Commercial jetliners never fly at 18,000 m (nor at 515 m/s). Max is about 11,000 m or so, and about 250 m/s. It would be interesting to know if consumer GPS ever worked aboard the Concorde.
Merry Christmas to everyone who hasn't read it yet.
Bonus story, also from Brian Shul: https://www.reddit.com/r/MilitaryPorn/comments/1mimvu/80000_...