Humble apologies stilton. (A definate case of RTFQ then). You are quite correct in your comment, inertial position is still the
'prime' source of navigation, in modern aircraft with GPS etc being used to refine this position. GPS can never on it's own replace an INS/IRS, in fact at the moment it's difficult to imagine what ever could. (And after all an aeroplane is not an aeroplane without
ATTITUDE 
).
Best Regards
Dude
