How many satellites are needed for a two-dimensional GPS fix?

For a two-dimensional GPS fix, two satellites are needed to determine the receiver's position in terms of latitude and longitude. The reason for this is based on the geometry of satellite positioning and trilateration.

When a GPS receiver interacts with a satellite, it calculates the distance to that satellite based on the time it takes for a signal to travel from the satellite to the receiver. However, to pinpoint a location on the Earth's surface, at least two distances are required to create an intersection point on a map.

The first satellite provides one distance measurement, which represents a sphere around that satellite. The second satellite’s signal allows for a second spherical measurement, and the intersection of these two spheres determines two potential locations on the Earth’s surface. However, due to the Earth's curvature, typically only one of these locations is valid based on the receiver's altitude or other known factors.

Using only one satellite offers no positional data beyond the spherical range of that single satellite, while three satellites are necessary for a three-dimensional fix, which includes altitude information. Four satellites are utilized for more accurate readings and corrections for atmospheric delays, but for basic latitude and longitude, two are sufficient.