As a cabin crew member i have stopped pax using hand held GPS systems, in this day and age post 9/11 i have to be a little bit suspicious of someone who wants to know EXACTLY where they are at any given time in the flight. Also other pax see someone sticking antenna to the window and they also get a bit worried.I do realise that most of the time its just someone with a spotters gene deficiancy but you never can tell.