I flew for PanAm - started flying early 1969, until their end 1991.
And I flew with their early INS systems - on the 707s...
And later flew the 747s, also equipped with the INS.
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Early INS systems were the "Litton LTN-51" or "Delco Carrousel C-IV"
We had a maximum of 9 waypoint memory... then had to enter subsequent waypoints.
Their accuracy degraded with time "in navigation", if not updated.
As an example, 1 NM error per hour of flight could be expected as average.
JFK to LHR - some 7 hours in "NAV" - INS generally would drift 7 NM off...
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We sometimes updated the INS over a known point, such as a VOR.
As an example, on the way from JFK to Europe, we would do it overhead Gander.
Then again maybe, overhead Shannon VOR, if our destination was still far.
Then later, INS were improved with "automatic update" operating with DMEs.
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With the 747, we had 3 INS systems (only 2 with the 707 generally).
So we could "triple mix" the navigation between the 3 systems.
It averaged the "inherent error" of the 3 systems and resulted in more NAV accuracy.
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I still fly the 747 - We have the Litton LTN-92s now.
Our 3 INS are extremely accurate now, in "triple mix" updated by 2 GPS receivers.
Happy contrails