4 satellites are required for a 3D fix, only 3 are required for 2D.
In a perfect world this is true (ish). To get anything close to an accurate fix you need more than four satellites - I would suggest at least 6 or 7.
If you could measure the doppler, distance, position (almanac), etc EXACTLY, then yes, all you need is 4, but we can't get close to that, so need more satellites in order to average out the errors.
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I personally hate the airspace alerts on my Garmin 795, and 695 before that. They're easy to miss. Especially when you compare them to the SkyDemon, which bleeps away, and puts up a big banner across the screen which requires you to acknowledge it.