GPS: A New Constellation

Closed on May 23, 2011

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 a New Constellation was the companion exhibition to Planetarium show A Star to Steer Her By.

The content of GPS: a New Constellation and the broader history of navigation is explored in much more detail in the Time and Navigation exhibition on display at the Museum in Washington, DC. 


Ramsden Sextant

Before 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.

GPS Block II Satellite

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 Constellation

How does GPS work?
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.

Land and Sea Navigation
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. Railroads are using GPS technology to replace older, maintenance-intensive mechanical signals.

Navigation in the Air
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.

Mapping the Earth
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).

Mapping the Land
The use of GPS is widespread 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.

New Frontiers in Science
GPS has made scientific field studies throughout the world more accurate and has allowed scientists to perform new types of geographic analyses. Geologists use GPS to measure expansion of volcanoes and movement along fault lines. Ecologists can use GPS to map differences in a forest canopy. Biologists can track animals using radio collars that transmit GPS data. Geographers use GPS to define spatial relationships between features of the Earth's surface.