THE GLOBAL POSITIONING SYSTEM

The Global Positioning System (GPS) was originally developed and deployed by the United States Air Force as a military application. GPS is now a U.S. Government-owned utility that provides users with positioning, navigation, and timing (PNT) services. The system consists of three segments: space, control, and user; the U.S. Air Force develops, maintains, and operates the space and control segments.

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 meet 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), and each satellite circles the Earth twice a day.

CONSTELLATION ARRANGEMENT

The Air Force flies more than 24 GPS satellites to maintain coverage whenever baseline satellites are serviced or decommissioned. Extra satellites may increase GPS performance, but are not considered part of the core constellation. 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.

GPS satellites provide services to both civilian and military users. The civilian service is freely available on a continuous, worldwide basis. The GPS satellites orbiting the earth transmit signals that allow GPS receivers to calculate and display accurate location, speed, date and time information to users. The U.S. Government allows companies and individuals to develop systems to augment the basic services provided by GPS by providing increased accuracy, integrity, availability, or other PNT improvements not inherently part of GPS.

A wide range of GPS augmentation systems have been developed in both the public and private sectors. The integration of GPS and Web-enabled information platforms, by companies such as PEGASAT, simplify user access to GPS data. The use of additional computing power, and data stored in memory to display road maps, topographic information and more, allow GPS receivers to convert location, speed, and time information into a more useful display format.

GPS CONTROL SEGMENT

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 includes a master control station, an alternate master control station, 12 command and control antennas, and 16 monitoring sites. The locations of these facilities are shown in the map below:



For official U.S. Government information about the GPS and related topics please see: http://www.gps.gov/.

GPS MODERNIZATION

It is the policy of the United States to maintain U.S. leadership in the service, provision, and use of satellite navigation systems. The U.S. government has additional policy goals to meet growing demands through the improved performance of GPS services, and seeks to remain competitive with international satellite navigation systems.

The GPS modernization program is an ongoing, multibillion-dollar effort to upgrade the GPS space and control segments, with new features to improve GPS performance. These features include new civilian and military signals and improvements to the space and control segments that will enhance overall performance. For example, legacy computers and communications systems are being replaced with a network-centric architecture, allowing more frequent and precise satellite commands that will improve accuracy for everyone.

For more information about the GPS modernization program please see http://www.gps.gov/systems/gps/modernization/ as well as the following videos:GPS Modernization Video - Air Force Space Command and How GPS Works - DNews.