Serious GPS question for Airline Pilots

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absit_omen

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This is a quote from another Poster on PBO Forum.

"Have you not experienced it yourself? It is very common for navies to interfere with GPS signals during exercises. In the UK they usually(?) warn the public first. It happens a lot in the Bristol Channel area, I think, and along the west coast? "

Can any pilot out there confirm that this does happen. If so, how do you deal with such 'interference'?

What is the nature of the'warning' Do Airlines receive this? I have never heard of such a 'warning'

Thanks in advance.
 
I don't know about pilots but this is sometimes the subject of a Navigation Warning from the MCA. Not very common in my experience, there were a couple of instances last year that I was aware of..
 
Presumably when the military play their games they don't interfere with their own GPSs so the spoiler must be coded in such a way that certain types of receiver are not affected. It could be that aircraft receivers are not affected by most naval interference. As you suggest, it could be an issue if aircraft were affected although it might be possible for surface vessels wishing to spoil the signals of other surface vessels to 'beam' the signal in the horizontal plane, reducing the risk of interference with aircraft flying at great altitude (as they mostly will be).

You need to understand that this is a huge military issue and they need to exercise.... if they have no way of disrupting the local GPS signals then an enemy can obtain the lat/long of a vessel by satellite - visually - and send that to the missile's guidance system which, if fitted with GPS, could send the missile right to the target. Remember the short range systems that require the soldier to keep a tank in the sights to guide the missile? A bit like that, but over continents and infallible unless you can knock out the GPS or shoot it down.

That's why the military has to have this kit. The military has to practice using the kit - from live bombs and missiles to advanced computer and radio systems.

One would hope that it is used responsibly in peace time.
 
I belive that airliners have inerial navigators so that they are independant of the GPS type of systems. They are basicly a dead reckoning system, they know where they start from.. and keep a track on where they are.
GPS's are making their way into the latest systems I think..
 
GPS is not considered adequate as the sole means of air navigation. It may be used as a backup to other methods (radio nav beacons or visual nav).

The possibility of interference, legitimate or otherwise, is one of the reasons for this cautious approach. The difficulty is that the user may be unaware of any problem. It does not "fail safe".
 
My wife is pilot, and they base their nav on inertial systems not GPS. The gps is now a feed to the system, but usually for initialisation. It needs to agree with the inertial system to be accepted. I think it really only acts as a cross reference. The errors typically associated with the military playing silly b' are rapid lateral shifts, which inertial systems do not suffer from and will reject. Slow drift on the inertial system is the main issue, and GPS can be used to correct for this.
 
Almost correct. Written before seeing Pyro James entry. Most shorthaul aircraft have what is known as an FMS (Flight management System). This takes input from various sources and presents navigational data, which in turn can be fed into the autopilot system. Most also carry a separate, stand-alone GPS system as back-up navigational data. ~This is not integrated with the autopilot. Long haul aircraft will also have an FMS, but taking inputs from an inertial system as well. They also will carry GPS as a stand-alone system.
NOTAMs (notice to Airmen) advise if the military is messing around with GPS signals in a specific area, which as you say they do in the Bristol Channel area from time to time. The GPS system will 'advise' if it cannot receive an adequate signal but not if the signal is being 'bent' to give spurious positions.
Aurigny don't provide GPS systems for the Trislander fleet. Anything you might have seen is the pilots personal bit of kit and 'is not to be used for navigational purposes'. Finding Alderney is relatively easy, you just look for the fog. Landing is another matter!
 
I suspect the area being 'messed around' must be bigger then we realise.

To relate - during one of the 'emergencies' related to Saddam and Iraq (must have been about 1993 or 94) I was belting accross the Zanzibar Channel in my Hartley 19 towards the south end of Zanzibar. Since you start out without sight of your target I had the GPS on bearing and range and was counting down the miles and sticking to the bearing at about 25 knots on a flat sea. Suddenly the Magellan was telling me I was three hundred odd miles away and doing several hundred knots. The readings returned to normal after about ten minutes and this never happened to me before or since.

So ..d'ye think this was the Bush playing with his knobs and sliders /forums/images/graemlins/grin.gif

/forums/images/graemlins/smirk.gif
 
That makes little sense to me. The problem is incoming ordnance. You'd have to influence the guidance system long before 'hundreds of feet' to make the missile change course sufficiently. In any case, guidance systems would know to ignore major interference hundreds of feet from the target. We are talking about tens of nm at least, but of course the military are hardly likely to enter this discussion or tell us /forums/images/graemlins/wink.gif
 
GPS signals are incredibly weak, and it is no great trick to interfere with them. I suspect that you were affected by anomalous conditions in the upper atmosphere, rather than by jamming.

As I had drummed into me very clearly when I were a lad, there is no such thing as a navigational instrument - there are only aids to navigation. By all means use the wonders of GPS, but always carry out sanity checks on the results!
 
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but of course the military are hardly likely to enter this discussion or tell us /forums/images/graemlins/wink.gif

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Which is why I asked for airline pilots to comment.

Also, FWIIW, as an ex-military of twenty years and whose last posting was to MOD Procurement Executive as a weapons staff officer I know a little bit about missile guidance systems and cannot recall the issue of GPS ever coming up. Perhaps I was out on a long lunch at the time!
 
I can confirm, as a very recently retired airline pilot, that there have been experiments with 'jamming'/bending GPS signals around various parts of the UK coast. These trials were notified via the NOTAM system. No mystery, it does happen and it is notified. Exactly what 'they' are trying to do I don't know. I know nothing of cruise missile guidance systems apart from what I've heard on the news... a mixture of an inertial system, topographical map recognition program and GPS updates, I believe.
 
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there was no gps signal over a distance of about 10 miles each time. How do they do it?

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I've just been talking to the mini-plod 'person-ing' the road barrier across the A36 near here ( the Highways Agency have closed the road for 3 months to rebuild a bridge ), and watched while over a score of car drivers turned up, following the instructions on their GPS 'Tom-Toms'
slavishly.

"But it says here I can follow this road!", bleat the car drivers. "There's simply no way through here. Turn around, go back a couple of miles, and follow the diversion signs....." The dimwits had each driven past over a dozen 'Warning- Road Closed Ahead' and 'Diversion' signs.....

I wish I could jam their ruddy signals over a 10 mile radius! /forums/images/graemlins/grin.gif
 
Talking of suspect GPS readings, I remember coming between Jura and Scarba one day and noticing that despite a water speed of 6kn the GPS said we were going the other way at 4 kn.
But I think it was right!
 
Years ago the military said they were experimenting with b**ggering around with the GPS signal. The experiment was centred in the middle of the Brecon Beacons.

At the time I was in Waterford about sail back to Swansea.

But I never had any problem with GPS on that trip.

Have not heard any notices of these experiments since.

As for aviation, I doubt they rely on GPS, don't they use LORAN oe similar?
 
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Also, FWIIW, as an ex-military of twenty years and whose last posting was to MOD Procurement Executive as a weapons staff officer I know a little bit about missile guidance systems and cannot recall the issue of GPS ever coming up. Perhaps I was out on a long lunch at the time!

[/ QUOTE ]I also worked for many years in MOD airborne contracts - airborne telemetry for fusing systems and aerials so got pretty involved. My experience was long before the days of GPS but unless things have changed you can't discuss the detail and all details are kept on a 'need to know' basis. I was working on JP233 when even the code JP233 was classified. The Beeb put on a documentary giving far more details than many of us designers actually knew. Many of the features had been discussed with potential customers and a huge amount of 'classified' information was being given out on the BAE stand at Farnborough! We learnt more from the Beeb and Farnborough than we were told in official briefings and documents.

My point is that could it be that you were not on a lunch break but they just didn't tell you, believing it to be unimportant? I would be surprised if GPS was not incorporated in missiles.
 
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