How bad is the best inertial navigation is? From what i see it would be pretty sufficient for civilian aviation (for fighter jets doing bunch of turns, etc. tha

How bad is the best inertial navigation is? From what i see it would be pretty sufficient for civilian aviation (for fighter jets doing bunch of turns, etc. that would be different of course) to get into the vicinity of the destination airport. Add correction by stars/Sun - any plane on long flight would get above clouds any way - and you'd get the precision of a modern ICBM - like 30m or so. Who needs GPS then? :) May be it is the time to make a startup to produce such a navigational device?