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This article was published in 1997 and has not been updated or revised. BEYONDBEYOND DISCOVERYDISCOVERY TM THE PATH FROM RESEARCH TO HUMAN BENEFIT THE GLOBAL POSITIONING SYSTEM The Role of Atomic Clocks here am I? The question seems simple; the the nature of time and ways to measure time accurately answer, historically, has proved not to be. For contributed to development of the GPS. It provides a dra- Wcenturies, navigators and explorers have matic example of how science works and how basic research searched the heavens for a system that would enable leads to technologies that were virtually unimaginable at them to locate their position on the globe with the accu- the time the research was done. racy 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, Where Is He? the U.S. Air Force launched the 24th Navstar satellite into orbit, completing a network of 24 satellites known It was 2:08 in the morning of June 6, 1995, when as the Global Positioning System, or GPS. With a GPS a U.S. Air Force pilot flying an F-16 fighter over receiver that costs less than a few hundred dollars you Serbian-held positions in Bosnia-Herzegovina first can instantly learn your location on the planet—your heard “Basher 52” coming over his radio. “Basher 52” latitude, longitude, and even altitude—to within a few was the call signal of American pilot Captain Scott hundred feet. O’Grady, whose own F-16 had been shot down by This incredible new technology was made possible by a Serbian forces in that area 4 days earlier. The pilot combination of scientific and engineering advances, par- would say later that hearing O’Grady’s call signal was ticularly development of the world’s most accurate time- like hearing a voice from beyond the grave. O’Grady’s pieces: atomic clocks that are pre- F-16 had been hit by a cise to within a billionth of a sec- Serbian ground-to-air missile ond. The clocks were created by and had exploded immediate- physicists seeking answers to ques- ly. Although the 29-year-old tions about the nature of the uni- pilot had managed to eject verse, with no conception that their technology would some day lead to a global system of naviga- American F-16 pilot Captain tion. Today, GPS is saving lives, Scott F. O'Grady arrives on the helping society in countless other deck of the USS Kearsarge in the ways, and generating 100,000 Adriatic Sea after his June 1995 jobs in a multi-billion-dollar rescue from Serb-controlled terri- tory in Bosnia-Herzegovina. A industry. The following article, GPS receiver concealed in his life adapted in part from an account vest enabled Marines to pinpoint by physicist Daniel Kleppner, the downed pilot's location, lead- describes how basic research into ing to a successful rescue opera- tion. (AP/Wide World Photos) NATIONAL ACADEMY OF SCIENCES safely, his wingman had seen no parachute come out of remarkable rescue possible had grown out of basic the flaming debris. research on the fundamental properties of atoms and Now O’Grady had been on the ground behind nuclei some 60 years earlier. 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, Time and Location, Precisely who verified his position and called in the Marines—in particular, the 24th Marine Expeditionary Unit and its GPS makes it possible to answer the simple ques- expert team for Tactical Recovery of Aircraft Personnel, tion “Where am I?” almost instantaneously and with or TRAP. Within 4 hours, the search and rescue team breathtaking precision. The new technology utilizes had lifted off from the USS Kearsarge in the Adriatic atomic clocks that keep time to within a billionth of a Sea and headed toward Bosnia. By 6:50 a.m., they had second. They were created by scientists who had no picked up O’Grady in a dramatic textbook rescue, had idea that the clocks would someday contribute to a weathered Serbian small-arms fire, and were heading global system of navigation. The system made its back home. Later that day in Alexandria, Virginia, public debut to rave William O’Grady, the young flier’s father, was informed reviews in the 1991 Gulf GPS makes it possible that his son was alive and safe. War. U.S. troops used it to answer the simple The press would hail O’Grady as a hero, and for navigation on land, question “Where am I?” O’Grady himself would give credit and thanks to the sea, and in the air, for Marines who “risked their lives to get me out.” But targeting of bombs, and almost instantaneously another factor allowed the Marines to perform their for on-board missile and with breathtaking crucial role in the rescue operation with surgical preci- guidance. GPS allowed precision. The new sion. When O’Grady had gone down, his life vest con- U.S. ground troops to technology utilizes tained a portable radio receiver tuned in to a network move swiftly and accu- of 24 satellites known as the Global Positioning System rately through the vast, atomic clocks that (GPS). O’Grady was able to determine his position featureless desert of the keep time to within a behind enemy lines—longitude, latitude, and altitude— Arabian Peninsula. billionth of a second. to within a few hundred feet, and he was then able to Since then, GPS They were created by signal that position to the Air Force fliers overhead and technology has moved to the Marines who were sent in to rescue him. One into the civilian sector. scientists who had no cannot help wondering whether O’Grady and his res- Today, GPS is saving idea that the clocks cuers knew that some of the technology that made this lives, helping society in would someday lead many other ways, and to a global system of generating jobs in a new navigation. 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 destina- tions and map their routes. • GPS is used to locate vessels lost at sea. •Trucking and transportation services use GPS to Originally conceived as a navigational tool for the military, keep track of their fleets and to speed deliveries. the Global Positioning System has spawned many commercial applications in an industry that some predict will reach $30 • Shipping companies equip their tankers and billion in annual revenues in the next decade: here, a built-in freighters with GPS for navigation and to record and locator device for automobiles. (Trimble Navigation Ltd.) control the movement of their vessels. 2 BEYOND DISCOVERY This article was published in 1997 and has not been updated or revised. THE GLOBAL POSITIONING SYSTEM: HOW IT WORKS A constellation of 24 satellites 1 orbiting 11,000 miles above Earth emits coded signals. Four atomic clocks in each satellite keep accurate time. The portable receiver 2 calculates latitude, longitude, altitude, and time by comparing signals from satellites; location is accurate to within 30 meters, or 100 feet. Rockwell International Adapted from The Washington Post • Pleasure boaters and owners of small commercial vehicles rely on GPS for navigation. • Civilian pilots use GPS for navigation, crop-dust- It Started with Basic ing, aerial photography, and surveying. Research . • Airlines have saved millions of dollars by using GPS to hone their flight plans; GPS can be used for The history of GPS is an account of how basic instrument landing at small, as well as large, airports research first made possible a vital defense technology and is making new air-avoidance systems possible. and then a variety of important commercial applications. • GPS is used regularly for mapping, measuring Many other technological advances also contributed to the earth, and surveying. GPS has been used to map the development of GPS, among them satellite launch- roads, to track forest fires, and to guide the blades of ing and control technologies, solid state devices, bulldozers in construction processes, making grading microchips, correlation circuitry, time-difference-of- accurate to within a few inches. arrival technology, microwave communication, and • Earth scientists use GPS to monitor earthquakes radionavigation. This account focuses on how the quest and the shifting of the earth’s tectonic plates. for understanding the nature of the atomic world, in • Telecommunications companies increasingly rely particular the creation of atomic clocks to study relativi- on GPS to synchronize their land-based digital net- ty and Einstein’s physics, led to the creation of highly works, comparing their reference clocks directly with accurate clocks and how those were later put to use, in GPS time. combination with satellite tracking technology, to satisfy • Satellite builders use GPS receivers to track the the basic human desire to know where we are and positions of their satellites. where we are going. • GPS is being installed in automobiles so that dri- For centuries, the only way to navigate was to look vers not only can find out where they are but also can at the position of the sun and stars and use dead reck- be given directions.

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