How many satellites are required to establish a 3D position accurately?

To accurately establish a 3D position using satellite-based navigation systems, four satellites are required. This is essential because determining a precise position in three dimensions (latitude, longitude, and altitude) involves calculating the intersection of signals from satellites.

When a receiver communicates with multiple satellites, it determines its distance from each one based on the time it takes for signals to travel. Three satellites can provide latitude, longitude, and an approximate altitude (a 2D position in a sphere), but they cannot uniquely define the altitude because there are multiple points on a sphere that would maintain the same distance from the three satellites.

The fourth satellite is critical as it provides the necessary information to resolve the ambiguity regarding altitude. It allows the system to establish a fourth measurement that helps pinpoint the exact position in 3D space. Thus, without the fourth satellite, a receiver cannot accurately determine its altitude and, consequently, its precise 3D location.