Mahatma Kote,
This unit is derived from full scale INS so I ask the question why there is a problem on modern aircraft when pitot tubes and whatever are blocked / misreading?
With well trained and experienced pilots in the cockpit, there isn't a problem when pitot tubes are blocked. They know what to do, they understand attitude and thrust.
I understand there is a difference between actual air speed components and true ground-speed, but why is this high precision information source not used? Or if it is, why are things like pitot tubes more important and overriding the absolute measurements? Especially over extended period of seconds to minutes?
High precision ground speed from the 3 IRS's on board is one of the many tools a well trained and experienced pilot can use to determine which airspeed readings are erroneous and which are accurate. But not before securing the aircraft in a safe attitude and thrust setting for the conditions.
Problems arise when airline CEO's start believing that aircraft fly themselves (and indeed fix themselves), and do not believe having well-trained and experienced flight crew is necessary. This is what we call the "race to the bottom", and it doesn't only happen in Asia.
AF447 crashed because the pilots crashed it. I'm not speculating on this incident but merely answering your questions.