How many satellites are required to calculate an exact 3D GPS location?

To accurately calculate a three-dimensional GPS location, including latitude, longitude, and altitude, a minimum of four satellites is required.

Each GPS satellite provides a signal that contains time and position information. When a GPS receiver calculates its distance from a satellite, it does so based on the time it takes for the satellite's signal to reach the receiver. This calculation essentially determines a sphere around each satellite where the receiver could potentially be located.

With one satellite, the receiver can identify two points (on either side of the Earth's surface), but it cannot determine a specific location without ambiguity. Adding a second satellite narrows down the location to two intersections of its two spheres. A third satellite further refines this intersection to a specific line where the first two spheres intersect. However, to pinpoint an exact point in three-dimensional space, which also includes altitude, a fourth satellite is needed to provide the necessary reference to eliminate any remaining ambiguity regarding the location.

This concept highlights the fundamental principle of triangulation used in GPS technology, where the more satellites available, the more accurate and reliable the positional data can be. Therefore, for precise 3D positioning, four satellites are essential.