Good God, some here are
still complaining about GPS being 'switched off' by the US DoD.
Get over it, ain't gonna happen, full stop.
Can it be used stand alone on the north Atlantic, as a primary means of navigation?
You bet...and I have done so many times.
Dual (or triple) Honeywell HT9100 units have been thus used for over ten years.
'Tis a fact.
Can an INS/IRS be updated with GPS?
You bet, as others have correctly indicated.
And as for BEagle...of course it was a
Collins unit, the best there is, bar none.
Except
...perhaps for Hamilton Sundstrand, the heart of the L1011 FMS.
Ahhh....Lockheed!