How many satellites are needed for a 2D position fix?

To achieve a 2D position fix using GPS technology, three satellites are required. A 2D fix provides horizontal position information (latitude and longitude), but does not provide elevation data. The reason three satellites are sufficient is that each satellite helps triangulate the position based on the distances from the satellites to the receiver.

The first satellite provides a distance that could be anywhere along a sphere around it; the second satellite's distance narrows that down to the intersection of two spheres, resulting in a circle. Adding a third satellite allows the position to be pinpointed at two locations on that circle, and the fourth dimension (the altitude or elevation) is determined by accounting for the time synchronization of the GPS signals. Thus, with three satellites, a 2D fix can be accurately determined.

Using one or two satellites would not provide enough information to resolve both dimensions in the horizontal plane, hence why those choices fall short of achieving a 2D position fix.