There's almost no such thing as a passive radio anymore... superheterodyne receivers are the norm now and they contain a local oscillator that can leak back out into the airwaves.
Directions require more than your satellite coordinates. Map services are frequently polling servers for traffic conditions, new tiles for the map, and so on. You'd hope that these can fall back gracefully but I wouldn't put it past them to not. If you activate airplane mode and disable your phone's cellular connection, even if your phone doesn't disable the GPS receiver, directions may stop working.
Works fine on my Android. I regularly lose cellular reception in the mountains, and it continues to work. Sometimes the tiles are low-res, but still readable. I would expect Apple would design around the same contingency, along with poor cellular service along more remote areas of the Interstate.
