WAAS Up?

The reason this doesn’t matter is that we don’t land airplanes with GPS really. We direct them down a path to the airport and then the pilot flys at minimum the last 200 feet with the runway in sight. If he can’t see it, he doesn’t land. Airliners are a different animal but they don’t use GPS for that either. (Mike C)

I couldn’t imagine anyone being silly enough to think that planes rely on GPS to figure out their altitude above the runway. (P)

WAAS-qualified avionics like Garmin’s 430W, 480W and 530W receivers are approved for precision instrument approaches based entirely on GPS. (Robert X. Cringely)

There's some truth mixed in with some misunderstanding there. While GPS is able to measure altitude, aircraft systems don't rely on it for a primary altitude measurement. They already have perfectly good barometric systems which measure air pressure and are adjusted for field conditions every hour (or so). On a GPS precision approach (that is, one with both horizontal and vertical guidance) the computer combines the (x,y) information from the GPS with the altimeter's data to produce ILS-type course and glideslope deviation indicators, showing how far from the desired approach course the aircraft is in the left/right and up/down axes.

So, while you can in fact fly an approach where your position data is based solely on GPS, that z-axis is still being measured by an altimeter. (Unless Mr. Cringely is certified to fly cat II or III approaches, in which case he needs a LAAS-certified receiver and suitably equipped airport, as unlike WAAS, it is ground-based: http://en.wikipedia.org/wiki/Local_Area_Augmentation_System)

Edit: apparently my info is out of date, the two new WAAS satellites have better positioning.

Isn't that kinda like proving a negative? Why would anyone write a story about something they have concluded has no news value?