The Global Positioning System (GPS) is a satellite-based navigation system consisting of three segments: the space segment, control segment, and user segment. The space segment includes a constellation of 24-31 GPS satellites transmitting radio signals. The control segment consists of ground facilities that track the satellites and maintain their orbits. The user segment includes GPS receivers that receive satellite signals to calculate location and time. GPS provides positioning, navigation, and timing services to both civilian and military users worldwide.

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The Global Positioning System
What is GPS?
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.
 Space Segment
The space segment consists of a nominal constellation of 24
operating satellites that transmit one-way signals that give the current GPS
satellite position and time.
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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ENLARGE
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.
 Control Segment
The control segment consists of worldwide monitor and
control stations that maintain the satellites in their proper orbits through
occasional command maneuvers, and adjust the satellite clocks. It tracks

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the GPS satellites, uploads updated navigational data, and maintains health
and status of the satellite constellation.
The GPS control segment consists of a global network of ground facilities
that track the GPS satellites, monitor their transmissions, perform analyses,
and send commands and data to the constellation.
The current Operational Control Segment (OCS) includes a master control
station, an alternate master control station, 11 command and control
antennas, and 16 monitoring sites. The locations of these facilities are
shown in the map above.
Air Force Personnel: Team Blackjack
Air Force video: 2SOPS keeps GPS flying! (2009)
The GPS constellation delivers consistently high performance
thanks to the dedicated efforts of its operators — the men and women of
the U.S. Air Force's 2nd Space Operations Squadron (2SOPS) and the Air

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Force Reserve's 19th Space Operations Squadron (19SOPS) at Schriever Air
Force Base, Colorado.
Together, 2SOPS and 19SOPS — nicknamed Team Blackjack — keep the GPS
satellites flying on a 24/7 basis, with continuous availability and high
accuracy for billions of civilian and military users.
 User Segment
The user segment consists of the GPS receiver equipment,
which receives the signals from the GPS satellites and uses the transmitted
information to calculate the user's three-dimensional position and time.
 ike the Internet, GPS is an essential element of the global information
infrastructure. The free, open, and dependable nature of GPS has led
to the development of hundreds of applications affecting every aspect
of modern life. GPS technology is now in everything from cell phones
and wristwatches to bulldozers, shipping containers, and ATM's.
 GPS boosts productivity across a wide swath of the economy, to
include farming, construction, mining, surveying, package delivery,
and logistical supply chain management. Major communications
networks, banking systems, financial markets, and power grids depend
heavily on GPS for precise time synchronization. Some wireless
services cannot operate without it.
 GPS saves lives by preventing transportation accidents, aiding search
and rescue efforts, and speeding the delivery of emergency services
and disaster relief. GPS is vital to the Next Generation Air
Transportation System (NextGen) that will enhance flight safety while
increasing airspace capacity. GPS also advances scientific aims such as
weather forecasting, earthquake monitoring, and environmental
protection.
 Finally, GPS remains critical to U.S. national security, and its
applications are integrated into virtually every facet of U.S. military
operations. Nearly all new military assets -- from vehicles to munitions
-- come equipped with GPS.

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GPS Services
GPS satellites provide service to civilian and military users. The civilian
service is freely available to all users on a continuous, worldwide basis. The
military service is available to U.S. and allied armed forces as well as
approved Government agencies.
 Augmentations
A variety of GPS augmentation systems and techniques are
available to enhance system performance to meet specific user
requirements. These improve signal availability, accuracy, and integrity,
allowing even better performance than is possible using the basic GPS
civilian service.
 A GPS augmentation is any system that aids GPS by providing
accuracy, integrity, availability, or any other improvement to
positioning, navigation, and timing that is not inherently part of GPS
itself.
 A wide range of different augmentation systems have been developed
by both the public and private sectors.
 To meet specific requirements, the U.S. government has fielded a
number of publicly available GPS augmentation systems, including (but
not limited to) the following systems.

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 Nationwide Differential GPS System (NDGPS)
 NDGPS is a ground-based augmentation
system that provides increased accuracy and integrity of GPS
information to users on U.S. waterways.
 The system consists of the Maritime Differential GPS System operated
by the U.S. Coast Guard.
 The inland component funded by the Department of Transportation
was decommissioned in August 2016.
 NDGPS is built to international standards, and similar systems have
been implemented by 50 countries around the world.
 Performance
The outstanding performance of GPS over many years has
earned the confidence of millions of civil users worldwide. It has proven its
dependability in the past and promises to be of benefit to users, throughout
the world, far into the future.
The Future of GPS
 Modernization
The United States is committed to an extensive
modernization program, including the implementation of a second and a

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third civil signal on GPS satellites. The second civil signal will improve the
accuracy of the civilian service and support some safety-of-life applications.
The third signal will further enhance civilian capability and is primarily
designed for safety-of-life applications, such as aviation.