How many satellites are needed for a 3D GPS fix?

To achieve a 3D GPS fix, which provides accurate information about position in three dimensions (latitude, longitude, and altitude), a minimum of four satellites is required. This is because, in addition to two-dimensional positioning (latitude and longitude), the altitude measurement requires an additional satellite for accurate triangulation.

When a GPS receiver connects to the signals from the satellites, it uses the time delay of the signals received to calculate the distance to each satellite. With three satellites, the receiver can determine its position on a two-dimensional plane. However, without a fourth satellite, the receiver cannot reliably establish the altitude above sea level, as there would be an ambiguity in determining the height without an additional data point to eliminate that variable.

Thus, four satellites are necessary to provide the additional information needed to resolve the altitude, resulting in a complete three-dimensional fix.