I take it that as latitude increases, the WGS84 ellipsoid model will cause a gradual increase in GPS indicated true altitude at a constant flight level, or have I got it the wrong way round ???

Greetings

GPS altitude is equal to true altitude +/- geodic correction, you can find some software computing the geodic for a given Lat Long.

And you can learn more than you actually want to know about WGS84 here.

Essentially, GPS altitudes are above a shape which is close to sea level, the errors from real sea level are small. Aircraft normally use pressure altitudes, and the daily variations from Interantional Standard Atmosphere (which a perfect altimeter would indicate) are considerable.

To avoid hitting a lump of cumulo granitus GPS altitude is more useful. To avoid hitting another airborne moving object pressure altitude is what you need.

We all know what Mr Clinton emraced and it wasn't always Hilary

Actually GPS height is equally good for terrain avoidance and aircraft separation - assuming everyone is working to a common system. It has in fact been mooted that aviation could, in time, move completely to using geometric height. Not only would this remove the complication of transition altitudes, but could save fuel by avoiding the invisible climbs and descents that are an inevitable consequence of flying through pressure systems. However many aspects of aircraft performance relate to pressure altitude. One could envisage the bizarre situation of aircraft having to request a descent simply as a result of flying into a region of low pressure.

Also, the algorithm used for the Z measurement (height) is different from that for X-Y (Lat-Long) because of the geometry of the system, and that it is optimised for land/sea use. The GPS altitude is subject to larger errors, and variation dependant on the satellite constellation being used.

I suspect we are stuck with pressure altitude as the standard for some years to come, in spite of the inherent inaccuracies.

No,the shape of the earth is known from WGS84, and the height of runway is known relaitive to this datum. Even without augmentation9e.g WAAS) GPS altitude is better than baro

Iforgot to say that vertical error is not much greater than horizontal, usually a factor around 1.5. It is simply geometry. In vertical , satellites are spread over 90 degrees, horizontal is 360degrees

Greetings

Vertical accuracy can be enhanced (for NPA, RNP...) by increasing the lateral accuracy (at that position, I should be at that height on that FPA) and through hybridation of GPS/accelerometers.
GPS is long term very precise, and IRS short term very precise, so we would have to verify if the acceleration measured by the IRS matches the change in position as given by the GPS.
:}

Ladies and gentlemen, we must not forget the effects of jamming on GPS. Because of the very low signal strength(transmitter is 10k miles away) GPS is dead easy to jam with amateur -made jammers--THIS MEANS SOME FORM OF BACK-UP

Any news on how LORAN-C is shaping as a back-up to GNSS?

Greetings

Ladies and gentlemen, we must not forget the effects of jamming on GPS. Because of the very low signal strength(transmitter is 10k miles away) GPS is dead easy to jam with amateur -made jammers--THIS MEANS SOME FORM OF BACK-UP

In fact it is a legacy from the cold war, to achieve the greatest precision the US located most of the GPS ephemeris over Moscow(In fact when we study cases of best GPS reception we call it the "Moscow case") So the Russians being very upset about the gps constellation invented hand held jammers, the story priced these jammers 150 US$ in the streets of Moscow.

The majority of the ephemeris may or may not be over Moscow, I don't know and I've not seen an authoritative cite that they were.

What I do know is that no US ICBM does or did use GPS for primary guidance. First of all, it is not needed. Inertial guidance with terminal phase radar guidance is extremely accurate. Secondly, as you said, GPS signals can be jammed with a USD 150 device and no ICBM would be risked by such a trivially defeated system.

Quote -

GPS guided weapons are provided with an integral multi-channel GPS receiver and Inertial Measurement Unit (IMU) which monitors the weapon's locations and attitude to adjust its flight path to accurately impact on the target. In low cost un-powered weapons, the guidance system adjusts the weapon's free fall to hit a pre-selected point fed into the weapon prior to takeoff. GPS is also used in guided missiles and cruise missiles, for mid-course navigation.

Further info from -
GPS/INS Precision Guidance System

Good news for once! More readable and up to date than the official sites:

International Loran Association

GPS Altitude
 

GPS Altitude
Gearpins is pretty close. GPS uses a spheroidal model of the earth (WGS 84) as a reference which is based on an equipotential surface worldwide. This averages all the peaks and troughs of the real earth's surface in terms of their gravitational effect. The altitude calculated by a GPS receiver will be the distance above this theoretical surface and may disagree with your real altitude above mean sea level. Neither can be equated to the reading on an altimeter (set to QNH) but it is probably close enough for most purposes. Just don't be surprised when you ditch in the Pacific and find your GPS reads -180ft even at high tide.
As far as GPS altitude variation with latitude is concerned, the satellites are not polar orbiting so there may be some reduction of accuracy (increased dilution of precision) in altitude near the poles. That is why you have triple IRS to iron out the wrinkles.
Hope that isn't too much information.
Cheers Chart8R

411A: I am a bit bemused by your statement:

On every aeroplane that I have flown for the last 10 years or so, and they have been both Airbus and Boeing, they have had GPS installed. I fly for a European airline, and I am European. How have we/ I not embraced it?

It is a wonderful tool, and the greatest single improvement to safety in my 34 year flying career.

No ICBM's in the USAF inventory use GPS for guidance, too easy to jam as posted above. Some aircraft dropped bombs/missiles use it for guidance though.

I am not a tin-foil hat wearing person but I believe the Euros really didn't like the fact that the USA could deny access to the system with the push of a button, one of the reasons for Galileo.

Greetings

I think we should stay on the main subject, and avoid unpleasant and useless remarks.:}
GPS was the key enabler for FANS, so whoever invented it doesnot matter, for the good of the entire industry we all have to be able to use it.:ok:

I'm not sure if where you are born affects the conclusion that the system is prone to interference in the EMI sense, perhaps the political control one.

It's an impressive implementation of a simple idea, but if you rely on it as the sole means of navigation and you'll get what you deserve one day! I think the civilian gps is flawed in security terms due to lack of signing of the data, but i welcome corrections on that. I imagine truly malicious practical exploits would be difficult. I've just read there are some simple software based approaches to anti-spoofing - it would be interesting if these criteria are required for aviation.

More so than ACAS/TCAS?