Global Positioning System

I

Introduction

Global Positioning System (GPS), space-based radio-navigation system, consisting of 24 satellites and ground support. Operated by the United States military but open to civilian uses, GPS provides users with accurate information about their location and velocity anywhere in the world. GPS is one of three satellite-based radio-navigation systems. The Russian Federation operates the Global Orbiting Navigation Satellite System (GLONASS), which also uses 24 satellites and provides accuracy similar to GPS. The European Union (EU) launched the first satellite in its planned Galileo program, also known as the Global Navigation Satellite System (GNSS), in December 2005.

II

Management and Purpose of GPS

GPS, formally known as the Navstar Global Positioning System, is operated and maintained by the United States Department of Defense. The National Space-Based Position, Navigation, and Timing Executive Committee manages GPS. The deputy secretaries of the Departments of Defense and Transportation lead the committee, which has a permanent staff that is responsible for the development of GPS.

GPS was initiated in 1973 to reduce the proliferation of navigation aids. By overcoming the limitations of many existing navigation systems, GPS became attractive to a broad spectrum of users. It was initially used as a navigational aid by military ground, sea, and air forces. In more recent years, GPS has been used by civilians in many new ways, such as in automobile and boat navigation, hiking, emergency rescue, and precision agriculture and mining.

III

How GPS Works

The GPS system was designed for 24 satellites. Each satellite lasts about ten years. Replacement satellites are placed in orbit regularly to ensure that at least 24 satellites are always functioning. The device that receives the GPS signal is known as a receiver. Handheld or wrist-mounted GPS receivers are available to the civilian population; GPS receivers can also be installed in automobiles and boats.

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An atomic clock synchronized to GPS is required in order to compute ranges from these three signals. However, by taking a measurement from a fourth satellite, the receiver avoids the need for an atomic clock. Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and velocity.

IV

The Components of GPS

GPS has three components: the space component, control component, and user component. The space component includes the satellites and the Delta rockets that launch the satellites from Cape Canaveral, in Florida. GPS satellites fly in circular orbits at an altitude of 20,100 km (12,500 mi) and with a period of 12 hours. The orbits are tilted to Earth's equator by 55 degrees to ensure coverage of polar regions. Powered by solar cells, the satellites continuously orient themselves to point their solar panels toward the Sun and their antennas toward Earth. Each satellite contains four atomic clocks.

The control component includes the master control station at Falcon Air Force Base in Colorado Springs, Colorado, and monitor stations at Falcon Air Force Base and on Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in the South Pacific Ocean. These stations monitor the GPS satellites. The control segment uses measurements collected by the monitor stations to predict the behavior of each satellite's orbit and clock. The prediction data is uplinked, or transmitted, to the satellites for transmission to the users. The control segment also ensures that the GPS satellite orbits and clocks remain within acceptable limits.

The user component includes the equipment, or receivers, used by military personnel and civilians to receive GPS signals. Military GPS receivers have been integrated into fighter aircraft, bombers, tankers, helicopters, ships, submarines, tanks, jeeps, and soldiers' equipment. In addition to basic navigation activities, military applications of GPS include target designation, close air support, “smart” weapons, and rendezvous.

The general population typically uses lightweight, handheld receivers or receivers that have been integrated into automobiles or boats. The general population uses GPS in many different ways. Surveyors use GPS to save time over standard survey methods. GPS is used in aircraft and ships for en route navigation and for airport or harbor approaches. GPS tracking systems are used to route and monitor delivery vans and emergency vehicles. In a method called precision farming, GPS is used to monitor and control the application of agricultural fertilizer and pesticides. GPS is available as an in-car navigation aid and as such, is used by vacationers and businesspeople who make frequent calls on clients or customers. Handheld GPS receivers are often used by hikers and hunters. Rescue crews use GPS to locate persons in emergency situations. A pastime known as caching or geocaching uses GPS to locate objects deliberately concealed in certain locations in an adult version of a child’s treasure hunt. GPS is also used on the space shuttle.

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