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Everyday Mysteries: Fun Science Facts from the Library of Congress

What is a GPS? How does it work?

The Global Positioning System (GPS) tells you where you are on Earth.

It's eleven o'clock ... do you know where your kids are? Would you like to? One way to track them would be to have a GPS receiver installed in the car! The GPS, or Global Positioning System, is one of the hottest technologies around, and no wonder. Consider these diverse uses:

Minnesota scientists use GPS to study movements and feeding habits of deer.
Surveyors used GPS to measure how the buildings shifted after the bombing in Oklahoma City.
GPS help settle property disputes between land owners.
Marine archaeologists use GPS to guide research vessels hunting for shipwrecks.
GPS data has revealed that Mt. Everest is getting taller!

GPS answers five questions simultaneously:

"Where am I?"
"Where am I going?"
"Where are you?"
"What's the best way to get there?
"When will I get there?"

GPS is the only system today that can show your exact position on the Earth anytime, in any weather, no matter where you are!

Development:
Like so many other high-tech developments, GPS was designed by the U. S. military. The concept started in the late '60s but the first satellite wasn't launched until February 1978. In 1989 the Magellan Corp. introduced the first hand-held GPS receiver. In 1992 GPS was used in Operation Desert Storm. On March 1996 the President decided to make GPS free for civilian users.

System Description:
GPS has three 'segments':

The space segment now consists of 28 satellites, each in its own orbit about 11,000 nautical miles above the Earth.
The user segment consists of receivers, which you can hold in your hand or mount in your car.
The control segment consists of ground stations (five of them, located around the world) that make sure the satellites are working properly.

Civilian Use:
At first, the military did not want to let civilians use GPS, fearing that smugglers, terrorists, or hostile forces would use it. Finally, bowing to pressure from the companies that built the equipment, The Defense Department made GPS available for non-military purposes, with some restrictions. On May 1, 2000, President Clinton lifted the restrictions, and announced that the option to degrade civil GPS signals during emergencies would be phased out by 2010. The federal government is committed to providing GPS technology for peaceful uses on a worldwide basis, free of charge.

Related Web Sites

Sam Wormley's Global Positioning System Resources - Sam Wormley's Web site provides numerous links to Web resources on Global Positioning Systems. His site includes an extensive bibliography of books on the subject of Global Positioning. This educational observatory site also helps K-12 students do real science via the Internet.
The Global Positioning System Overview - These materials were developed by Peter H. Dana, The Geographer's Craft Project, and the Department of Geography at the University of Colorado in Boulder. It provides an overview of global positioning systems, a reference list, and related links and documents.
U.S. Coast Guard Navigation Center Global Positioning System (GPS) - This Web site has general information about GPS. It includes the United States policy statement regarding GPS availability.
Geocaching (a GPS online game) - "A GPS device and a hunger for adventure are all you need for high tech treasure hunting. Here you can find the latest caches in your area, how to hide your own cache, and what you need to get started in this fun and exciting sport."