Season One Research Files
Episode Twelve
11:00am-12:00pm
GLOBAL POSITIONING SYSTEM
On June 26, 1993, the U.S. Air Force launched the 24th Navstar satellite into orbit, completing a network of 24 satellites known as the Global Positioning System, or GPS. Although designed for the U.S. military, and funded and operated by the U. S. Department of Defense, there are many thousands of civil users of GPS worldwide. Civil users are permitted to use the system without charge or restrictions. With a low-cost GPS receiver or a download to a handheld device, anyone can instantly learn their location on the planet -- latitude, longitude, and even altitude -- to within a few hundred feet.
This incredible technology was made possible by a combination of scientific and engineering advances, particularly development of the world's most accurate timepieces: atomic clocks that are precise to within a billionth of a second. The clocks were created by physicists seeking answers to questions about the nature of the universe, with no conception that their technology would some day lead to a global system of navigation.
GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. The entire operational constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The Master Control facility for the GPS is located at Schriever Air Force Base (formerly Falcon AFB) in Colorado.
As a military device built by the Department of Defense at a cost of $12 billion and intended primarily for military use, the GPS encountered some controversies. As with any new technology, progress brings risk, and GPS potentially could be used to aid smugglers, terrorists, or hostile forces. The Pentagon made the GPS system available for commercial use only after being pressured by the companies that built the equipment and saw the enormous potential market for it. As a compromise, the Pentagon initiated a policy known as selective availability, whereby the most accurate signals broadcast by GPS satellites would be reserved strictly for military and other authorized users. GPS satellites broadcast two signals: a civilian signal that is accurate to within 100 feet and a second signal that only the military can decode that is accurate to within 60 feet. The Pentagon has also reserved the ability to introduce errors at any time into the civilian signal to reduce its accuracy to about 300 feet. In March 1996, the White House announced that a higher level of GPS accuracy will be made available to everyone, and the practice of degrading civil GPS signals will be phased out within a decade. The White House also reaffirmed the federal government's commitment to providing GPS services for peaceful civil, commercial, and scientific use on a worldwide basis and free of charge.
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