I use a Garmin GPS II+, which is a handheld, non aviation model. I attach it to the window using a suction mount from Halfords (designed for mobile phones). The only time it has ever lost signal was due to interference from the radio when I had a particular VOR tuned in. I usually fly low winged aircraft, obviously high winged aircraft may blank more of the signal from the satelite. Maplin sell suction mounts for the aerial which allows you to remotely mount the aerial on the inside of a window.