Establishment of the Global Positioning System, which consists of twenty-four Navstar satellites that orbit Earth every twelve hours, provided an innovative method of navigation.
The Global Positioning System (GPS) consists of twenty-four Navstar satellites that orbit Earth every twelve hours. Radio contact with at least four satellites is available at all times from any point on the planet. The first operational GPS satellite was launched in 1978, and by 1994, the system included twenty-four satellites.
The system, operated by the U.S. military, provides an innovative method of navigation. The satellites orbit at a height of about 20,200 kilometers (12,600 miles). People access satellite signals by using a receiver, which can be a unit solely for navigational purposes or a part of a watch or cell phone.
The technology began in 1973 when the military decided to create a satellite navigation system using the timing systems of the Air Force and Navy. The military spent several years testing the navigation system and finally sent up satellites between 1978 and 1985. In the early 1980’s, the project was in financial jeopardy; people wondered about its usefulness. It took a tragedy to finally open the technology to civilian use. In 1983, the Soviet Union shot down Korean Air Lines Flight 007 when the plane lost its way in Soviet airspace.
The flotilla of GPS satellites eventually grew to about thirty. These four-thousand-pound satellites transmit signals that can be picked up by anyone on Earth with a proper receiver. Each satellite rotates around the Earth twice each day.
The math involved in the GPS three-dimensional system is called trilateration. The system works because each satellite uses a different transmission code and carries an atomic clock. A radio signal from a satellite is of high frequency and transmits several facts to the receiver: time, satellite status, orbit data, and clock data. When a user operates a receiver, the unit is not just finding its position by one satellite; rather, the unit measures the distance from the user to at least three satellites.
A receiver determines distance by calculating the time delay between the transmission and reception of its signals. In this process, the receiver counts how long it takes for a signal to arrive. Because radio waves move at about the speed of light, the receiver can figure out how far away the satellite is. Each satellite has its own code pattern, called a coarse acquisition (C/A). The receiver can issue the same C/A by using the satellite’s number. The receiver lines up both patterns, measures the delay, and thus can tell how far away it is from the satellite.
Once the receiver knows where and how far away a particular satellite is, the receiver checks several satellites in the same manner. Signaling to three satellites allows the receiver to construct three imaginary spheres. The spheres connect at two points, one in space and one at the location of the receiver. That location information may be presented to the user as a display, telling a boater, for instance, the boat’s speed and location. Some units can show the user’s location on a map and provide turn-by-turn directions. The receiver usually has an antenna, internal processors, and an ordinary clock. Although there are some inaccuracies in how GPS works, adjustments are made by the receivers.
When a receiver is part of a wireless cell phone, distance to the nearest tower is also known and helps pinpoint location. Cell phone technology sometimes works faster than regular receivers because cell phone signals are not blocked by buildings or trees. Because GPS phones work well in buildings, businesses have used them to track employees. The U.S. government requires that all cell phones be capable of transmitting their location when the user dials 911 for emergency services.
One in a constellation of twenty-four satellites that transmits radio signals to Earth from space.
Global positioning technology has advanced clarity in navigation and positioning. It can both help the world and harm it: GPS increases the safety of travel, but, at the same time, enables greater accuracy in the use of weapons. The use of GPS by Amazonian tribes is a good example of the value of the technology to humanity. In the past, these tribes were less able to manage their homeland resources. With the advent of GPS, they became able to map their lands and monitor and help curb deforestation. This process has helped to preserve the tribes’ resources.
Because GPS can reveal the receiver’s direction, speed, altitude, and location, many uses for the technology have been developed. Recreational sports such as dogsledding, car racing, bicycling, hot-air ballooning, sailing, wind surfing, skiing, and hiking have been enhanced by the implementation of GPS. The technology is essential for pilots and boaters, and by the early twenty-first century it was becoming commonly used in cars and trucks.
GPS gives the user an exact time reference; this is useful, for example, to researchers seeking to know more about earthquakes. By utilizing GPS, researchers know exactly where a recording station was prior to an earthquake. After the earthquake, GPS can tell the researchers how far the recording station has moved.
GPS is also useful as a synchronization tool. Many systems that rely on knowledge of the exact time—such as 911 emergency computers, security devices for buildings and computers, military missiles and radar, computer clocks, and telecommunication networks—use GPS. In contrast to these peaceful and productive uses, GPS is used by the military to guide missiles. Weather and radar countermeasures can throw missiles off course, but with an onboard computer linked to a satellite, GPS-guided missiles are capable of destroying any target on Earth.
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Andrade, Alessandra A. L. The Global Navigation Satellite System: Navigating into the New Millennium. Burlington, Vt.: Ashgate, 2001. Provides an international view of issues of availability, cooperation, and reliability of air navigation services. Focuses on the American GPS and Russian GLONASS systems. -
Cooke, Donald. Fun with GPS. Redlands, Calif.: Esri Press, 2005. Illustrated volume for young people, teachers, and hobbyists outlines how to use GPS for recreational activities. -
Farrell, Jay A. Aided Navigation: GPS with High Rate Sensors. New York: McGraw-Hill, 2008. Explains the new, small sensors useful in overcoming errors such as those caused by acceleration. -
Hinch, Stephen W. Outdoor Navigation with GPS. Santa Rosa, Calif.: Annadel, 2004. Explains how to choose a GPS system for outdoor journeys. Teaches compass and map reading and how to use map datums and position formats. -
Kaplan, Elliot D., and Christopher Hegarty, eds. Understanding GPS. 2d ed. Norwood, Mass.: Artech House, 2006. Guide for engineers, students, and others who want to be knowledgeable about the technology, applications, and systems of GPS. -
Logsdon, Tom. Understanding the Navstar: GPS, GIS, and IVHS. New York: Van Nostrand Reinhold, 1995. Comprehensive explanation of the entire Navstar system and its multiple functions. -
Owings, Rich. GPS Mapping: Make Your Own Maps. Fort Bragg, Calif.: Ten Mile Press, 2005. Discusses how to choose GPS software and how to make maps. -
Rabbany, Ahmed el-. Introduction to GPS: The Global Positioning System. Norwood, Mass.: Artech House, 2006. For professionals and students who want a comprehensive introduction to GPS technology. -
Sweet, Robert J. GPS for Mariners. Camden, Maine: International Marine/McGraw-Hill, 2003. Guide for boaters explains how to plot courses, where to place the GPS antenna, and how to navigate using waypoints and routes.
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