Global Positioning System

Global Positioning System

Launching a GPS Satellite

A Navstar global positioning system (GPS) satellite is launched into orbit by a Delta rocket. GPS satellites continuously transmit data about the satellite’s position and the current time. Military and civilian navigators use the information gathered from several satellites to compute their own position.

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.

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 Receiver with Map

A Global Positioning System (GPS) receiver links with an array of satellites to give users their location. Many GPS units have enough memory capacity to store maps so that users can pinpoint their map location and use it to plot routes to their next destination.

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.

V

GPS CAPABILITIES

Global Positioning System (GPS)

The Navstar Global Positioning System (GPS) is a network of 24 satellites in orbit around the earth that provides users with information about their position and movement. A GPS receiver computes position information by comparing the time taken by signals from three or four different GPS satellites to reach the receiver.

GPS is available in two basic forms: the standard positioning service (SPS), or civilian signal, and the precise positioning service (PPS), or military signal. Prior to 2000 the U.S. military intentionally corrupted or degraded the SPS signal for national security purposes by using a process known as Selective Availability. As a result, the SPS signal was much less accurate than PPS. In May 2000 President Bill Clinton announced that the military would stop Selective Availability. This increased the accuracy and reliability of SPS by a factor of ten. Today, the military and civilian GPS signals are believed to be of the same accuracy. For national security reasons the Defense Department retained the ability to jam the SPS signal on a regional basis if necessary. Both the SPS and the PPS signals provide a horizontal position that is accurate to about 10 m (about 33 ft).

Finding Location with GPS

Global Positioning System (GPS) satellites orbit high above the surface of Earth at precise locations. They allow a user with a GPS receiver to determine latitude, longitude, and altitude. The receiver measures the time it takes for signals sent from the different satellites (A, B, and C) to reach the receiver. From this data, the receiver triangulates an exact position. At any given time there are multiple satellites within the range of any location on Earth. Three satellites are needed to determine latitude and longitude, while a fourth satellite (D) is necessary to determine altitude.

Several techniques have been developed to enhance the performance of GPS. One technique, known as differential GPS (DGPS), employs two fixed stations on Earth as well as satellites. DGPS provides a horizontal position accurate to about 3 m (about 10 ft). Another technique, known as WAAS, or Wide Area Augmentation System, was developed by the Federal Aviation Administration (FAA) to improve the safety of aircraft navigation. WAAS monitoring stations around the United States catch GPS signals, correct errors, and send out more-accurate signals. A technique involving the use of carrier frequency processing (see Carrier Wave), known as survey grade GPS, was pioneered by surveyors to compute positions to within about 1 cm (about 0.4 in). SPS, DGPS, WAAS, and carrier techniques are accessible to all users.

VI

EUROPEAN RIVALRY AND COOPERATION

The European Union (EU) launched the first satellite in its planned space-based radio-navigation system in December 2005. The EU system will eventually have 30 satellites and is expected to become operational in 2009. It will have an accuracy of 1 m (3.3 ft) and will be operated as a strictly civilian enterprise. Other countries outside the EU, such as China and Israel, are contributing financially to the creation of the system. Known as the Galileo program and also as the Global Navigation Satellite System (GNSS), the system will be compatible with GPS. The United States negotiated an agreement under which satellite signals could be jammed over a battlefield area without shutting down the entire GNSS or GPS system. The two systems, however, were expected to compete for commercial applications.