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Global Positioning System: the Role of Atomic Clocks

Global Positioning System: the Role of Atomic Clocks

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 The Role of Atomic

here am I? The question seems simple; the the of 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 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 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 , Where Is He? the U.S. Air 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 on the planet—your heard “Basher 52” coming over his . “Basher 52” , , 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 in that 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--old tions about the nature of the uni- pilot had managed to eject verse, with no conception that their technology would some 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 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 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 and Now O’Grady had been on the ground behind nuclei some 60 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 , 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 . 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 . The system made its back home. Later that day in , 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 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 the size of credit cards, so small and so inexpensive that virtually any 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 use GPS to pinpoint destina- tions and 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 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 : 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 above 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 . Research . . . • Airlines have saved millions of dollars by using GPS to hone their flight plans; GPS can be used for The 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 . arrival technology, 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 • 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 , the only way to navigate was to look vers not only can find out where they are but also can at the and and use dead reck- be given directions. In Japan, 500,000 automobiles oning. Even after modern clocks were developed, mak- have already been equipped with a GPS-based naviga- ing it possible to find one’s longitude, the most accurate tion system. instruments could yield a position that was accurate That’s just the beginning. The current worldwide only to within a few miles. However, when the Soviet market for GPS receivers and technology is estimated Union launched Sputnik on October 4, 1957, it was at over $2 billion and is expected to grow to over $30 immediately recognized that this “artificial ” could billion in the next 10 years. be used as a navigational tool. The very next evening,

NATIONAL ACADEMY OF SCIENCES 3

This article was published in 1997 and has not been updated or revised. researchers at the Lincoln Laboratory of the radio signals transmitted by the satellites—in essence, Massachusetts Institute of Technology (MIT) were able measuring the Doppler shifts of the signals—a subma- to determine the satellite’s orbit precisely by observing rine could accurately determine its location in 10 or how the apparent of its radio signal increased 15 . In 1973, the Department of Defense as it approached and was looking for a foolproof method of satellite navi- The history of GPS decreased as it departed—an gation. A brainstorming session at the Pentagon over effect known as the Doppler the Labor Day weekend produced the concept of is an account of shift. The proof that a satel- GPS on the basis of the department’s experience with how basic research lite’s orbit could be precisely all its satellite predecessors. The essential components first made possible determined from the ground of GPS are the 24 Navstar satellites built by Rockwell a vital defense was the first step in establish- International, each the size of a large automobile and ing that positions on the weighing some 1,900 pounds. Each satellite orbits technology and ground could be determined the earth every 12 hours in a formation that ensures then a variety of by homing in on the signals that every point on the planet will always be in radio important broadcast by satellites. contact with at least four satellites. The first opera- commercial In the years that fol- tional GPS satellite was launched in 1978, and the lowed, the U.S. Navy exper- system reached full 24-satellite capability in 1993. applications. imented with a series of Considering how extraordinarily sophisticated the systems, technology is, the operating principle of GPS is beginning with the system in 1965, which was remarkably simple. Each satellite continuously broad- developed to meet the navigational needs of sub- casts a digital radio signal that includes both its own marines carrying Polaris nuclear missiles. These sub- position and the time, exact to a billionth of a sec- marines needed to remain hidden and submerged for ond. A GPS receiver takes this information—from at a time, but -based navigation, four satellites—and uses it to calculate its position on known as inertial navigation, could not sustain its the planet to within a few hundred feet. The receiver accuracy over such long periods. The Transit system compares its own time with the time sent by a satellite comprised a half-dozen satellites that would circle the and uses the difference between the two to cal- earth continuously in polar orbits. By analyzing the culate its distance from the satellite. ( travels at

A of Selected Events in the Development of GPS. This of selected events emphasizes early research in physics, notably atomic clocks, that contributed to the development of the Global Positioning System and illustrates the value of such long- basic research in the ultimate achievement of important benefits to society. It does not provide a complete portrait of the development of GPS.

1949 1954 1959 Norman Ramsey invents separated- Charles Townes at Columbia Albert Kastler and Jean oscillatory-field method at University demonstrates Brossel, working in Paris 1938-1940 Harvard University, for which he was operation of the first and at MIT, develop awarded the Nobel Prize in 1989. based on emission of radia- methods of optical I.I. Rabi invents molecu- Jerrold Zacharias proposes using tion from - pumping. Kastler is lar-beam magnetic reso- Ramsey’s method to create cesium- cules. Townes shared the awarded the Nobel Prize nance at Columbia beam “fountain” that would be 1964 . for this . University in 1938. He accurate enough to measure gravita- and his colleagues apply tional red shift. magnetic resonance to fundamental studies of atoms and molecules. Possibility of atomic 1949 1954-1956 1957 clock to measure gravi- National Bureau of Standards Zacharias and Sputnik is launched in October by tational red shift is dis- operates based National Company the Soviet Union. Satellite Doppler cussed. Rabi is awarded on microwave absorption in develop the first self- tracking is inaugurated at MIT the Nobel Prize for this ammonia gas. Work starts on contained portable Lincoln Laboratory and Johns work in 1944. cesium-beam atomic clock. atomic clock, the Hopkins Applied Physics Laboratory Atomichron. (APL). Navy Transit program is started at APL in December.

This article was published in 1997 and has not been updated or revised. 186,000 miles per second: if the satellite time hap- pened to be, for example, one-thousandth of a second behind the GPS receiver’s time, then the receiver A Tool to Study Nature would calculate that it was 186 miles from that satel- lite.) By checking its time against the time of three GPS itself was born as a military tool, but the satellites whose positions are known, a receiver could atomic clocks that made GPS possible originated in pinpoint its longitude, latitude, and altitude. basic research shortly before the Second World War. The method just described would require that It was then that scientists found that high-precision both the satellites and the receiver carry clocks of techniques developed to study fundamental atomic remarkable accuracy. However, having a receiver pick structure could be used to make an atomic clock. up a signal from a fourth satellite allows the receiver Their inspiration had to do not with ultraprecise navi- to get by with a relatively simple —like gation, but rather with the dream of making a clock that used in most . Once the receiver has good enough to study the nature of time itself—in made contact with four satellites, the system takes particular, the effect of gravity on time predicted by over and computes its position almost instantaneously. Einstein’s theory of gravity and known as the gravita- For the system to work, the receiver has to know tional red shift. exactly where the satellites are and the satellites have to Until the late 1920s, the most accurate timepieces be able to keep reliable and extraordinarily accurate depended on the regular swing of a . They time. Accuracy is ensured by having each satellite carry were superseded by more accurate clocks based on the four atomic clocks, the most accurate timing devices regular vibrations of a quartz crystal, which could ever made. Reliability is ensured by the satellites’ keep time to within less than one-thousandth of a sec- 11,000--high orbits, which put them far above the ond per day. Even that kind of precision, however, atmosphere and keep them moving in very predictable would not suffice for scientists who wanted to study trajectories. The Department of Defense monitors the Einstein’s theory of gravity. According to Einstein, a satellites as they pass overhead twice a day and mea- gravitational field would distort both and time. sures their speed, position, and altitude precisely. That Thus, a clock on top of Mount Everest, for instance, information is sent back to the satellites, which broad- was predicted to run 30 millionths of a second per cast it along with their timing signals. day faster than an identical clock at sea level. The

1977 1961 Test satellite 1989-1993 Development of 1973 incorporating Series of 24 satellites are 1960 GPS begins at 1967 Development of principal features launched at Ramsey and students Aerospace Navstar GPS is of later GPS Transit system about 6 per year. Kleppner and Corporation as a approved by the satellites, includ- is made avail- Final satellite is Goldenberg operate system designed Department of ing first cesium able to civilian launched on hydrogen maser at to meet military Defense. clocks in space, community. June 26, 1993. Harvard University. needs. is launched.

1974 1996 1968 1964-1965 First GPS test 1978-1985 White House Standards of Ten announces 1960-1965 First position satellite, from a Defense GPS satellites that a higher optically fix from a program, Navigation are launched, level of GPS pumped clock is Transit satellite is launched to test Satellite System built by Rockwell accuracy will introduced. Cesium is computed rubidium clocks frequency standards are defined. and time-dissemi- International. be available aboard Polaris to everyone. are installed in submarine. nation techniques. most international time-standard labo- ratories.

This article was published in 1997 and has not been updated or revised. only way to make this accurate was to Barton Silverman, New York Times control a clock by the infinitesimal oscillations of the

itself. Harvard University

Rabi’s Clock

According to the laws of quantum physics, atoms absorb or emit electromagnetic energy in discrete amounts that correspond to the differences in energy between the different electronic configurations of the atoms, i.e., different configurations of the Two pioneering surrounding their nuclei. When an atom undergoes a scientists whose transition from one such “energy state” to a lower one—it emits an electromagnetic wave of a discrete work contributed to the Global Positioning characteristic frequency, known as the resonant fre- System: left, I.I. Rabi’s research on the funda- quency. These resonant are identical for mental properties of atoms and nuclei led to every atom of a given type—cesium 133 atoms, for example, all have a resonant frequency of exactly his invention of a technique called magnetic 9,192,631,770 cycles per second. For this reason, resonance on which the first atomic clock the cesium atom can be used as a metronome with was based; right, Rabi’s former student, which to keep extraordinarily precise time. The first substantial progress toward developing Norman Ramsey, laid the groundwork for the clocks based on such an atomic was development of the cesium-beam “fountain” achieved in the 1930s at a Columbia University labora- clock and invented the hydrogen maser, tory in which I.I. Rabi and his students studied the devices that redefined timekeeping. fundamental properties of atoms and nuclei. In the course of his research, Rabi invented the technique known as magnetic resonance, by which he could mea- sure the natural resonant frequencies of atoms. Rabi fectly in step with the precise resonant frequency of won the 1944 Nobel Prize for his work. It was in that the atoms—as a metronome to generate time pulses. year that he first suggested—“tossed off the idea,” as Rabi himself never pursued the development of his students put it—that the precision of these reso- such a clock, but other researchers went on to improve nances are so great that they could be used to make a on the idea and perfect the technology. In 1949, for clock of extraordinary accuracy. In particular, he pro- instance, research by Rabi’s student Norman Ramsey posed using the frequencies of what are known as suggested that making the atoms pass twice through “hyperfine transitions” of the atoms—transitions the oscillating electromagnetic field could result in a between two states of slightly different energy corre- much more accurate clock. In 1989 Ramsey was sponding to different magnetic interactions between awarded the Nobel Prize for his work. the nucleus of an atom and its electrons. In such a clock, a beam of atoms in one particular hyperfine state passes through an oscillating electro- . The closer the oscillation frequency of Practical Applications that field to the frequency of the hyperfine transition of the atom, the more atoms absorb energy from the After the war, the U.S. National Bureau of field and thereby undergo a transition from the origi- Standards and the British National Physical nal hyperfine state to another one. A feedback loop Laboratory both set out to create atomic-time stan- adjusts the frequency of the oscillating field until virtu- dards based on the atomic-resonance work of Rabi ally all the atoms make the transition. An atomic clock and his students. The first atomic clock was estab- uses the frequency of the oscillating field—now per- lished at the National Physical Laboratory by Louis

6 BEYOND DISCOVERY

This article was published in 1997 and has not been updated or revised. Essen and John V.L. Parry, but this clock required a maser in 1960 that operates with hydrogen and roomful of equipment. Another of Rabi’s former serves as an atomic clock of extreme precision. associates, Jerrold Zacharias of MIT, managed to turn By 1967, research in atomic clocks had proved so the atomic clocks into practical devices. Zacharias fruitful that the second was redefined in terms of the had plans for building what he called an atomic foun- oscillations of a cesium atom. Today’s atomic clocks tain, a visionary type of atomic clock that would be are typically accurate to within 1 second in 100,000 accurate enough to study the effect of gravity on time years. Our nation’s primary is the that had been predicted by Einstein. In the process, recently inaugurated atomic clock at the National he developed an atomic clock small enough to be Institute of Standards and Technology, called NIST-7. wheeled from one laborato- Its estimated accuracy is to within 1 second in 3 mil- ry to another. In 1954, lion years. By 1967, research Zacharias joined with the Over the years, all three clocks—the cesium-beam in atomic clocks National Company in clock, the hydrogen-maser clock, and the rubidium had proved so Malden, Massachusetts, to clock—have seen service in space, either in satellites or fruitful that the build a commercial atomic in ground control systems. GPS satellites ultimately clock based on his portable rely on cesium clocks that resemble those conceptual- second was rede- device. The company pro- ized by Rabi 60 years ago. fined in terms of duced the Atomichron, the In 1993, 2 after it was conceived in the the oscillations of first commercial atomic Pentagon, GPS became fully functional with the a cesium atom. clock, 2 years later and sold launching of its 24th satellite. The satellites are 50 within 4 years. The operated by the U.S. Air Force, which monitors Today’s atomic cesium atomic clocks used them from five ground stations around the world. clocks are typically in GPS today are all descen- The data gathered are analyzed at the Air Force accurate to within dants of the Atomichron. Consolidated Space Operations Center in , 1 second in Physicists have contin- which transmits daily updates to each satellite, cor- 100,000 years. ued to experiment with recting their clocks and their orbital data. novel variations on the atomic-resonance ideas of Rabi and his students and to put them to work in atomic clocks. Rather than using magnets, one tech- GPS and the nique makes use of a phenomenon known as optical pumping to select out the energy levels of the atoms It is often forgotten that GPS is still a military that will do the timekeeping and employs a beam of device built by the Department of Defense at a cost light to force all the atoms in the beam into the of $12 billion and intended primarily for military use. desired state. This work led to a Nobel Prize for That fact has led to one of the few controversies sur- Alfred Kastler of the École Normal Supérieure in rounding the remarkably successful system. As with Paris. Today, many atomic clocks use optically any new technology, progress brings risk, and GPS pumped rubidium atoms instead of cesium. The potentially could be used to aid smugglers, terrorists, rubidium clocks are considerably less expensive and or hostile forces. The Pentagon made the GPS sys- smaller than cesium clocks, but they are not quite as tem available for commercial use only after being accurate. pressured by the companies that built the equipment Another type of atomic clock is known as the and saw the enormous potential market for it. As a hydrogen maser. originated in research on compromise, however, the Pentagon initiated a policy the structure of molecules by Charles Townes and his known as selective availability, whereby the most accu- colleagues at Columbia University in 1954, work for rate signals broadcast by GPS satellites would be which Townes shared the 1964 Nobel Prize in reserved strictly for military and other authorized physics. The maser, which is the precursor of the users. GPS satellites now broadcast two signals: a laser, is a microwave device that generates its signal civilian signal that is accurate to within 100 feet and a by direct emission of radiation from atoms or mole- second signal that only the military can decode that is cules. While Townes’s original maser used ammonia, accurate to within 60 feet. The Pentagon has also Ramsey and his colleagues at Harvard developed a reserved the ability to introduce errors at any time

NATIONAL ACADEMY OF SCIENCES 7 This article was published in 1997 and has not been updated or revised. ing civil GPS signals In March 1996, the will be phased out White House within a decade. The announced that the White House also reaf- firmed the federal gov- higher level of GPS ernment’s commitment accuracy will be made to providing GPS ser- available to everyone, vices for peaceful civil, and the practice of commercial, and scien- tific use on a world- degrading civil GPS sig- wide basis and free of nals will be phased out charge. within a decade. The The future of GPS White House also reaf- appears to be virtually unlimited; technologi- firmed the federal gov- cal fantasies abound. ernment’s commitment The system provides a to providing GPS ser- novel, unique, and vices for peaceful civil, instantly available commercial, and scien- address for every square yard on the sur- tific use on a world- face of the planet—a wide basis and free new international stan- of charge. dard for and distances. To the com- puters of the world, at least, our locations may be defined not by a street address, a city, and a state, Twenty-four Navstar satellites—each the size of a large but by a longitude and a latitude. With the GPS automobile and weighing some 1,900 pounds—circle the earth in 11,000-mile-high orbits. The satellite system, built location of services stored with phone numbers in by Rockwell International and operated by the U.S. Air computerized “yellow pages,” the search for a Force, was completed in 1993, 20 years after it was first local restaurant or the nearest gas station in any conceived in the Pentagon. (Lockheed Martin Astro Space) city, town, or suburb will be completed in an instant. With GPS, the world has been given a into the civilian signal to reduce its accuracy to technology of unbounded promise, born in the about 300 feet. laboratories of scientists who were motivated by In March 1996, the White House announced their own to probe the nature of the uni- that the higher level of GPS accuracy will be made verse and our world, and built on the fruits of available to everyone, and the practice of degrad- publically supported basic research.

This article, which was published in 1997 and has not been updated or revised, was adapted by Gary Taubes from an article written by MIT scientist Daniel Kleppner for Beyond Discovery: The Path from Research to Human BenefitTM, a project of the National Academy of Sciences. The Academy, located in Washington, D.C., is a society of distinguished scholars engaged in scientific and engineering research and dedicated to the use of science and technology for the public welfare. For more than a , it has provided independent, objective scientific advice to the nation.

© 1997 by the National Academy of Sciences April 1997

8 BEYOND DISCOVERY

This article was published in 1997 and has not been updated or revised.

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