How does a GPS unit work?

Why won't a personal camping/hiking GPS unit (like Garmin, Magellan, PC style) work inside an airplane?

• I've tried this several times with different devices and can never get a satellite signal.  This confuses me because the same units that I've tried this with can receive a signal indoors with many floors above me.  As long as I'm near a window, I can get a signal.  I would think that being 30,000 feet up, the satellite signal would be even stronger.  The only answer I've heard that I've found a slightly believable is that GPS units available to the public can't track satellites while moving at 500-600mph during flight, but aviation GPS units can.

• Answer:

  Speed and altitude aren't the issue here, because COCOM limits only come into play at 1,000 knots or 60,000ft; commercial aircraft cruise at ~500 knots and ~35,000ft, respectively. The more likely culprit is the metal fuselage of the aircraft: GPS signal was designed to be hidden in noise, so it is a very faint (-130dBm) signal by the time it gets to you. The metal skin of the plane is very good at blocking it. Sticking the GPS antenna near the window will only help a little bit, because most of the satellite constellation will still be blocked (a minimum of 4 GPS satellites are needed for GPS to calculate position - not just 3 for triangulation).