Thanks Graybeard.
I actually looked it up. Cape Hallet to McMurdo is 337 n.miles, a track check 15 minutes into that leg would have shown a cross track error of 4 or 5 miles or thereabouts - enough to get your attention, I would've thought.
Tks for the heads-up on the INS - I was going on the Carousel figures, dimly remembered. Allowable drift three miles per hour plus three, I think it was.
Chz.,
Jafa.