GPS Satellite Constellation

The satellites in the GPS constellation are arranged into six equally spaced orbital planes surrounding the Earth. Each plane contains four "slots" occupied by baseline satellites. This 24-slot arrangement ensures users can view at least four satellites from virtually any point on the planet.

The Space Force normally flies more than 24 GPS satellites to maintain coverage whenever the baseline satellites are serviced or decommissioned. The extra satellites may increase GPS performance but are not considered part of the core constellation.

In June 2011, the Air Force successfully completed a GPS constellation expansion known as the "Expandable 24" configuration. Three of the 24 slots were expanded, and six satellites were repositioned so that three of the extra satellites became part of the constellation baseline. As a result, GPS now effectively operates as a 27-slot constellation with improved coverage in most parts of the world. Learn more about it here: http://www.schriever.spaceforce.mil/News/Article-Display/Article/277054/50-sw-completes-gps-constellation-expansion/

Technical details about the orbits, coverage, and performance of the GPS satellite constellation are documented in the GPS Performance Standards here: https://www.gps.gov/technical/ps/