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.

You can use offline GPS navigation software on a plane. I did it several times, just as my friends. I use LG G3 and OsmAnd app.

LG G3 has very poor GPS antenna so you have to press the phone against the plane window and wait for 4-5 minutes before it finds sufficient number of sattelites.

A couple of years back my kid got the GPS working on his Windows phone, using some 3rd party app that cut through all of the other location detection crap. We were able to track our speed and location in a passenger jet flying out of Maui.