Where Am I?
courtesy of aerospace corporation2007_4_21 In October 1957, when the Soviet Union launched Sputnik , the world’s first artificial satellite, its eerie beeping from space sent researchers scrambling to track the orbit of the high-flying little radio transmitter. Most of them followed the location of Sputnik ’s pulsed signal with dish antennas and used the huge but modestly powered computers of the day to calculate its course. But at Johns Hopkins University’s Applied Physics Laboratory (APL), in Laurel, Maryland, the physicists William H. Guier and George C. Weiffenbach took a different approach.
Over several sleepless nights, as the satellite passed overhead, the two scientists monitored the Doppler shift of the signal, increasing in apparent frequency as it approached and decreasing as it departed. These measurements, along with publicly available data on the satellite, yielded its position and velocity. The scientists knew that Doppler phase-difference information was already providing reliable data for astronomers studying the expansion of the universe and for state troopers recording the speed of motorists. When combined with publicly available data, could it reveal the path of a satellite?
From their single location, Guier and Weiffenbach not only plotted the satellite’s path from horizon to horizon but calculated its entire orbit around the planet. Other tracking locations soon confirmed their findings. With a shortwave radio receiver, a tape recorder, a mechanical calculator, and a borrowed wave analyzer, they had solved the problem ahead of other observatories that had far more sophisticated equipment. The APL team tracked Sputnik until its transmitter failed on October 26. Then, for the next six months, they tracked both Sputnik II and America’s first satellite, Explorer I .
In March 1958 the two men were summoned to meet with their boss, Frank T. McClure, who stunned them with his interpretation of their data. McClure, who would serve as director of APL’s Research Center for nearly 25 years, offered a reverse premise: If measuring a satellite’s Doppler shift could determine its precise orbit, then the satellite’s signals should also permit a properly equipped observer to fix his or her location on the planet. “If you can find the orbit of a satellite,” he said, “you sure as hell can find the listening station on Earth from the orbit.” In 1961 McClure’s studies supporting this idea earned him NASA’s first “invention award” of $3,000 for contributions to space development.
McClure and his colleagues were among thescientists whose activities following Sputnik led to the development of the Global Positioning System, or GPS. Along the way the measurement of Doppler shifts would be abandoned, but the idea of using satellites hundreds of miles away to determine one’s position on earth can be traced directly back to McClure’s insight, which in turn was based on Guier and Weiffenbach’s resourcefulness.
The revolutionary concept would vault navigational science far beyond the realm of sextants, magnetic compasses, and even the World War II breakthrough, LORAN (LOng RAnge Navigation). What we now call GPS was developed as a weapon of war that could direct ballistic missiles to specific targets or guide ground troops through trackless deserts, but it would be offered to the world for civilian use after a Korean airliner’s apparent navigational error led it to destruction in Soviet airspace.
Commercial applications have helped law-enforcement officers track stolen vehicles, given cellular phones lifesaving capabilities, calculated the movement of the earth’s tectonic plates within millimeters, and assisted hikers in finding their way through the wilderness. Today a new generation of sophisticated satellites is being launched to further refine military navigational capabilities, provide more accurate tracking of storms, and expand the civilian GPS market to a projected $68 billion annually by 2010.
courtesy of aerospace corporation2007_4_26 Soon after his eureka moment, McClure shared his conclusions with another APL researcher, Richard Kershner, a rumpled, slightly built physicist whose group had developed the Navy’s Terrier missile system. McClure knew that Kershner inspired intense loyalty and affection in his associates. He was just the man to build a team that could use APL’s revolutionary findings for the benefit of the Navy’s submarine fleet. Over a long weekend McClure and Kershner worked out the preliminary details of Transit, the first satellite-based navigational system, which would help missile-armed subs navigate the world’s oceans and zero in on distant targets.
The first Transit satellite was placed in a polar orbit in 1960. When the system became operational in 1962, despite the opposition of Navy officers who doubted it would work, it had seven satellites orbiting at an altitude of about 600 nautical miles. These satellites broadcast signals to ground-based users, who could locate themselves by measuring the signals’ Doppler shifts. The satellites also transmitted information about their orbital positions, which they obtained from a set of four ground-based tracking stations.
By interpreting Doppler measurements from one satellite, a user could narrow his location down to an arc along the earth’s surface. Readings from two satellites yielded intersecting arcs that showed where the user was; using three satellites gave even better results. The Navy hoped for accuracy within about half a nautical mile, but it proved to be much better, around 80 feet. A crystal-based clock in each satellite helped ensure the accuracy of the time and location data being transmitted.
Transit was so valuable to the Navy’s submarines and surface vessels that it was released to civilian users in 1967. Under the name NAVSAT, it would help guide both weekend sailors and commercial shipping crews until the mid-1990s. Transit scored its greatest coup on November 24, 1969. When Apollo 12 splashed down in the Pacific, the USS Hornet was waiting, guided by satellite to the touchdown site.
Although Transit demonstrated that navigational satellites could be helpful and reliable, it was hardly user-friendly. It required long observation times, and the small number of satellites made for spotty access; sometimes it remained silent for hours at a time. Users on vessels that were in motion had to make time-consuming corrections. And it yielded data in only two dimensions, latitude and longitude. For the third dimension, altitude, which would be crucial for aviation, a more advanced system was needed.
Fortunately, other scientists were already pushing the technology into new dimensions. Three visionaries in particular made vitally important advances. One, a physicist working under the sponsorship of the Navy, would champion the use of atomic clocks to achieve unprecedented accuracy. Another would guide the Air Force’s efforts to create three-dimensional navigational aids for its fast-moving aircraft. The third, a young officer outranked in stripes but not in scientific know-how, would show the feuding military services how to work together to create a consolidated system better than any of their individual concepts.
Roger L. Easton had joined the wartime Naval Research Laboratory (NRL) in Washington, D.C., in 1943 as a physicist researching radar beacons and blind-landing systems for aircraft. As an active-duty naval officer he conducted research at the lab and aboard vessels until the end of the war. A decade later he helped write NRL’s Project Vanguard proposal for a scientific satellite program, which President Dwight D. Eisenhower would select as America’s scientific contribution to the International Geophysical Year. To measure the Vanguard satellite’s orbit, Easton invented the Minitrack system. This worked by monitoring the satellite’s emitted signal at two different ground stations and measuring the phase difference between them. Later the technology was expanded to track satellites that did not send signals, by reflecting signals off them.
Within a few years, tracking satellites from earth had become fairly routine. Now it was time to turn the tables and track vessels on earth with satellites. In 1964 Easton was leading a team in NRL’s Space Systems Division that developed an improved space-based navigation system called Timation. This was a more direct predecessor of today’s GPS. Easton envisioned a constellation of 24 signal-emitting satellites carrying clocks synchronized to a master clock on earth. Doppler shifts would not be used; instead, by measuring how long it took the signal to arrive, users could tell how far they were from the satellite. When repeated with other satellites and combined with data about their orbits, this procedure could yield the user’s position in three dimensions.
The use of time readings transmitted from space was Timation’s key innovation. Obviously, it would require extremely accurate clocks. When the first two Timation satellites were placed in orbit, in 1967 and 1969, they carried clocks with stable quartz-crystal oscillators. Two later satellites would be equipped with atomic clocks, which eventually set the GPS standard. In 1964 Easton received a U.S. patent for Timation, one of 11 he was awarded during his long and productive career.
Another physicist who laid groundwork for GPS was Ivan Getting. He had graduated at the top of his class from MIT in 1933, but “it was the lowest part of the Depression, and I couldn’t get a job,” he later recalled. “So instead I got a Rhodes Scholarship and spent delightful years in England.” After five years as an instructor at Harvard, he repaid the favor in 1940 by working in the MIT Radiation Laboratory group that developed the first automatic microwave-tracking fire-control radar, which would help save London from V-1 buzz bombs during World War II. At the Rad Lab he became “very much aware” that other researchers at the university were developing the LORAN system. He never forgot how LORAN used time difference in the arrival of radio signals to calculate position.
Getting joined Raytheon’s missile division in 1951 as vice president of engineering and research. In 1960 he was invited to serve as founding president of the Aerospace Corporation, a nonprofit organization created to help the Air Force apply modern science and technology to the development of ballistic missiles and military space systems.
u.s. navy photo2007_4_27 After accepting the presidency (he would hold the position until he retired in 1977), Getting began assembling nearly 1,500 scientists to identify areas where space systems might prove most valuable. One of their first projects, led by Phillip Diamond, looked at developing a three-dimensional satellite navigation system without the limitations of Transit. In this study, Getting said later, “the GPS concept was born.”
The Air Force considered the research promising, and in 1963 it began supporting the program, which was designated System 621B. By the early 1970s Aerospace had recommended a concept that would place 20 satellites in a geosynchronous inclined orbit, circling the globe every 24 hours while ranging between 30 degrees north and 30 degrees south. All the satellites would broadcast on the same frequency, transmitting a “spread spectrum” signal based on pseudo-random noise to make them resistant to jamming and interference.
Meanwhile, the army, trying to recover from the Eisenhower administration’s cancellation of its pioneering satellite program in 1956, embarked on a program called SECOR (SEquential Collation Of Range). Between 1964 and 1969 the Army launched 13 small satellites, 4 of which failed to reach orbit. These spacecraft were used primarily for research and mapping, to precisely locate points remote from the continents, such as Pacific islands. To make a reading, SECOR used a single satellite in combination with four ground stations, three whose positions were known exactly and a fourth on the island being pinpointed. Each of the three fixed stations measured its range electronically from a transponder in the satellite. They did this sequentially, as reflected in the system’s name. The readings were combined to calculate the satellite’s precise location, which was then used to establish the position of the fourth ground station.
Despite this flurry of activity, none of the service programs could muster crucial Department of Defense support. DoD officials were concerned over both the cost and the inefficiency of having multiple satellite programs run by teams from the individual services. They wanted to spend defense funds on one program, not three or four. In 1968 the department established the tri-service Navigational Satellite Executive Committee to encourage coordination. It made some progress, contracting for several studies to fine-tune the basic concept of satellite guidance. However, it was unable to break the impasse, as each development team pushed to expand its own system. To end that deadlock, in April 1973 the DoD’s powerful deputy secretary for research and engineering, Malcolm Currie, instructed the services to create a single consolidated system. The Air Force was designated program manager.
The task of maneuvering a compromise was assigned to the Joint Program Office (JPO), headed by a young Air Force colonel named Bradford W. Parkinson. Parkinson, who had been overseeing the 621B program, was selected to fill the hot seat for both his scientific expertise and his unusual multi-service background. At the Naval Academy he had volunteered to serve in the Air Force. After graduating in 1957, just months before Sputnik was launched, he continued his studies in navigation, aeronautics, and astronautics at MIT and Stanford. After graduating with distinction from both the Air Force Air Command and Staff College and the Naval War College, he taught at the Air Force Test Pilot School and then headed the departments of astronautics and computer science at the Air Force Academy.
As the Vietnam War heated up, a colleague enlisted Parkinson’s expertise to help develop a gunship (a cargo plane modified to provide concentrated aerial firepower over a battle-field). Taking leave from teaching to help perfect the lethal AC-130 Spectre, he logged 170 hours of night combat testing it. Parkinson still posts a photograph of that gunship on his office wall, explaining how it helped block the infiltration of North Vietnamese supplies through Laos. “I love the [A]C-130,” he told an interviewer.
While directing the JPO, Parkinson says, he came under “enormous pressure” from the competing teams. Each one was motivated by service pride, fear of lost funding, and a passionate belief that its approach was the best. The Navy teams that had conceived Transit and Timation were feuding with each other to build improved versions of their systems. Even the Army argued somewhat haplessly on behalf of its more specialized SECOR program. But some of the strongest pressure came from the Air Force and the Aerospace Corporation, as personified by Ivan Getting, to support 621B. Generals admonished Parkinson that he was shortening his Air Force career by promoting a composite program. “On occasion I was ‘braced up’ in the halls of the Pentagon as if I was still a plebe with the Naval Academy,” he reminisced in an Institute of Electrical and Electronics Engineers interview.
Parkinson himself believed that System 621B, though yet to be tested through an actual launch, was superior to the Navy’s Transit and Timation programs in several ways. System 621B’s spread-spectrum signal type, for instance, would allow all satellites in a constellation to transmit on the same frequency and resist jamming and other interference. Its digital structure would permit miniaturization and reduce cost. By contrast, each of Timation’s satellites had to broadcast on a different frequency, creating various problems. Timation’s “side-tone ranging” signal, though arguably stronger than 621B’s, was more vulnerable to jamming, and the same was true for Transit’s signal structure.
But Parkinson also knew that the Air Force’s concept had vulnerabilities as well. Its elliptical 24-hour orbits, favored by the Air Force because they would direct satellites over critical geographic areas for longer periods, seemed less logical for a true global navigation system than Timation’s inclined circular orbits. Timation’s creators had also developed an orbital constellation structure and tested highly accurate clocks designed to survive the stress of a launch and the perils of space.
Transit, the Navy program developed from the Applied Physics Laboratory’s Sputnik revelations, had America’s longest satellite development and operating history behind it. Among its strengths was a rich store of software for predicting and determining satellite orbits. Yet it too had a number of weaknesses that would have to be corrected if it became the centerpiece of GPS.
When the JPO submitted its first concept proposal to the DoD’s Defense System Acquisition and Review Council in August 1973, Parkinson said, “the answer was a thumbs down,” because council members considered it a repackaged 621B. “Brad, you can get this right,” Currie told him, specifying that he wanted a true joint program—and soon.
Parkinson assembled a talented group of Air Force officers with advanced degrees in engineering. He invited a few contractors to join them and told the group to work out a compromise fast. They met on neutral turf at the Pentagon over Labor Day weekend. Working through the holiday in the almost deserted building, they drafted a seven-page document that synthesized the best of the Air Force and Navy concepts and technology into a superior joint system.
It would use the digital signal structures and frequencies from the Air Force’s 621B, allowing for miniaturization and reduced cost. But 621B’s orbits would be replaced with ones more similar to those of the Navy’s Timation system, circling the planet every 12 hours. Timation’s inclined orbits would be boosted higher, to 11,000 miles, to provide broader coverage and minimize atmospheric drag. Satellites would carry multiple atomic clocks based on rubidium and cesium, providing accuracy of one second in 317,000 years. The system would need 24 satellites, the minimum necessary for 4 of them to reach every point on earth with simultaneous signals. They would be supported by a master control station in Colorado Springs and a network of monitoring and relaying stations around the globe.
The synthesis that the Pentagon meeting came up with was dubbed Navstar GPS. It won DoD approval in December 1973 and, not long afterward, a $104 million initial budget. To gain the support of tradition-bound military brass, who could still delay projects they didn’t like, Parkinson emphasized its combat capabilities, including dramatically improved targeting accuracy. To ensure an Army buy-in on the project, he chose the Army’s Yuma Proving Grounds in Arizona for the testing of user equipment.
Employing solar-powered transmitters on the ground and in tethered balloons, the tests stunned observers as the “pseudolites” (simulated satellites) demonstrated accuracy within 50 feet. But the Air Force hierarchy remained skeptical that Navstar GPS could achieve the same accuracy in combat and was fearful that it would divert funding from more cherished aircraft programs. “The Air Force never fully backed the program,” says Parkinson, noting that over the years it tried three times to cancel the system that later helped it win wars. Ivan Getting, who had squabbled with Parkinson over the joint program, became an advocate who helped him save it. Critics in the Air Force were ultimately overruled by DoD civilians, though they did manage to slow full implementation for years.
courtesy of aerospace corporation2007_4_30 The Navy, despite its own dissenters, had a supportive team ready to get the program off the ground in a hurry. Just months after the JPO won approval for its synthesized program, the Navy launched a modified Timation satellite (renamed Navigational Technology Satellite 1) on July 14, 1974, as the first Navstar prototype. It carried two rubidium-based atomic clocks, which failed soon after launch. NTS-2 followed on June 23, 1977, with clocks based on cesium. Such clocks not only could provide precise times to ground stations but could accurately forecast satellite orbits, thus reducing the need for frequent satellite updates.
These tests cleared the way for the launch of four Navstar Block I satellites in 1978. The first-generation Block I constellation, with 11 satellites built by Rockwell International, was meant to validate the system; it would be completed with a final launch in 1985. The fifth satellite in this series included a new wrinkle: sensors to detect the detonation of nuclear weapons and indications of a pending nuclear attack. All later Navstar satellites, and some other DoD satellites, would help monitor compliance with nuclear proliferation treaties.
With the program validated and satellites going aloft, Parkinson retired from the Air Force in 1978 to teach at Colorado State University. He soon moved into the business world, working at Rockwell International, Intermetrics, and Trimble Navigation. He was appointed to the Aerospace Corporation board in 1997 and became its chairman three years later.
But he had found his true home at Stanford University, where he became a professor of aeronautics and astronautics in 1984. There he studied innovative civilian uses of GPS (as the system had become best known), pioneering a Federal Aviation Administration (FAA) program to help aircraft take off and land safely in poor visibility. Another program showed how robotic tractors could plow a furrow within two inches on a rough field. From his Stanford office, he still helps manage NASA’s Gravity Probe B, a spacecraft launched in 2004 to test unverified predictions of Einstein’s theory of relativity. He also chairs the Advisory Council and the GPS III Independent Review Team.
Eight years after Parkinson retired from the Air Force, Navstar’s planned next phase, Block II, suffered a major setback in 1986 when the Challenger disaster suspended all Space Shuttle flights and trashed NASA’s timetable. The Navstar program, which had been relying on the Shuttle program to launch a series of Block II satellites, had to rework its plans and use Delta boosters instead. It couldn’t launch the first of Navstar’s 28 Rockwell-built satellites (24 plus 4 in-orbit spares) until February 1989 and the last until March 1994.
Even before Challenger , another tragedy had forced a rethinking of GPS’s original emphasis on serving only the defense establishment. On September 1, 1983, Soviet fighter jets shot down Korean Airlines Flight 007, which had strayed into Soviet airspace. All 269 aboard the jumbo jet died. To help avert future tragedies, President Ronald Reagan announced that Navstar’s signals would be made available for international civilian use as the system came on line. U.S. military interests would be protected through coding of military signals and by applying perturbations to reduce the precision of readings made available to civilian users, a system called Selective Availability (SA).
That scheme didn’t hold up very long, as electronics manufacturers quickly found ways to override SA with “differential systems.” These calibrated the errors from GPS in real time, using readings from a base with known location as a standard, and relayed the corrections to users, like Differential GPS today (see sidebar on page 25). The government made little effort to prevent such measures, since experts, including Parkinson, had argued from the beginning that SA would limit GPS’s benefit to some civilian users without preventing hostile forces from using the system for targeting. Parkinson’s team even demonstrated a differential system to show how easily SA could be bypassed.
Many among the general public first became aware of GPS during Operation Desert Storm, the 1991 liberation of Kuwait. According to one assessment, “GPS satellites enabled coalition forces to navigate, maneuver and fire with unprecedented accuracy in the vast desert terrain almost 24 hours a day.” The technology helped overcome Iraq’s and Kuwait’s primitive roads and blinding sandstorms. Allied commanders cited GPS and night-vision equipment as two new weapons that helped ensure the speedy triumph. GPS precision minimized civilian casualties by locating military targets accurately.
On the ground, soldiers and tankers recognized GPS’s value and requested many more receivers than the Army had in stock. Officials turned to the private sector, ordering more than 10,000 units from commercial suppliers. With so many commercial receivers being used by the troops, the government had to disengage Selective Availability. (The degradation of civilian signals was restored after the war ended but was turned off entirely in 2000 by order of President Bill Clinton.) The much-publicized usefulness of GPS in Desert Storm triggered a general demand for receivers, as civilians came to realize that the technology could help them meet needs ranging from measuring fluctuations in the San Andreas Fault to tracking municipal garbage trucks.
Researchers have continued developing systems to improve the accuracy of GPS, whose civilian applications are coordinated by the Department of Transportation. The FAA’s Wide Area Augmentation System (WAAS) is one such enhancement. Its combination of geosynchronous satellites and ground stations provides extremely accurate guidance for all-weather landings at any airport. The Coast Guard also maintains a system covering much of the U.S. coastline that provides differential GPS data.
The prospect of an even broader civilian market beckoned in September 2005, when the Air Force launched the first major upgrade of the GPS constellation. Seven Lockheed Martin Block 11-RM satellites will be in orbit by late 2007, replacing aging satellites in the current constellation. The 11-RM satellites, for the first time, offer civilian users two separate channels. In addition to increasing the accuracy of GPS by compensating for ionosphere distortion, the new satellites will enhance GPS reception for low-power cell phones and other personal communications devices. The Lockheed satellites will be joined in 2009 by at least a dozen even more advanced 11F spacecraft, built by Boeing.
Dual channels should boost civilian accuracy to approximate that enjoyed by the military. Improved signals, with greater power and precision, should help weather forecasters do a better job tracking hurricanes and other natural threats. The Defense Department will get an upgrade with greater power and improved resistance to jamming, letting it transmit stronger signals without interfering with civilian receivers.
Inevitably, other nations have decided to get into the act. Both Europe and Russia are establishing GPS-like constellations. The European Space Agency’s Galileo system is expected to be in service by 2010 with a 30-satellite constellation. Eventually, anyone with a civilian receiver should be able to establish a position from either Galileo or Navstar. American users may someday also be able to accept information from Russia’s GLONASS (GLObal NAvigation Satellite System). This system put its first operational satellite into orbit in 1982, but the Soviet Union’s (and later Russia’s) economic chaos brought progress to a temporary halt. A joint venture with the Indian government should help Russia complete a 24-satellite constellation by 2010.
The scientists who made gps possible have received many honors for their contributions. Most recently, in November 2005, President George W. Bush awarded Roger Easton the National Medal of Technology. After retiring from federal service in 1980, Easton remained a consultant to NRL, exploring new ways to improve GPS accuracy and helping create the national Space Surveillance System. That system, an extension of Easton’s 1950s Minitrack scheme for Vanguard, was the first one capable of detecting and tracking every type of satellite orbiting earth. It continues to catalogue all known man-made space objects to this day.
After retiring to New Hampshire, Easton was elected twice to the state assembly. He ran for governor in 1986 but lost in the Republican primary to John Sununu (the father of the state’s current senator of the same name). Now in his mid-eighties, he vigorously asserts his claim to be the principal inventor of GPS, contending that Navstar is based largely on Timation. “It’s been 41 years since I came up with the invention,” he told his local newspaper, the Valley News . “A lot of other people have claimed to invent it, but they aren’t the first inventors.”
Parkinson and Getting, whose own disagreement over a GPS compromise didn’t reduce their mutual admiration, were honored jointly in 2003 with the National Academy of Engineering’s $500,000 Draper Prize, named for Parkinson’s mentor at MIT, Charles Stark Draper. They were also inducted jointly into the National Inventors Hall of Fame in February 2004. Getting’s induction was posthumous; he died five months earlier.
Appropriately, the National Aeronautic Association, in presenting its Robert J. Collier Trophy, the nation’s most prestigious aviation award, in 1992, didn’t attempt to single out one honoree but bestowed it instead on what it called “GPS team.” It lauded researchers from the Naval Research Laboratory, the Air Force, the Aerospace Corporation, Rockwell International, and IBM Federal Systems Company. Together, the citation said, those researchers created “the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago.”
Parkinson, who at 72 owns several handheld receivers that help him find his way along trails in California’s High Sierra, gladly shares credit for the joint effort. “I am amazed at how many people made contributions to this effort,” he says. “It is not a one-person show.”
