Is WAAS Accuracy Superior To ILS?
 

Does WAAS generate more accurate position information?

"Raytheon, a WAAS contractor, measured horizontal accuracy at three meters and vertical accuracy at four meters. By comparison, a Cat III ILS is accurate only to 7.6 meters in both planes at the middle marker."

Replacing the ILS: the Wide-Area Augmentation System (WAAS) will provide ILS-like accuracy with GPS. Can it replace the familiar ground-based system on which we depend?(Instrument Landing System) | Article from Aviation Safety | HighBeam Research

If this is true, why is WAAS not deemed sufficient for all Cat III approaches?

:confused:

Wide area augmentation is not new, its been under consideration since the mid 90s at least, and mainly refered, then, to on-route navigation unless local area augmentation is included such as airport located differential GPS reference station or psuedolites the latter used to provide additional ranging signals to improve dilution of precision (DOP) geometries. This is the main problem with GPS in the vertical channel. DOP is a geometric satelite orbital configuration issue as seen at the user location that constantly changes as the satelites move in their 12 hour orbits. This is one reason why there is a push to increase the number of space vehicles in orbit to mitigate occurance of DOP chimneys (peaks). Prediction of DOP chimneys is possible so that during unaceptable DOP periods aircraft can be duely forewarned caveating GPS aided navigation - not good if you're caught in the middle of a CAT II/III landing.

GPS as a sole means system down to CAT III is a delicate issue. One of the areas WAAS tries to adress is the integrity and aviability of the GPS signal for life critical appications. CAT III requires stringent availability and integrity figures which are better met by ILS systems. Getting a consensus on these figures is one poblem, DoD supreme control of GPS is another. The civil WAAS system is built around a free in-times-of-peace military system hardly a solid foundation on which to build. Things may change when dedicated civilian systems come online, designed from the outset with cooperative overlay systems in mind suitable for aviation.

Accuracy figures provided are likely 2 sigma averages under good DOP conditions.

About the same as CAT I.

Far too early...not nearly enough time in service for a determination.

A distinctly European/UK problem...the not invented here syndrome.:rolleyes:

.:rolleyes:


Agreed a combination of the not invented here, they may take it away , signals not good enough problem ... Free GPS is great and also a great way to be totally had by the short and curlys.

ILS approaches to a CAT I level will be available in europe only with a ground based augmentation system, WAAS (or EGNOS here) is not deemed sufficient for that. Of course there is a certain hesitation to rely solely on a system where none of the participating parties has any control over, same as most probably nobody in the US is going to use GALILEO once it is live (if ever).

But of course the positive sides of using GPS technology are seen, after all the current GBAS stations can provide for up to 48 precision approaches in around 30NM around the station, very worthwile and cost effective all around. However trials are quite scarce with only two that i know of operating in the last few years, one of them currently withdrawn as the beta level station is being replaced with one that can be certified for normal operational standard (EDDW).

I don't want to be pedantic, but wouldn't it be of greater benefit to other readers, particularly to students, if we used the term SBAS (Satellite Based Augmentation Service) to describe what we are talking about, rather than the terms WAAS (Wide Area Augmentation Service), which is the US FAA implementation of SBAS or EGNOS (European Geostationary Navigation Overlay Service), which is the European Space Agency implementation of SBAS?

There are other SBASs up and active, the Japanese have a system and I believe that the Indians have a system as well. No doubt others will be established in the future.

If this was an exclusively American or exclusively European forum, it would make sense to refer to the specific regional system - but our forum, and this discussion, is global.

One must never forget that it's an integrity issue as much as an accuracy one.

Any GA pilot will brag about how the GPS brought them right down on the numbers time and time again. But of course that one time in a thousand when it drops out at the critcal moment...

...doesn't bear thinking about.

First question, yes, but I suspect you've confused things a bit. 7.6m/7.6m is the spec accuracy for WAAS. Measured accuracy is <3m horizontal, high-end systems about twice as good. Vertical accuracy is not good enough. Cat III will require <1m accuracy. The proposed, well, in development system for Cat III approaches is currently called LAAS (Local Area Augmentation System) They're both forms of differential GPS (dGPS).

LAAS works a lot like the old dGPS. dGPS was to transmit correction data on the piggy-backed signals of high-powered FM stations. Two problems: it degraded in accuracy the further you were from the station limiting its certified accuracy to 10m and, high-powered FM stations tended to only be available in large metro areas. This limited its usefulness for long range navigation even in the con-US. It also had the problem that you had to have a separate receiver for the differential data and a plug in your GPSr to accept that data, meaning everybody (land,sea and air) had to upgrade their equipment including the manufacturers. WAAS got around that by piggybacking on a GPS signal and using geosynchronous satellites to deliver the correction data.

As I said, LAAS uses the same concept, but with a few extra twists. Each LAAS-equipped airport will have its own augmentation reciever stations and serve as its own master station and corrections transmitter all-in-one. It won't be dependent upon GEO satellites. This should mitigate the high latitude deficiencies of WAAS. It accepts the range limitation, even embraces it. By limiting the certified range the certified accuracy goes up. You'll still use WAAS for route navigation. One piece of technology can serve the whole airport and because you wouldn't be following a beam, straight in approaches aren't neccessary. Every runway could be Cat III as long as they met the lighting, etc. requirements.:ok:

Read second post

Pity the Europeans.
They appear to be so far behind with the acceptance of GPS, it is a real shame.
One wonders just when they will get their own system up and running.
Don't hold your breath.:rolleyes:

I share your sentiments about the irksome, small minded Europeans ( amoung which naturally the Brits don't count themselves :confused:) but, just this once, their retiscence to fall hook line and sinker for GPS is understandable.

Had GLONAS or Galileo been fully established before GPS I doubt the US would have adopted either of them for exactly the same reasons the Euros don't put all their huevos in the GPS basket.

However had Galileo predated GPS the great difference to the world would have been that the system would not be have been freely available to all - eurobeancounters would be counting royalties on a per reciever basis just like they collect TV license money.

Not every one is as generous as the Americans.

The highlighted goes beyond the parochial view of who invented it/who controls it/whose d:mad: is bigger:rolleyes:

The problem has more to do with what happens if the signal is lost?

Partly right of course, but it is planned, as far as i know, that the basic services are free. Only the high integrity, high precision services are gonna cost money, but as a paying customer of course you have certains rights and the relevant contracts will focus on guaranteed integrity and quality of service, especially in the aviation sector. However, at the moment Galileo is way behind its schedules and cost overruns are huge, so we have to wait and see when it will be available.

It is certainly a shame that the Europeans are so far behind in the development and acceptance of GPS based systems.

But do spare a thought for the Americans.

They are completely and utterly clueless as to how they are perceived by the rest of the world, which condemns them to never understand why others might not trust them wholeheartedly.

You just content yourselves with the opinion that we are all just jealous......

Tester07, how does the rest of the world see the USA aviation wise pray tell?

It's the administration not the aviation, and you may not like the answer to that question :E

In any case, in the usual holy-war that ensues when GPS is mentioned, I think we may be missing the point here:

With an ILS you fly down the localiser beam - the closer you get to the kit, the narrower the cone, and the more sensitive *and ACCURATE* it will get. GPS is an area system, so the 3-4m accuracy will persist wherever you are.

7.6m positional error at the middle marker wouldn't seem a big deal - I would think 3-4 metres in the touchdown area could be quite significant. Seems this might be a case of choosing what you measure to make the results show what you want..

Those on the eastern side of the great Atlantic divide might like to consider the origins of the Instrument Landing System....never mind GPS.

A joint development by Sperry (an American company) and Reed Pigman...the latter individual was also greatly involved in the development of the VOR system of navigation.

Now, having said this, would I personally like to operate to CATIII with (augmented) GPS?

Ahhhh, no...not just yet.:uhoh:

NB.
Pigman also colaborated with Sperry on the development of the first 'true' flight director...the Sperry Zero Reader.

This tread is boring. Let's discuss GPS-jammers and their possible use during civilian GPS CAT III approaches :uhoh:

B 377:

Does GPS DOP apply to very specific locations, at specific times, at a high rate of change or to broader regions at lesser, more predictable intervals?

Thanx.

In early days of GPS ( early 90s, say) when a full satelite constellation had not matured DOP was more of an issue than today. GPS coverage is lacking in the polar regions (as semi synchronous orbits are only inclinded 55 deg) so DOP will worsen with extreme latitudes with virtually no coverage at the poles. GLONAS which uses more inclined orbits provides much better coverage than GPS in the polar regions something of interest to the Soviets.

Yes, as the satelites move in their orbits DOP geometry changes at a given location on earth with time. Some places may suffer bad DOP while others enjoy good DOP. Should for any reason satelites fail or be shut down for maintenace or have health flags set by ground control, less SVs will be available potentially creating DOP issues in certain places and times. SVs can also be moved reducing coverage at some places while improving in others ( for tactical purposes say remembering GPS is a military system)

Overdetermined solutions are always the aim in GPS (a minimum of 4 of sats will provide position and corrected GPS time - 3 satelites position only) but accuracy and integruty improve as more satelites are added to the solution (the so called all-in-view recievers process all visible satelites - involving 12 or more processing channels) either using standard linear algebra techniques or with a Kalman filter that models user motion. In fact Kalman filters help any GPS rx coast thru periods of bad DOP or loss of signals as can happen when a plane banks or a road vehicle moves into built up or wooded areas that eclipse some of the satelites - the urban canyon effect.

Not surprisingly there is a push , as I write this, to increase the number of active space vehicles for DOP and integrity reasons and of course to have quick deploy orbitting spares.