peplum: The pitot-static system provides Air Data, relative to the medium which the aircraft flies in, unlike the other sources you listed. An analogy; think of a boat on a river - if the river is moving at 30kts and the boat is going against the flow at 30kts, then it's GPS and INS might show 0kts... but the boat would see a flow of 30kts at it's hull.
I've finally caught up on the day's messages, and I'm intrigued and saddened that with such safety-critical data sources (Air Data sensors), there is still little redundancy in design offered. I.e. Are there 3 different air data sensors designed by 3 different companies, based on 3 different sampling principles, and then 3 of each onboard?! Pilots, and computers/software, can only compute and respond effectively (without error) if their senses / sensors can guarantee dependable data.
This could well be a systemic disease inflicting mankind lately, take for example the design of Fukushima and other BWR's around the globe... listen to what ArnieG has to say about that...
The Implications of the Fukushima Accident on the World's Operating Reactors | Fairewinds Associates, Inc