Why does a GPS receiver need four satellites instead of three in normal use?

While it is a common misconception that three satellites are sufficient for a precise fix, a GPS receiver requires at least four satellites to function accurately in practice.1

While three satellites could theoretically determine a position in three-dimensional space by finding the intersection of three spheres, this assumes the receiver has a perfectly synchronized clock. Because GPS receivers typically use less expensive, non-atomic clocks, they cannot measure the signal travel time with the extreme precision required to match the atomic clocks on board the satellites.12

The fourth satellite is essential to resolve this timing discrepancy. By processing signals from a fourth satellite, the receiver’s computer can calculate and offset the error in its own internal clock, allowing for an accurate determination of longitude, latitude, and altitude. Additionally, using four satellites enables the receiver to calculate velocity by measuring Doppler effect shifts.1