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.
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.
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
Current and Future Satellite Generations
The GPS constellation is a mix of old and new satellites. The following table summarizes 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.
The satellite counts below were current as of June 25, 2014. They do not include the 3-5 decommissioned GPS satellites ("residuals") kept in orbit in case there is a need to reactivate them. For today's operational constellation status, visit the NAVCEN website. Go there
|Legacy Satellites||Modernized Satellites|
|Block IIA||Block IIR||Block IIR(M)||Block IIF||GPS III|
|Now in production|
GPS Block IIF
All 12 satellites in the GPS IIF series have completed production. The Air Force plans to launch the remaining IIF satellites by 2016.
UPDATE On August 1, 2014, the Air Force successfully launched the seventh GPS IIF satellite into orbit. The spacecraft will spend several weeks in check-out before going operational.
- Post-launch news release 1
- Post-launch news release 2
- Pre-launch news release
- ULA Launch mission booklet (PDF)
The Air Force launched the sixth GPS IIF satellite on May 16, 2014, and set it operational on June 10, 2014.
The Air Force launched the fifth GPS IIF satellite on February 20, 2014, and set it operational on May 30, 2014.
Get more information about the GPS IIF program from the prime contractor (Boeing):
On March 31, 2014, the Air Force announced a contract modification for Lockheed Martin to produce the seventh and eighth GPS III satellites. The Air Force previously announced a separate contract modification to purchase long-lead items for those satellites.
The Air Force exercised options for the fifth and sixth GPS III satellites in February and December 2013, and for the third and fourth satellites in January 2012.
- Dec 2013 contract info at defense.gov
- Feb 2013 contract info at defense.gov
- Jan 2012 contract info at defense.gov
Get more information about the GPS III program from the prime contractor (Lockheed Martin):