k and that has a FOG IRS, a jam-resistant military GPS, servos, you name it.

EVO

Yes, I think training is the #1 problem. But the powers to be, as well as much of the old crowd, have dug themselves so deep that bringing it in formally is just about impossible.

Putting a GNS430 or similar in a plane flown by a new PPL just makes matters a lot worse... but that's what happens when technology moves along but the regulatory powers pretend it doesn't.

The best single thing the CAA could do now is mandate a rooftop GPS aerial. But then they would be admitting that GPS is OK to use in some circumstances, and they can't possibly do that... and lots of people would be sure to moan about the £500 cost. And they can't be seen to regulate portable equipment so they would have to mandate a panel mounted unit, and who will pay for that? It's Mode-S cost, roughly.