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GPS or the Global Positioning System was invented by the U.S. Department of Defense (D.O.D) and Ivan Getting, at the cost of twelve billion taxpayer dollars. The Global Positioning System is a satellite navigational system, predominantly designed for navigation. GPS is now gaining prominence as a timing tool.
"For centuries, navigators and explorers have searched the heavens for a system that would enable them to locate their position on the globe with the accuracy necessary to avoid tragedy and to reach their intended destinations. On June 26, 1993, however, the answer became as simple as the question. On that date, 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. With a GPS receiver that costs less than a few hundred dollars you can instantly learn your location on the planet--your latitude, longitude, and even altitude--to within a few hundred feet.
This incredible new 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. Today, GPS is saving lives, helping society in countless other ways, and generating 100,000 jobs in a multi-billion-dollar industry."
Eighteen satellites, six in each of three orbital planes spaced 120ยบ apart, and their ground stations, formed the original GPS. GPS uses these "man-made stars" or satellites as reference points to calculate geographical positions, accurate to a matter of meters. In fact, with advanced forms of GPS, you can make measurements to better than a centimeter
Gradually, the technology became a household name with a broad range of commercial and everyday applications. It helps pilots navigate without crashing into other aircraft, guides drivers to their destinations and helps commercial fishermen find the spots where they can net the biggest catches. Hand-held G.P.S. receivers are increasingly popular among hikers, boaters and other outdoor enthusiasts.
where is he?
It was 2:08 in the morning of June 6, 1995, when a U.S. Air Force pilot flying an F-16 fighter over Serbian-held positions in Bosnia-Herzegovina first heard "Basher 52" coming over his radio. "Basher 52" was the call signal of American pilot Captain Scott O'Grady, whose own F-16 had been shot down by Serbian forces in that area 4 days earlier. The pilot would say later that hearing O'Grady's call signal was like hearing a voice from beyond the grave. O'Grady's F-16 had been hit by a Serbian ground-to-air missile and had exploded immediately. Although the 29-year-old pilot had managed to eject safely, his wingman had seen no parachute come out of the flaming debris.
Now O'Grady had been on the ground behind enemy lines for 4 days, surviving on grass and insects, sleeping by day under camouflage netting, and moving by night. He had finally risked radio contact with fliers, who verified his position and called in the Marines--in particular, the 24th Marine Expeditionary Unit and its expert team for Tactical Recovery of Aircraft Personnel, or TRAP. Within 4 hours, the search and rescue team had lifted off from the USS Kearsarge in the Adriatic Sea and headed toward Bosnia. By 6:50 a.m., they had picked up O'Grady in a dramatic textbook rescue, had weathered Serbian small-arms fire, and were heading back home. Later that day in Alexandria, Virginia, William O'Grady, the young flier's father, was informed that his son was alive and safe.
The press would hail O'Grady as a hero, and O'Grady himself would give credit and thanks to the Marines who "risked their lives to get me out." But another factor allowed the Marines to perform their crucial role in the rescue operation with surgical precision. When O'Grady had gone down, his life vest contained a portable radio receiver tuned in to a network of 24 satellites known as the Global Positioning System (GPS). O'Grady was able to determine his position behind enemy lines--longitude, latitude, and altitude--to within a few hundred feet, and he was then able to signal that position to the Air Force fliers overhead and to the Marines who were sent in to rescue him. One cannot help wondering whether O'Grady and his rescuers knew that some of the technology that made this remarkable rescue possible had grown out of basic research on the fundamental properties of atoms and nuclei some 60 years earlier.
Time and Location, Precisely
GPS makes it possible to answer the simple question "Where am I?" almost instantaneously and with breathtaking precision. The new technology utilizes atomic clocks that keep time to within a billionth of a second. They were created by scientists who had no idea that the clocks would someday contribute to a global system of navigation. The system made its public debut to rave reviews in the 1991 Gulf War. U.S. troops used it for navigation on land, sea, and in the air, for targeting of bombs, and for on-board missile guidance. GPS allowed U.S. ground troops to move swiftly and accurately through the vast, featureless desert of the Arabian Peninsula.
Since then, GPS technology has moved into the civilian sector. Today, GPS is saving lives, helping society in many other ways, and generating jobs in a new multi-billion-dollar industry. Advances in integrated-circuit technology--the technology used to make computer chips--soon will lead to GPS receivers and transmitters the size of credit cards, so small and so inexpensive that virtually any vehicle can have one installed and any person can carry one.
In just a few short years, applications for GPS already have become almost limitless:
* Emergency vehicles use GPS to pinpoint destinations and map their routes.
* GPS is used to locate vessels lost at sea.
* Trucking and transportation services use GPS to keep track of their fleets and to speed deliveries.
* Shipping companies equip their tankers and freighters with GPS for navigation and to record and control the movement of their vessels.
* Pleasure boaters and owners of small commercial vehicles rely on GPS for navigation.
* Civilian pilots use GPS for navigation, crop-dusting, aerial photography, and surveying.
* Airlines have saved millions of dollars by using GPS to hone their flight plans; GPS can be used for instrument landing at small, as well as large, airports and is making new air-avoidance systems possible.
* GPS is used regularly for mapping, measuring the earth, and surveying. GPS has been used to map roads, to track forest fires, and to guide the blades of bulldozers in construction processes, making grading accurate to within a few inches.
* Earth scientists use GPS to monitor earthquakes and the shifting of the earth's tectonic plates.
* Telecommunications companies increasingly rely on GPS to synchronize their land-based digital networks, comparing their reference clocks directly with GPS time.
* Satellite builders use GPS receivers to track the positions of their satellites.
* GPS is being installed in automobiles so that drivers not only can find out where they are but also can be given directions. In Japan, 500,000 automobiles have already been equipped with a GPS-based navigation system.
That's just the beginning. The current worldwide market for GPS receivers and technology is estimated at more than $2 billion and is expected to grow to more than $30 billion during the next 10 years.
It Started with Basic Research...
The history of GPS is an account of how basic research made possible first a vital defense technology and then a variety of important commercial applications. Many other technological advances also contributed to the development of GPS, among them satellite launching and control technologies, solid state devices, microchips, correlation circuitry, time-difference-of-arrival technology, microwave communication, and radionavigation. This account focuses on how the quest for understanding the nature of the atomic world, in particular the creation of atomic clocks to study relativity and Einstein's physics, led to the creation of highly accurate clocks and how those were later put to use, in combination with satellite tracking technology, to satisfy the basic human desire to know where we are and where we are going.
For centuries, the only way to navigate was to look at the position of the sun and stars and use dead reckoning. Even after modern clocks were developed, making it possible to find one's longitude, the most accurate instruments could yield a position that was accurate only to within a few miles. However, when the Soviet Union launched Sputnik on October 4, 1957, it was immediately recognized that this "artificial star" could be used as a navigational tool. The very next evening, researchers at the Lincoln Laboratory of the Massachusetts Institute of Technology (MIT) were able to determine the satellite's orbit precisely by observing how the apparent frequency of its radio signal increased as it approached and decreased as it departed--an effect known as the Doppler shift. The proof that a satellite's orbit could be precisely determined from the ground was the first step in establishing that positions on the ground could be determined by homing in on the signals broadcast by satellites.
In the years that followed, the U.S. Navy experimented with a series of satellite navigation systems, beginning with the Transit system in 1965, which was developed to meet the navigational needs of submarines carrying Polaris nuclear missiles. These submarines needed to remain hidden and submerged for months at a time, but gyroscope-based navigation, known as inertial navigation, could not sustain its accuracy over such long periods. The Transit system comprised a half-dozen satellites that would circle the earth continuously in polar orbits. By analyzing the radio signals transmitted by the satellites--in essence, measuring the Doppler shifts of the signals--a submarine could accurately determine its location in 10 or 15 minutes. In 1973, the Department of Defense was looking for a foolproof method of satellite navigation. A brainstorming session at the Pentagon over the Labor Day weekend produced the concept of GPS on the basis of the department's experience with all its satellite predecessors. The essential components of GPS are the 24 Navstar satellites built by Rockwell International, each the size of a large automobile and weighing some 1,900 pounds. Each satellite orbits the earth every 12 hours in a formation that ensures that every point on the planet will always be in radio contact with at least four satellites. The first operational GPS satellite was launched in 1978, and the system reached full 24-satellite capability in 1993.
Considering how extraordinarily sophisticated the technology is, the operating principle of GPS is remarkably simple. Each satellite continuously broadcasts a digital radio signal that includes both its own position and the time, exact to a billionth of a second. A GPS receiver takes this information--from four satellites--and uses it to calculate its position on the planet to within a few hundred feet. The receiver compares its own time with the time sent by a satellite and uses the difference between the two times to calculate its distance from the satellite. (Light travels at 186,000 miles per second: if the satellite time happened to be, for example, one-thousandth of a second behind the GPS receiver's time, then the receiver would calculate that it was 186 miles from that satellite.) By checking its time against the time of three satellites whose positions are known, a receiver could pinpoint its longitude, latitude, and altitude.
The method just described would require that both the satellites and the receiver carry clocks of remarkable accuracy. However, having a receiver pick up a signal from a fourth satellite allows the receiver to get by with a relatively simple quartz clock--like that used in most watches. Once the receiver has made contact with four satellites, the system takes over and computes its position almost instantaneously.
For the system to work, the receiver has to know exactly where the satellites are and the satellites have to be able to keep reliable and extraordinarily accurate time. Accuracy is ensured by having each satellite carry four atomic clocks, the most accurate timing devices ever made. Reliability is ensured by the satellites' 11,000-mile-high orbits, which put them far above the atmosphere and keep them moving in very predictable trajectories. The Department of Defense monitors the satellites as they pass overhead twice a day and measures their speed, position, and altitude precisely. That information is sent back to the satellites, which broadcast it along with their timing signals.
