^ yes, the windows. No doubt about it. With my GPS, which picks up much weaker signals in bands (frequencies) similar to cell phones, the nearer I am to windows (on regular aircraft of course) the stronger the signal. It also depends on angle to the window, since GPS signals propagate roughly on line of sight. If I move my GPS away from the window, say a foot below, I lose most or all signals. Middle seat it may work on the tray table (line of sight again) but not below. None of this is any surprise if you understand electromagnetic wave propagation and shielding. As others have said, the fuselage is a great shield.