GPS.gov: Space Segment

The GPS space segment consists of a constellation of satellites transmitting radio signals to users.

The United States is committed to maintaining the availability of at least 24 operational GPS satellites, 95% of the time.

To ensure this commitment, the Air Force has been flying 31 operational GPS satellites for the past few years.

Constellation Arrangement

GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km (12,550 miles). Each satellite circles the Earth twice a day.

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Expandable 24-Slot satellite constellation, as defined in the SPS Performance Standard.

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 Air 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 at AF.mil

Technical details about the orbits, coverage, and performance of the GPS satellite constellation are documented in the GPS Performance Standards. View document

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Current and Future Satellite Generations

The GPS constellation is a mix of old and new satellites. The following table summarizes the features of the current and future generations of GPS satellites, including Block IIA (2nd generation, "Advanced"), Block IIR ("Replenishment"), Block IIR-M ("Modernized"), Block IIF ("Follow-on"), GPS III, and GPS IIIF ("Follow-on").

Learn about GPS modernization View full-size images

As of October 18, 2018, there were a total of 31 operational satellites in the GPS constellation, not including the decommissioned, on-orbit spares.

For more up-to-date constellation status information, visit the NAVCEN website. Go there

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Recent Developments