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I had 2 tricks - really high-order filter borrowed from some paper and the assumption that a walking human doesn't gain constant acceleration. Even in an elevator you stop on a floor which resets your speed to 0.

I agree that there is no way you could extend this directly for flying, but with modern devices and things like ground-distance radar, relative airspeed indicators and so on I don't think it is beyond the realm of possibility. Plus we have detailed hightmap of the world, which, when combined with a radar should allow for terrain tracking. That makes the accuracy of sole INS much less crucial.