The Global Positioning System (GPS) is a U.S.-owned utility that provides users with positioning, navigation, and timing (PNT) services. This system consists of three segments: the space segment, the control segment, and the user segment. The U.S. Air Force develops, maintains, and operates the space and control segments.
GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km. Each satellite circles the Earth twice a day.
The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the Earth, each containing four “slots” occupied by baseline satellites. This 24-slot arrangement ensures there are at least four satellites in view 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.
Current and Future Satellite Generations
The GPS constellation is a mix of new and legacy satellites. The following text describes the various generations, or blocks, of GPS satellites that are currently flying. It also describes the satellites under development as part of the GPS modernization program.
GPS Block IIA
Block IIA is an upgraded version of the GPS Block II satellites launched in 1989-1990. The “II” refers to the second generation of GPS satellites, although Block II was actually the first series of operational GPS satellites. The “A” stands for advanced.
Developed by Rockwell International (now Boeing), the IIA series production comprised a total of 19 satellites: Space Vehicle Number (SVN) 22 through SVN-40. The first IIA was launched in November 1990, and the last launch occurred in November 1997. As of January 2012, there were 10 Block IIA satellites remaining in the GPS constellation, including two that have operated for over 20 years.
• Coarse/Acquisition (C/A) code navigation for civil users.
• Precision P(Y) code navigation for military users.
• 7.5-year design lifespan.
GPS Block IIR
The IIR series were produced to replace the II/IIA series as the II/IIA satellites gradually degraded or exceeded their intended design life. The “R” in Block IIR stands for replenishment.
Developed by Lockheed Martin, the production consisted of a total of 13 satellites: SVN-41 through SVN-47, SVN-51, SVN-54, SVN-56, and SVN-59 though SVN-61. The first successful launch occurred in July 1997, and the last in November 2004. As of January 2012, there were 12 IIR satellites in the GPS constellation, forming the backbone of today’s GPS along with the IIR(M) series.
• On-board clock monitoring.
GPS Block IIR(M)
The IIR(M) series of satellites are an upgraded version of the IIR series, completing the backbone of today’s GPS constellation. The “M” in IIR(M) stands for modernized, referring to the new civil and military GPS signals added with this generation of spacecraft.
Developed by Lockheed Martin, there are eight IIR(M) satellites: SVN-48 through SVN-50, SVN-52, SVN-53, SVN-55, SVN-57, and SVN-58. The first IIR(M) was launched in September 2005, and the last launch occurred in August 2009. As of February 1, 2012, there were seven healthy IIR(M) satellites in the GPS constellation, with the final one (SVN-49) set to “unusable” status but transmitting signals for test purposes.
• Second civilian GPS signal (L2C) for improved performance in commercial applications.
• Two new military signals providing enhanced military jam-resistance.
• Flexible power levels for military signals.
GPS Block IIF
The IIF series expand on the capabilities of the IIR(M) series with the addition of a third civil signal in a frequency protected for safety-of-life transportation. The “F” in IIF stands for follow-on. Compared to previous generations, GPS IIF satellites have a longer life expectancy and a higher accuracy requirement. Each spacecraft uses a mix of rubidium and cesium atomic clocks to keep time within 8 billionths of a second per day. The IIF series will improve the accuracy, signal strength, and quality of GPS.
Developed by Boeing, the IIF series includes a total of 12 satellites: SVN-62 through SVN-73. The first IIF satellite launched in May 2010. As of January 2012, there were two operational IIF satellites in the GPS constellation.
• Operational version of the third civilian GPS signal (L5) for transportation safety.
• 12-year design lifespan.
• Extremely accurate atomic clocks.
• Additional information about the GPS IIF satellites is available from the manufacturer’s website.
GPS Block III
Currently under development by Lockheed Martin, the GPS III series (fact sheet) is the newest block of GPS satellites (SVN-74 and up). GPS III will provide more powerful signals in addition to enhanced signal reliability, accuracy, and integrity — all of which will support precision, navigation, and timing services. Based on the current contracts and funding, four GPS III satellites will be produced with options to purchase an additional eight satellites. Future versions will feature increased capabilities to meet demands of military and civilian users alike.
• Fourth civilian GPS signal (L1C) for international interoperability.
• 15-year design lifespan.
• Future: Distress Alerting Satellite System (DASS) for search and rescue.
• Future: Satellite crosslinks for rapid command and reduced age of data.
Update On January 12, 2012, the Air Force awarded Lockheed Martin a contract for production of the third and fourth GPS III satellites.