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.

Graphic showing the six orbital planes of the constellation
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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"), and GPS III.

Learn about GPS modernization View full-size images
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Legacy Satellites Modernized Satellites
GPS IIA satellite GPS IIR satellite GPS IIR-M satellite GPS IIF satellite GPS III satellite
Block IIA Block IIR Block IIR-M Block IIF GPS III
0
operational
12
operational
7
operational
12
operational
In production
  • Coarse Acquisition (C/A) code on L1 frequency for civil users
  • Precise P(Y) code on L1 & L2 frequencies for military users
  • 7.5-year design lifespan
  • Launched in 1990-1997
  • Last one decommissioned in 2016
  • C/A code on L1
  • P(Y) code on L1 & L2
  • On-board clock monitoring
  • 7.5-year design lifespan
  • Launched in 1997-2004
Learn more at af.mil
  • All legacy signals
  • 2nd civil signal on L2 (L2C)
    Learn more
  • New military M code signals for enhanced jam resistance
  • Flexible power levels for military signals
  • 7.5-year design lifespan
  • Launched in 2005-2009
Learn more at af.mil
  • All Block IIR-M signals
  • 3rd civil signal on L5 frequency (L5)
    Learn more
  • Advanced atomic clocks
  • Improved accuracy, signal strength, and quality
  • 12-year design lifespan
  • Launched in 2010-2016
Learn more at af.mil
  • All Block IIF signals
  • 4th civil signal on L1 (L1C)
    Learn more
  • Enhanced signal reliability, accuracy, and integrity
  • No Selective Availability
    Learn more
  • Satellites 11+: laser reflectors; search & rescue payload
  • 15-year design lifespan
  • First launch no earlier than 2018
Learn more at af.mil

As of October 17, 2017, 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

GPS III

Get more info from the prime contractor (lockheedmartin.com) View GPS III videos

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