Global Positioning System

Introduction to GPS

The Global Positioning System (GPS) is the most significant recent advance in navigation and positioning technology. In the past, the stars were used for navigation.

Today's world requires greater accuracy. The new constellation of artificial stars provided by the Global Positioning System serves this important need.

GPS is an aerospace technology that uses satellites and ground equipment to determine position anywhere on Earth. Anyone with a small receiver can use the system at no cost. GPS has drastically changed methods of navigation and is fast becoming important in everyday life.

The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). Utilizing a constellation of at least 24 Medium Earth Orbit satellites that transmit precise microwave signals, the system enables a GPS receiver to determine its location, speed, direction, and time.

Other similar systems are the Russian GLONASS (incomplete as of 2007), the upcoming European Galileo positioning system, the proposed COMPASS navigation system of China, and IRNSS of India.

Developed by the United States Department of Defense, GPS is officially named NAVSTAR GPS (Contrary to popular belief, NAVSTAR is not an acronym, but simply a name given by Mr. John Walsh, a key decision maker when it came to the budget for the GPS program).

[1] The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is approximately US$750 million per year, [2] including the replacement of ageing satellites, and research and development.

Following the shootdown of Korean Air Lines Flight 007 in 1983, President Ronald Reagan issued a directive making the system available for free for civilian use as a common good.

[3] Since then, GPS has become a widely used aid to navigation worldwide, and a useful tool for map-making, land surveying, commerce, and scientific uses. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Pre-GPS For thousands of years, speed was limited to a walking pace and landmarks were used to find location. At sea, early navigators limited their voyages to coastal routes to avoid becoming lost. New methods for determining position arose as trade between distant ports increased. Polaris, the North Star, was used to determine north-south distance (latitude) in the northern hemisphere. But mariners also had to find latitude when sailing in the southern hemisphere, and they lacked a method for determining east-west position (longitude). The solution, celestial navigation, required accurate time. In the late 18th century this led to the development of the marine chronometer, an accurate sea-going timepiece. Beginning in the 19th century the U.S. Naval Observatory, the nation's official timekeeper, provided accurate time for navigators from an array of chronometers. A sextant is used to make precise celestial observations of the Sun, stars, and planets. It measures height, in degrees, above the horizon, which is used with the exact time to calculate position. The sextant shown here was made in the first half of the 19th century. "Shooting the stars" remains a basic skill for the sea-going navigator. Electronic Innovations Electronic navigation introduced all-weather capability, ease of use, and eventually, increased accuracy. In the 1930's radio beacons were used to provide bearings from airfields. During World War II radio navigation systems were developed, the best known being LORAN, or Long Range Aid to Navigation. Positions were determined by the timing of signals received from different LORAN transmitter stations. In the 1960s the Omega system provided worldwide electronic navigation coverage for the first time. These land-based electronic navigation systems were accurate to within several miles, equivalent to celestial navigation. In the mid-1960's the U.S. Navy's NAVigation SATellite System (NAVSAT), also known as TRANSIT, was developed to provide more accurate positions for ships and submarines. TRANSIT Satellite TRANSIT was the first operational satellite positioning system. Six satellites gave worldwide coverage every 90 minutes and provided positions that were accurate to within 200 meters (660 feet). Positions were obtained by measuring the Doppler shift of the satellite signal. TRANSIT was effective, but it was limited by low accuracy and lack of 24-hour availability. The TRANSIT system operated until 1996. GPS Revolution Throughout the 1960s the U.S. Navy and Air Force worked on a number of systems that would provide navigation capability for a variety of applications. Many of these systems were incompatible with one another. In 1973 the Department of Defense directed the services to unify their systems. The basis for the new system would be atomic clocks carried on satellites, a concept successfully tested in an earlier Navy program called TIMATION. The Air Force would operate the new system, which it called the Navstar Global Positioning System. It has since come to be known simply as GPS. The new system called for three components: ground stations that controlled the system, a "constellation" of satellites in Earth orbit, and receivers carried by users. The system was designed so that receivers did not require atomic clocks, and so could be made small and inexpensively. The Soviet Union also developed a satellite-based navigation system, called GLONASS, which is in operation today. GPS Satellite GPS satellite launches began in 1978, and a second-generation set of satellites ("Block II") was launched beginning in 1989. Today's GPS constellation consists of at least 24 Block II satellites. The system became fully operational in 1995. MANPACK GPS Receiver One of the first portable GPS units available to soldiers in the field was the PSN-8 "Manpack" receiver. About 1,400 were manufactured between 1988 and 1993. PLGR GPS Receiver The Manpack was replaced in 1993 by the hand-held Precision Lightweight GPS Receiver (PLGR), popularly known as the "Plugger." These units are similar to civilian receivers, but they can use higher-precision GPS signals. GPS Goes Public GPS was designed so that civilian users would not be able to obtain the same accuracy that the military could. Nevertheless, civilian as well as military applications were intended from the start. After the downing of Korean Flight 007 in 1983 -a tragedy that might have been prevented if its crew had access to better navigational tools- President Ronald Reagan issued a directive that guaranteed that GPS signals would be available at no charge to the world. That directive helped open up a commercial market. Deployment of GPS continued at a steady pace through the 1990s, with growing numbers of civilian and military users. GPS burst into public awareness during the Persian Gulf War in 1991. GPS was used extensively during that conflict, so much so that not enough military-equipped GPS receivers were available. To satisfy demand, the Department of Defense acquired civilian GPS units and temporarily changed GPS transmissions to give civilian receivers access to higher-accuracy military signals. Working of GPS Global Positioning System satellites transmit signals to equipment on the ground. GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. GPS operations depend on a very accurate time reference, which is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board. Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The signals, moving at the speed of light, arrive at a GPS receiver at slightly different times because some satellites are farther away than others. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver. When the receiver estimates the distance to at least four GPS satellites, it can calculate its position in three dimensions. There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite's orbit. Determining Position A GPS receiver "knows" the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect. Navigation through GPS GPS is a powerful tool that can save a ship's navigator hours of celestial observation and calculation. GPS has improved efficient routing of vessels and enhanced safety at sea by making it possible to report a precise position to rescuers when disaster strikes. GPS improves efficiency on land as well. Delivery trucks can receive GPS signals and instantly transmit their position to a central dispatcher. Police and fire departments can use GPS to dispatch their vehicles efficiently, reducing response time. GPS helps motorists find their way by showing their position and intended route on dashboard displays. RailroXads are using GPS technology to replace older, maintenance-intensive mechanical signals. GPS in Vehicles Many types of GPS systems can be used on vehicles, providing the driver with the current position and a local map. Nautical Chart Error The data collected from satellite navigation systems provide more accurate information for maps and nautical and aeronautical charts. This example demonstrates how charts are updated to prevent navigational mishaps. Air Navigation with GPS GPS offers an inexpensive and reliable supplement to existing navigation techniques for aircraft. Civil aircraft typically fly from one ground beacon, or waypoint, to another. With GPS, an aircraft's computers can be programmed to fly a direct route to a destination. The savings in fuel and time can be significant. GPS can simplify and improve the method of guiding planes to a safe landing, especially in poor weather. With advanced GPS systems, airplanes can be guided to touchdown even when visibility is poor. For the private pilot, inexpensive GPS systems provide position information in a practical, simple, and useful form. Earth Mapping Surveyors and map makers use GPS for precision positioning. GPS is often used to map the location of such facilities as telephone poles, sewer lines, and fire hydrants. Surveyors use GPS to map construction sites and property lines. Forestry, mineral exploration, and wildlife habitat management all use GPS to precisely define positions of important assets and to identify changes. During data collection, GPS points can be assigned codes to identify them as roads, streams, or other objects. These data can then be compared and analyzed in computer programs called Geographic Information Systems (GIS). Surveying With GPS Surveying that previously required hours or even days using conventional methods can be done in minutes with GPS. Correct Your Watch! Because GPS includes a very accurate time reference, the system is also widely used for timekeeping. GPS receivers can display time accurate to within 150 billionths of a second. ManagingLand The use of GPS is widepread in field that require geospatial information for managing assests over large areas. Forestry, mineral exploration, and wildlife habitat management all use GPS to precisely define positions of important assets and to identify changes. GPS and Agriculture GPS receivers installed in farm equipment provide accurate position information. This enables farmers to apply fertilizers and harvest crops with great precision.