Yes - as I said in my original reply, the aerial is line-of sight - the more sky (or runway) it can directly see, the better. Trees, walls, and roof tiles all greatly cut down its range.
I wonder if the signals do reflect off walls, though, because I originally had the AirNav antenna on my study windowsill and I still got some signals that would have otherwise come through most of my house - anyone know about signal reflections at this frequency?
Rod