Since 1st October EGNOS (European equivalent of WAAS) has been fully operational and certified for "safety critical applications such as flying aircraft". It is accurate to 1 to 2 meters horizontally and 2 to 4 meters vertically so ideal for precision approaches for which it has been designed. (I believe the first trial precision IAP is underway at Heathrow).
Do you have a reference for it being certified for "safety of life" applications? The website still says 2010 or 2011 or something like that. IOW, can a GNS530
W use the EGNOS signal today?
Google turns up
this and
this and many others.
EGNOS has been operational for a number of years now. The signal has been transmitted with a TEST flag set which prevents any IFR GPS from using the signal. Perversely only handhelds can use the signal.
Beagle - I am aware (as most people are) that barometric altimetry has its place, for vertical traffic separation. I posted that image to make the point that the widespread garbage that GPS altitude is way off is just that, garbage.
The baro altimetry errors are complicated. Even a perfect altimeter, set to the perfectly accurate QNH by making it read the exact known elevation prior to departure, will misread substantially under non-ISA conditions at high altitudes. That is why when flying at say 10,000ft the altimeter can be 500ft out, whereas the GPS will still be accurate to, whatever, 20ft.
An IFR GPS contains a means of correcting for the geoid shape. Traditionally this is done using a lookup table an interpolating it as necessary, but one can do it using a polynomial surface. I once found some US Navy website which had some polynomials which delivered the geoid correction, within not many feet, over most of the earth's surface. But this variation is small; of the order of 50ft in N Europe.
In that example on my initial post, I obviously set the altimeter to read the airport's elevation (actually from the ATIS QNH which this time happened to be spot on) and had only flown about 50nm away (pretty well parallel to the isobars, as it happened) and climbed to only 2000ft. I also set the Southampton QNH in that area. So the opportunity for the altimeter to accumulate a significant error was minimal.