View Full Version : Gps

31st Aug 2002, 20:06
for GPS availability ,explane aircraft integrity monitoring extrapolation and how to get it ,thanks

31st Aug 2002, 21:50
It ain't "aircraft integrity monitoring"....'tis RAIM, and is done IN the receiver, not the aeroplane systems.
All GPS systems explain this quite well in the manual...suggest you read same, then come back if you have questions.
Have used GPS for years now, and have found it...superb.

1st Sep 2002, 09:13
411A, you are priceless.

RAIM - Receiver Autonomous Integrity Monitoring.

Basically, a system to monitor the integrity of GPS nav. Why do we need it? Well, a VOR monitors its own integrity at the transmitter. In terms of quality and speed for use as an approved aircraft navigation system, a GPS satellite doesn't.

So its done onboard the aircraft, in the receiver. For GPS nav you need to track 4 satellites. For RAIM you need to track 5, so that the system can sense a duff signal. However, the full monty is FDE (Fault Detection and Exclusion), for that you need to track 6 satellites in order to identify, and ignore, the duff one.

Some recievers will predict and display RAIM availability for a certain position and time. Some authorities also require you to prove RAIM availability for your flight before despatch. There are a number of websites that offer the facility, one is the AUGER tool on the ECACNAV site. If you want to get really smart, you can input a mask angle to the equation to take account of high terrain on GPS appoaches.

Although the GPS constellation is based on 24 satellites, there were 27 airborne last time I checked. Preflight RAIM checks are, I think, only required when the constellation drops below 22 satellites.

A somewhat general reply, but I can't find my paper with all the info on it. Despite what 411A says, there is little detail in most flight manuals that are available to line crews.

2nd Sep 2002, 09:48
You will find that www.trimble.com gives a good explanation of GPS. Described adequately in layman's terms.

Check 6
2nd Sep 2002, 14:49
Here is the link the the ECACNAV GPS Site:

ECACNAV GPS Site (http://augur.ecacnav.com/)

2nd Sep 2002, 17:03

I think the term you are referring to is actually "Autonomous Integrity Monitoring Extrapolation" not Aircraft. Its more commonly known as AIME. Its common fit on a lot of the Airbus fleet these days (as well as some CRJ's) and its not simply RAIM as described above.

AIME is a patented technique for the integration of GPS and Inertial guidance to produce a robust position solution with high availability even in situations in which the GPS guidance is lost (for example in an area in which GPS is jammed or in those rare cases where there are not enough satellites to generate a position solution). AIME is capable of providing RNP 0.3 accuracy guidance with an availability of 99.999% and forms the basis of the Airbus RNP certification. Its seriously impressive nav technology imho, and I'm not a salesman :-)

AIME was developed my Litton Aero Products now a part of Northrop Grumman, the URL is http://www.ngnavsys.com/Navigation/# and look for the LTN-101 product. Theres plenty more info around on it if you are interested, e.g. here (http://www.siae.org.sg/aiem/40/pres-08.pdf) .

Hope this helps a bit, (perhaps 411A could read the above and come back with any questions :D )



2nd Sep 2002, 18:52

As I don't fly Airboos equipment, not really interested in AIME...but thanks for the invite.
RAIM works in our aeroplanes very nicely, actually. And, it (GPS) is dead on, every time.
Never have understood why the "Eurpoeans" have not used it to a greater extent, ie: how many GPS (now RNAV) approaches have been approved in, for example, the UK?:rolleyes:
On the other hand, in the USA, several thousand. Business jet operators especially find it very useful.:D

Captain Stable
3rd Sep 2002, 17:43
411A, two reasons the UK has not yet approved either GPS approaches or the use of GPS as a primary navigation aid are (1) the system can be degraded at any time without notice and (2) trials of the system's resistance to interference are still ongoing.

3rd Sep 2002, 23:00

Whilst what you've said is perfectly true it doesn't actually answer 411's point at all. Why is it ok for Americans and not for Europeans is the mystery. I must admit I can't think of a reason.

4th Sep 2002, 01:08
Or alternatively a first class case of not invented here.

As if VOR's & NDB's can't be interfered with by external
agencies...perhaps rather than setting up the ludicrous
duplication of Galileo we might negotiate with the US
on a guaranteed pan-European quality of service and
contribute some satellites to the GPS constellation.

-- Andrew

4th Sep 2002, 01:35
GPS is a classic case of the operators knowing it works in practice, and now the regulators want to make it work in theory...

The truth is that the operating environment in Europe (including the UK) does not allow many of the benefits of GPS to be seen.

4th Sep 2002, 18:29

Would you take a look at these plates?

HDN ILS/DME RWY 10 (http://edj.net/cgi-bin/echoplate.pl?SouthWest/HDN_idr10.GIF)
Not authorised if local altimeter setting unavailable

HDN VOR A (http://edj.net/cgi-bin/echoplate.pl?SouthWest/HDN_vA.GIF)
Not authorised if local altimeter setting unavailable


HDN GPS C (http://edj.net/cgi-bin/echoplate.pl?SouthWest/HDN_gC.GIF)
Nothing to "un"authorise the approach if local altimeter setting unavailable.

So GPS alone is being used to work out your altitude? Would you shoot the GPS approach if local altimeter was unavailable, and it was crappy weather all the way down, but you wanted to see whether you could get in? My reading of the plate says you can, but experienced airline pilots I speak to cannot agree.

4th Sep 2002, 18:44
Here I have a question for the whizzkids: while flying last week over Africa I brought a "backup" GPS in addition to our on board UNS-1C GPS. Lateral position always agreed, but 3-D alt read out on back-up GPS (on board gps has no alt info) differed 1800 ft at fl 330 with altimeters. regional qnh 1016..isa +12...

4th Sep 2002, 20:41

Have no idea why the note is not on the GPS chart.
Also, ALL approaches (except Cat II, III ILS) must use the baro altimeter for minima determination, and definately NOT GPS indicated altitude (unless baro corrected, some higher end sets, and even then, not official).


Not unusual. GPS indicates absolute altitude above mean sea level and unless baro corrected, will not agree with the pressure altimeter.

Is GPS foolproof? Of course not, and the same can be said for VOR (beam bending), NDB (night or coastal effect), LORAN (precip static, lane skip), etc. Just another aid to put in the pilots pocket when needed. I can recall when pressure-pattern navigation was used, and at the time figured it was the cats pajamas.
But THEN we had a Navigator...a whole 'nother subject.

4th Sep 2002, 21:49
GPS vertical measurements are referenced to the WGS84 ellipsoid model, not sea level or the local geoid. Additionally, the accuracy of the GPS vertical measurement is in the order of x 1.5 less than the lateral measurement.

It is possible to convert the height above the ellipsoid to the height above the geoid (i.e. the actual earth), but this still involves using a mathematical model with inherent inaccuracies.

Elliot Moose
4th Sep 2002, 22:44
Why no GPS in the EU? One phrase "George W. Bush"!

Few people dispute the unbelievable universal accuracy of the GPS system. I have used it for primary navigation in remote areas for several years--even before the Americans stopped degrading the signal four or five years ago it was generally accurate within about 10-30 feet-- and it really is the best.

However, the assurances made by the Americans that they will never again degrade the system are only that--promises. No matter what, they still control the switch on the whole damn thing and I must admit that the rest of the world (i.e. not North America) has a point in being reluctant to to take them at their word.
An ideal world would see GPS as the sole source for enroute and non-precision approach flying, but unlike the present system where everybody controls their own air navigation facilities totally, the yankees are the sole owners of the GPS system. Unless they somehow "give up control" either by sharing control or allowing other countries to own some of the satelites, they will have ultimate control over the world's skies in a GPS only future. I for one don't trust their assurances for a minute; if they feel it is in their interest to flip the switch, they will. That said, for me it is a risk I'm willing to take in exchange for the benefits of the system. I can certainly understand why some people aren't willing to take that risk though.;)

5th Sep 2002, 07:32

Have no idea why the note is not on the GPS chart.

I looked in the AIM, and it might be better to ask why the ILS and VOR plates have the 'procedure not authorised without local altimeter setting' notes.

Section 5-4-5 a 4 (http://www.faa.gov/atpubs/aim/Chap5/aim0504.html#5-4-5a) . Instrument Approach Procedure Charts, starts

Approach minimums are based on the local altimeter setting for that airport, unless annotated otherwise;
So the default is to use the local altimeter.

and the section ends

When the altimeter setting(s) on which the approach is based is not available, the approach is not authorized.
So the GPS plate is not authorised without local altimeter because the AIM says so. I guess the ILS and VOR notes are just helping out the people who have forgotten that part of the AIM. :)

6th Sep 2002, 13:57
To say GPS can't be used because it might be switched off or degraded is not a valid argument. ILS or VOR can become unavailable.

The problem with GPS is that it offers no consistent, guaranteed service. It is perfect on 9x% of occasions, quite good on 9y% but not reliable in 100% of situations. This means you have to have Plan B for the 0.000z% of occasions, and if you have a Plan B you might as well use that all the time.

NASA or FAA have analysed this in great depth and if I remember correctly they think several new things will have to be added to permit primary en-route use.

6th Sep 2002, 16:29
It may come as a great surprise to many to learn that several airlines are already using GPS as a primary navaid for Oceanic operations (also in Europe on the RNAV routes), as well as many highend business jet users. 'Tis a shame that the Europeans have such a negative attitude, but hey thats OK....
The term second fiddle comes to mind with these folks.
The benefits of GPS far outweigh the small chance of any problems, IMHO.

Elliot Moose
6th Sep 2002, 21:51

I think your info is a bit out of date.

The (extremely) rare cases where GPS is not available are readily predictable with the present generation of receivers, and therefore the FAA and Transport Canada have approved GPS for both primary enroute and approach phases of flight. TC requires that you must be able to fly a conventional approach (NDB VOR ILS, etc) with onboard equipment at alternate aerodromes only.

As far as comparing the ability of governments to "shutdown" or "degrade" signals, you are correct except for one important point. If I am motoring along over the EU and say France decides to make pipi in my cornflakes by shutting down the nav aids, I will likely know about it by conventional means. The shutdown would also take some time as there is no "master switch" controlling all enroute and approach aids. Besides, I can always say "Je ne give a damn pas" and keep right on going to someplace where things are working.

GPS however does have a master switch, (and a fine tuning knob to boot) and that switch is OWNED at present by the little paranoid guy from Texas (no offense there Texans, but you have to admit his isolationist tendencies even outstrip those of his father). Now ole Dubya (and Slick Willy before him) have said "We ain't gonna touch dat switch", but they also haven't handed it over to anybody either. I guess I can't blame them, they built the system. They could effectively shut down all or part of the world at a moment's notice. During the gulf war (before they made the big promise) they did just that. THAT is the difference. If the Brits decide to do this, they can only affect their own airspace. The USA can affect everybody's GPS, and that is where it is unreliable to the rest of the world.

I've said it before......they have a point there!:eek:

10th Sep 2002, 13:53
I think that what twistedenginestarter meant to say is ... "NASA or FAA have analysed this in great depth and if I remember correctly they think several new things will have to be added to permit sole enroute use." Primary use is already available in the USA, Australia and, yes, even in little ol' Papua New Guinea.

Nobody's game to bite the bullet on SOLE use, for reasons of reliability, as already stated here. The problem faced by the US FAA is that they've pinned their finances and reputations on WAAS. With this in place, sole use becomes feasible. Sadly for them, the work on WAAS hasn't been going well and, if memory serves me correctly, the US Vice President (perhaps the previous one) stated some time ago that it was highly unlikely that SOLE use would ever be possible.

Even so, the technology is pretty good the way it is. The experience in PNG, aside from a few cowboys who didn't live long enough to appreciate the system's limitations, is good. We get excellent satellite coverage here, being so close to the equator, so we're very happy with the level of operational approval that exists right now.

Of course, we also don't get the sort of interference that has happened, for example, in Italy. Very few FM radio stations and very few radio-controlled taxis and buses. I know that it's horses for courses, but as far as we're concerned, this horse is a real winner!

10th Sep 2002, 18:24
Wonderful subject.

A few points at random:

1. GPS is in widespread use in Europe. The incentive to create GPS approaches has always been much less than in the USA because of the WX. Far, far more European runway ends have at least Cat 1 ILS so the business case (which is what it's all about) is poor for the incremental advantages. And there is far, far less GA/bizav looking for GPS non-precision approaches which has been a huge US driver.

2. As has now become apparent, GPS Cat 3 approaches, which are what are needed by the people who actually pay the bills in Europe are a long way off - if do-able at all.

3. I spent hundreds of hours discussing GPS with US FAA and related officials in the mid-90s. They frequently, honestly could not understand why Europe wouldn't just jump for it. I used to say: take a look at all the literature on GPS, substitute the word French (or Austrian, or Greek or even British) everywhere it says US and then consider how the USA would feel about signing up to a system entirely controlled by those people. An effective argument I found.

4. Aviation is a tiny user of GPS/Glonass/GNSS. It only matters at all because a) it's got an exceptionally fierce safety requirement and b) it's got deep pockets. But GPS etc at the moment is very heavily about job and technology creation and protection - there are public domain reports on both the US and European sides making this totally and unashamedly clear. In government circles nobody really pretends otherwise. It's a multi-billion dollar/euro industry for a good few years to come and there is absolutely no way Europe is going to let it pass by.

Stop me, I could go on...

11th Sep 2002, 02:12
Some sensible comments by Algy. The European operating environment is, indeed, not the ideal place to take full advantage of GPS's many benefits. At present, I find EMS heli ops to remote locations under IFR are seeing some of the greatest rewards.

As an aside, I think the ownership/liability/legal issue has been exaggerated for political ends. There wasn't a lot of fuss made about using Omega/Loran....

Check 6
11th Sep 2002, 06:03
The U.S. has probably ten times (maybe more) as many airports as Europe.

It would not be economically feasible, or justifiable, to install an ILS at all of them.

GPS allows many U.S. airports that would not otherwise have an approach, or only an NDB approach, to have a more precise approach with lower minimums.

12th Sep 2002, 15:23
What's the status of GLONASS? Is it kaput? There was a bit of talk about using it as a 'European owned' GPS alternative, if I recall correctly.

12th Sep 2002, 15:57
GLONASS is not European, it's Russian. Here is a link to their official site http://www.rssi.ru/SFCSIC/english.html

13th Sep 2002, 07:59
The European one is called Galileo. Have a look at this (http://www.esa.int/export/esaSA/navigation.html)

13th Sep 2002, 12:48
The last I heard about Galileo - admittedly quite a while ago - it didn't look like a starter. The word I had was that the French were looking for contributions from other EU member States to get it off the ground, so to speak. The thoughts expressed to me suggested that most other EU partners weren't happy to part with the serious sort of money that France was looking for, especially as GPS already exists and is free, albeit with the sort of limitations and reservations that have already been expressed in this thread.

Has the situation changed in respect of Galileo in the last 12 months?

14th Sep 2002, 05:09
Just a little correction. The LTN-101 IRUs fitted to just about all European CRJs do not have the built-in GPS and thus AIME. The RAIM is done in their Collins GPS-4000 with overall checks in the FMS. Most North American CRJs are not fitted with IRUs but have AHRS instead (the driver is that IRUs are required for the CAT IIIa HGS).

Shore Guy
14th Sep 2002, 12:13
Looks like the US DOT (FAA, Coast Guard, etc) are finally realizing that ground based navaids should be left intact and maintained because of the relative ease to degrade and/or scramble the GPS signal. Was at a boat show not long ago speaking with a Raytheon sales/engineering type, and they had received a number of complaints from boaters in the Tampa Bay area (Florida, USA). Seems the D.O.D. had affected the signal near MacDill AFB, home of the US Central Command, which was/is controlling the effort in Afghanistan.

As to the accuracy of Differential GPS, I remember reading of a NASA project to drive an attitude indicator in a general aviation aircraft using DGPS. As I recall, the differential transmitter was in the center of the aircraft with receivers at each extremity – the initial test flight resulted in a slightly jumpy display of the attitude display. After downloading all the data, the culprit was found…..wing flex.