Originally Posted by chuzwuza
Airbourn gps aims to acquire 4 satellites to triangulate lat, long positions and also altitude. Ground based gps such as cell phones obviously dont require altitude hence will acquire 3 satelites.
Ground based receivers do require 4 satellites to compute a position. In order to compute a position with only 3 satellites, the receiver elevation must be known to the receiver. Being "ground based" is not the same as at a known elevation. A "ground based" receiver can have an elevation anywhere from -400m to +8800m. That's not close enough to calculate an accurate horizontal position. Now, in situations as on a boat on the ocean, where you know that your elevation will be sea level (or close enough, anyway) that information can be entered into the receiver, and it is then able to compute a position based on only 3 satellites. Now, it may be that there is some trickery going on in a cell phone that allows the gps receiver within to know it's approximate elevation based on the network cell it's in (I'm locked to cell XX56Mx, therefore, my elevation is approximately 85 m above sea level) IN that case a receiver could determine a position using only 3 satellites. But it is not generally true that a "ground based" receiver only needs 3 satellites for a position.