This article was published in 1997 and has not been updated or revised.
`
`BEYOND DISCOVERY
`BEYOND DISCOVERY
`
`T H E PAT H F R O M R E S E A R C H T O H U M A N B E N E F I T T M
`
`THE GLOBAL POSITIONING SYSTEM
`The Role of Atomic Clocks
`Where am I? The question seems simple; the
`
`the nature of time and ways to measure time accurately
`contributed to development of the GPS. It provides a dra-
`matic example of how science works and how basic research
`leads to technologies that were virtually unimaginable at
`the time the research was done.
`
`answer, historically, has proved not to be. 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 accu-
`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,
`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, par-
`ticularly development of the world’s most accurate time-
`pieces: atomic clocks that are pre-
`cise to within a billionth of a sec-
`ond. The clocks were created by
`physicists seeking answers to ques-
`tions about the nature of the uni-
`verse, with no conception that
`their technology would some day
`lead to a global system of naviga-
`tion. Today, GPS is saving lives,
`helping society in countless other
`ways, and generating 100,000
`jobs in a multi-billion-dollar
`industry. The following article,
`adapted in part from an account
`by physicist Daniel Kleppner,
`describes how basic research into
`
`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 immediate-
`ly. Although the 29-year-old
`pilot had managed to eject
`
`American F-16 pilot Captain
`Scott F. O'Grady arrives on the
`deck of the USS Kearsarge in the
`Adriatic Sea after his June 1995
`rescue from Serb-controlled terri-
`tory in Bosnia-Herzegovina. A
`GPS receiver concealed in his life
`vest enabled Marines to pinpoint
`the downed pilot's location, lead-
`ing to a successful rescue opera-
`tion. (AP/Wide World Photos)
`
`N A T I O N A L
`
`A C A D E M Y
`
`O F
`
`S C I E N C E S
`
`IPR2020-00408
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`

`

`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 preci-
`sion. When O’Grady had gone down, his life vest con-
`tained 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 res-
`cuers knew that some of the technology that made this
`
`Originally conceived as a navigational tool for the military,
`the Global Positioning System has spawned many commercial
`applications in an industry that some predict will reach $30
`billion in annual revenues in the next decade: here, a built-in
`locator device for automobiles. (Trimble Navigation Ltd.)
`
`2
`
`B E Y O N D D I S C O V E R Y
`
`remarkable rescue possible had grown out of basic
`research on the fundamental properties of atoms and
`nuclei some 60 years earlier.
`
`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 lead
`to a global system of
`navigation.
`
`Time and Location, Precisely
`GPS makes it possible to answer the simple ques-
`tion “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 accu-
`rately 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 destina-
`tions 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.
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
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`

`

`THE GLOBAL POSITIONING SYSTEM: HOW IT WORKS
`
`1
`
`2
`
` A constellation of 24 satellites
` orbiting 11,000 miles
`above Earth emits coded
`signals. Four atomic clocks
`in each satellite keep
`accurate time.
`
` The portable receiver
` 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-
`ing, 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 net-
`works, 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 dri-
`vers 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 naviga-
`tion system.
`That’s just the beginning. The current worldwide
`market for GPS receivers and technology is estimated
`at over $2 billion and is expected to grow to over $30
`billion in the next 10 years.
`
`It Started with Basic
`Research . . .
`
`The history of GPS is an account of how basic
`research first made possible 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 launch-
`ing 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 relativi-
`ty 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 reck-
`oning. Even after modern clocks were developed, mak-
`ing 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,
`
`N A T
`
`I O N A L
`
` A C A D E M Y
`
` O F
`
`
`
`S
`
`C
`
`I
`
`E N C
`
`E
`
`S
`
`3
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
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`
`

`

`The history of GPS
`is an account of
`how basic research
`first made possible
`a vital defense
`technology and
`then a variety of
`important
`commercial
`applications.
`
`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 satel-
`lite’s orbit could be precisely
`determined from the ground
`was the first step in establish-
`ing that positions on the
`ground could be determined
`by homing in on the signals
`broadcast by satellites.
`In the years that fol-
`lowed, the U.S. Navy exper-
`imented with a series of
`satellite navigation systems,
`beginning with the Transit system in 1965, which was
`developed to meet the navigational needs of sub-
`marines carrying Polaris nuclear missiles. These sub-
`marines 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 subma-
`rine could accurately determine its location in 10 or
`15 minutes. In 1973, the Department of Defense
`was looking for a foolproof method of satellite navi-
`gation. 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 opera-
`tional 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 broad-
`casts a digital radio signal that includes both its own
`position and the time, exact to a billionth of a sec-
`ond. 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 cal-
`culate its distance from the satellite. (Light travels at
`
`A Chronology of Selected Events in the Development of GPS.
`This timeline 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-term basic research in the ultimate
`achievement of important benefits to society. It does not provide a complete portrait of the development of GPS.
`
`1938-1940
`I.I. Rabi invents molecu-
`lar-beam magnetic reso-
`nance at Columbia
`University in 1938. He
`and his colleagues apply
`magnetic resonance to
`fundamental studies of
`atoms and molecules.
`Possibility of atomic
`clock to measure gravi-
`tational red shift is dis-
`cussed. Rabi is awarded
`the Nobel Prize for this
`work in 1944.
`
`1949
`Norman Ramsey invents separated-
`oscillatory-field resonance method at
`Harvard University, for which he was
`awarded the Nobel Prize in 1989.
`Jerrold Zacharias proposes using
`Ramsey’s method to create cesium-
`beam “fountain” clock that would be
`accurate enough to measure gravita-
`tional red shift.
`
`1949
`National Bureau of Standards
`operates atomic clock based
`on microwave absorption in
`ammonia gas. Work starts on
`cesium-beam atomic clock.
`
`1954
`Charles Townes at Columbia
`University demonstrates
`operation of the first maser
`based on emission of radia-
`tion from ammonia mole-
`cules. Townes shared the
`1964 Nobel Prize in physics.
`
`1959
`Albert Kastler and Jean
`Brossel, working in Paris
`and at MIT, develop
`methods of optical
`pumping. Kastler is
`awarded the Nobel Prize
`for this work.
`
`1954-1956
`Zacharias and
`National Company
`develop the first self-
`contained portable
`atomic clock, the
`Atomichron.
`
`1957
`Sputnikis launched in October by
`the Soviet Union. Satellite Doppler
`tracking is inaugurated at MIT
`Lincoln Laboratory and Johns
`Hopkins Applied Physics Laboratory
`(APL). Navy Transit program is
`started at APL in December.
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
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`

`

`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
`would calculate that it was 186 miles from that satel-
`lite.) 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 mea-
`sures their speed, position, and altitude precisely. That
`information is sent back to the satellites, which broad-
`cast it along with their timing signals.
`
`A Tool to Study Nature
`GPS itself was born as a military tool, but the
`atomic clocks that made GPS possible originated in
`basic research shortly before the Second World War.
`It was then that scientists found that high-precision
`techniques developed to study fundamental atomic
`structure could be used to make an atomic clock.
`Their inspiration had to do not with ultraprecise navi-
`gation, but rather with the dream of making a clock
`good enough to study the nature of time itself—in
`particular, the effect of gravity on time predicted by
`Einstein’s theory of gravity and known as the gravita-
`tional red shift.
`Until the late 1920s, the most accurate timepieces
`depended on the regular swing of a pendulum. They
`were superseded by more accurate clocks based on the
`regular vibrations of a quartz crystal, which could
`keep time to within less than one-thousandth of a sec-
`ond per day. Even that kind of precision, however,
`would not suffice for scientists who wanted to study
`Einstein’s theory of gravity. According to Einstein, a
`gravitational field would distort both space and time.
`Thus, a clock on top of Mount Everest, for instance,
`was predicted to run 30 millionths of a second per
`day faster than an identical clock at sea level. The
`
`1960
`Ramsey and students
`Kleppner and
`Goldenberg operate
`hydrogen maser at
`Harvard University.
`
`1961
`Development of
`GPS begins at
`Aerospace
`Corporation as a
`system designed
`to meet military
`needs.
`
`1967
`Transit system
`is made avail-
`able to civilian
`community.
`
`1973
`Development of
`Navstar GPS is
`approved by the
`Department of
`Defense.
`
`1977
`Test satellite
`incorporating
`principal features
`of later GPS
`satellites, includ-
`ing first cesium
`clocks in space,
`is launched.
`
`1964-1965
`First position
`fix from a
`Transit satellite
`is computed
`aboard Polaris
`submarine.
`
`1968
`Standards of
`a Defense
`Navigation
`Satellite System
`are defined.
`
`1974
`First GPS test
`satellite, from
`Timation program,
`is launched to test
`rubidium clocks
`and time-dissemi-
`nation techniques.
`
`1978-1985
`Ten prototype
`GPS satellites
`are launched,
`built by Rockwell
`International.
`
`1960-1965
`Rubidium optically
`pumped clock is
`introduced. Cesium
`frequency standards
`are installed in
`most international
`time-standard labo-
`ratories.
`
`1989-1993
`Series of 24
`satellites are
`launched at
`about 6 per year.
`Final satellite is
`launched on
`June 26, 1993.
`
`1996
`White House
`announces
`that a higher
`level of GPS
`accuracy will
`be available
`to everyone.
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
`Apple EX1016 Page 5
`
`

`

`only way to make measurements this accurate was to
`control a clock by the infinitesimal oscillations of the
`atom itself.
`
`Barton Silverman, New York Times
`
`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 electrons
`surrounding their nuclei. When an atom undergoes a
`transition from one such “energy state” to a lower
`one—it emits an electromagnetic wave of a discrete
`characteristic frequency, known as the resonant fre-
`quency. These resonant frequencies are identical for
`every atom of a given type—cesium 133 atoms, for
`example, all have a resonant frequency of exactly
`9,192,631,770 cycles per second. For this reason,
`the cesium atom can be used as a metronome with
`which to keep extraordinarily precise time.
`The first substantial progress toward developing
`clocks based on such an atomic timekeeper was
`achieved in the 1930s at a Columbia University labora-
`tory in which I.I. Rabi and his students studied the
`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
`won the 1944 Nobel Prize for his work. It was in that
`year that he first suggested—“tossed off the idea,” as
`his students put it—that the precision of these reso-
`nances are so great that they could be used to make a
`clock of extraordinary accuracy. In particular, he pro-
`posed using the frequencies of what are known as
`“hyperfine transitions” of the atoms—transitions
`between two states of slightly different energy corre-
`sponding to different magnetic interactions between
`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-
`magnetic field. The closer the oscillation frequency of
`that field to the frequency of the hyperfine transition
`of the atom, the more atoms absorb energy from the
`field and thereby undergo a transition from the origi-
`nal hyperfine state to another one. A feedback loop
`adjusts the frequency of the oscillating field until virtu-
`ally all the atoms make the transition. An atomic clock
`uses the frequency of the oscillating field—now per-
`
`Two pioneering
`
`scientists whose
`
`work contributed to the Global Positioning
`
`System: left, I.I. Rabi’s research on the funda-
`
`mental properties of atoms and nuclei led to
`
`his invention of a technique called magnetic
`
`resonance on which the first atomic clock
`
`was based; right, Rabi’s former student,
`
`Norman Ramsey, laid the groundwork for the
`
`development of the cesium-beam “fountain”
`
`clock and invented the hydrogen maser,
`
`devices that redefined timekeeping.
`
`fectly in step with the precise resonant frequency of
`the atoms—as a metronome to generate time pulses.
`Rabi himself never pursued the development of
`such a clock, but other researchers went on to improve
`on the idea and perfect the technology. In 1949, for
`instance, research by Rabi’s student Norman Ramsey
`suggested that making the atoms pass twice through
`the oscillating electromagnetic field could result in a
`much more accurate clock. In 1989 Ramsey was
`awarded the Nobel Prize for his work.
`
`Practical Applications
`
`After the war, the U.S. National Bureau of
`Standards and the British National Physical
`Laboratory both set out to create atomic-time stan-
`dards based on the atomic-resonance work of Rabi
`and his students. The first atomic clock was estab-
`lished at the National Physical Laboratory by Louis
`
`6
`
`B E Y O N D D I S C O V E R Y
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
`Apple EX1016 Page 6
`
`

`

`By 1967, research
`in atomic clocks
`had proved so
`fruitful that the
`second was rede-
`fined in terms of
`the oscillations of
`a cesium atom.
`Today’s atomic
`clocks are typically
`accurate to within
`1 second in
`100,000 years.
`
`Essen and John V.L. Parry, but this clock required a
`roomful of equipment. Another of Rabi’s former
`associates, Jerrold Zacharias of MIT, managed to turn
`the atomic clocks into practical devices. Zacharias
`had plans for building what he called an atomic foun-
`tain, a visionary type of atomic clock that would be
`accurate enough to study the effect of gravity on time
`that had been predicted by Einstein. In the process,
`he developed an atomic clock small enough to be
`wheeled from one laborato-
`ry to another. In 1954,
`Zacharias joined with the
`National Company in
`Malden, Massachusetts, to
`build a commercial atomic
`clock based on his portable
`device. The company pro-
`duced the Atomichron, the
`first commercial atomic
`clock, 2 years later and sold
`50 within 4 years. The
`cesium atomic clocks used
`in GPS today are all descen-
`dants of the Atomichron.
`Physicists have contin-
`ued to experiment with
`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-
`nique makes use of a phenomenon known as optical
`pumping to select out the energy levels of the atoms
`that will do the timekeeping and employs a beam of
`light to force all the atoms in the beam into the
`desired state. This work led to a Nobel Prize for
`Alfred Kastler of the École Normal Supérieure in
`Paris. Today, many atomic clocks use optically
`pumped rubidium atoms instead of cesium. The
`rubidium clocks are considerably less expensive and
`smaller than cesium clocks, but they are not quite as
`accurate.
`Another type of atomic clock is known as the
`hydrogen maser. Masers originated in research on
`the structure of molecules by Charles Townes and his
`colleagues at Columbia University in 1954, work for
`which Townes shared the 1964 Nobel Prize in
`physics. The maser, which is the precursor of the
`laser, is a microwave device that generates its signal
`by direct emission of radiation from atoms or mole-
`cules. While Townes’s original maser used ammonia,
`Ramsey and his colleagues at Harvard developed a
`
`maser in 1960 that operates with hydrogen and
`serves as an atomic clock of extreme precision.
`By 1967, research in atomic clocks had proved so
`fruitful that the second was redefined in terms of the
`oscillations of a cesium atom. Today’s atomic clocks
`are typically accurate to within 1 second in 100,000
`years. Our nation’s primary time standard is the
`recently inaugurated atomic clock at the National
`Institute of Standards and Technology, called NIST-7.
`Its estimated accuracy is to within 1 second in 3 mil-
`lion years.
`Over the years, all three clocks—the cesium-beam
`clock, the hydrogen-maser clock, and the rubidium
`clock—have seen service in space, either in satellites or
`in ground control systems. GPS satellites ultimately
`rely on cesium clocks that resemble those conceptual-
`ized by Rabi 60 years ago.
`In 1993, 2 decades after it was conceived in the
`Pentagon, GPS became fully functional with the
`launching of its 24th satellite. The satellites are
`operated by the U.S. Air Force, which monitors
`them from five ground stations around the world.
`The data gathered are analyzed at the Air Force
`Consolidated Space Operations Center in Colorado,
`which transmits daily updates to each satellite, cor-
`recting their clocks and their orbital data.
`
`GPS and the Future
`
`It is often forgotten that GPS is still a military
`device built by the Department of Defense at a cost
`of $12 billion and intended primarily for military use.
`That fact has led to one of the few controversies sur-
`rounding the remarkably successful system. 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 sys-
`tem 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, however, the Pentagon initiated a policy
`known as selective availability, whereby the most accu-
`rate signals broadcast by GPS satellites would be
`reserved strictly for military and other authorized
`users. GPS satellites now 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
`
`N A T
`
`I O N A L
`
` A C A D E M Y
`
` O F
`
`
`
`S
`
`C
`
`I
`
`E N C
`
`E
`
`S
`
`7
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
`Apple EX1016 Page 7
`
`

`

`In March 1996, the
`White House
`announced that the
`higher level of GPS
`accuracy will be made
`available to everyone,
`and the practice of
`degrading civil GPS sig-
`nals will be phased out
`within a decade. The
`White House also reaf-
`firmed the federal gov-
`ernment’s commitment
`to providing GPS ser-
`vices for peaceful civil,
`commercial, and scien-
`tific use on a world-
`wide basis and free
`of charge.
`
`ing civil GPS signals
`will be phased out
`within a decade. The
`White House also reaf-
`firmed the federal gov-
`ernment’s commitment
`to providing GPS ser-
`vices for peaceful civil,
`commercial, and scien-
`tific use on a world-
`wide basis and free of
`charge.
`The future of GPS
`appears to be virtually
`unlimited; technologi-
`cal fantasies abound.
`The system provides a
`novel, unique, and
`instantly available
`address for every
`square yard on the sur-
`face of the planet—a
`new international stan-
`dard for locations 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,
`but by a longitude and a latitude. With the GPS
`location of services stored with phone numbers in
`computerized “yellow pages,” the search for a
`local restaurant or the nearest gas station in any
`city, town, or suburb will be completed in an
`instant. With GPS, the world has been given a
`technology of unbounded promise, born in the
`laboratories of scientists who were motivated by
`their own curiosity to probe the nature of the uni-
`verse and our world, and built on the fruits of
`publically supported basic research.
`
`Twenty-four Navstar satellites—each the size of a large
`automobile and weighing some 1,900 pounds—circle the
`earth in 11,000-mile-high orbits. The satellite system, built
`by Rockwell International and operated by the U.S. Air
`Force, was completed in 1993, 20 years after it was first
`conceived in the Pentagon. (Lockheed Martin Astro Space)
`
`into the civilian signal to reduce its accuracy to
`about 300 feet.
`In March 1996, the White House announced
`that the higher level of GPS accuracy will be made
`available to everyone, and the practice of degrad-
`
`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 century, it has
`provided independent, objective scientific advice to the nation.
`
`© 1997 by the National Academy of Sciences
`April 1997
`
`8
`
`B E Y O N D D I S C O V E R Y
`
`This article was published in 1997 and has not been updated or revised.
`
`IPR2020-00408
`Apple EX1016 Page 8
`
`