INS is certified as SOLE MEAN of NAVIGATION
GPS is certified as PRIMARY MEAN of NAVIGATION.
Well, as in many things in aviation, it actually
depends.
In 1997, I was operating L1011 aircraft with a dual Honeywell HT9100 GPS installation, and....it was certified as a
sole means of navigation, on oceanic sectors, worldwide.
Yes, even on the crowded north Atlantic.
Gosh, what an absolute surprise.
As to the lack of map shift that Old Smokey refers to...I have, for the last forteen years had my private twin engine aircraft fitted with a single GPS with a moving map, and....its accuracy is
unsurpassed, and now with WAAS, as accurate as a category one ILS, depending of course, on the designated instrument procedure....fully IFR approved.
As the Brits would say...the bees knees of navigation equipment.
A very few RAIM failures...perhaps half a dozen during that 14 year period, due mostly to DoD exercises, out west, in the USA (Holloman AFB and China Lake NAS.)