GPS Declassified

by Richard D. Easton

  Synchronicity

  Aboard each satellite is an atomic clock, along with two or more backup clocks in case of malfunction. Extremely precise timing is essential for GPS accuracy, and a number of technological advances in timekeeping had to occur before satellite navigation could become a reality. Just as pendulum clocks were unsuitable for use at sea, the first timekeeping approaches that followed pendulums were not accurate enough to transmit precise time across the vast distance from a satellite in orbit to a user below. To appreciate how precise the time signal must be for GPS, consider that an error of one nanosecond—a billionth of a second—results in an error of about one foot.33 An error of two microseconds—two millionths of a second—can send a speeding jet off course by a third of a mile.34

  Pendulum clocks, with the addition of a second pendulum, reached their performance limit by the early 1920s, keeping accurate time to within ten seconds a year.35 However, the growing radio industry, which uses frequencies in the millions of cycles per second, needed more precisely measured seconds to prevent one station’s signal from drifting over another’s.36 The National Bureau of Standards (NBS; renamed the National Institute of Standards and Technology, or NIST, in 1988) searched for a material that would vibrate—oscillate—at a constant rate, like electrically driven tuning forks of the era, but much faster. The answer was to apply an electric current to a thin quartz crystal. Scientists learned to make quartz crystals vibrate at high frequencies that varied depending on the size and shape of the crystal. Quartz oscillators became the primary frequency standard at NBS, which set up four 100-kHz quartz oscillators that achieved accuracies of about one second in three years on average.37 By 1950 quartz accuracy had improved by a factor of one hundred, and crystal oscillators replaced pendulum clocks at NBS as the primary standard for time intervals.38 Quartz oscillators have some drawbacks. No two crystals are exactly alike or have identical resonant frequencies, and they wear out. Their frequency changes slowly due to aging and environmental variables such as temperature, humidity, and pressure.39

  Scientists turned next to the atom. Physicists knew that atoms absorb or emit energy at specific frequencies, giving each atom a resonant frequency. Atoms represented an oscillator that would never waver or wear out, and all atoms of a given element would be identical. News of their potential use in clocks produced wonderment typical of atomic age discoveries. “‘Cosmic Pendulum’ for Clock Planned: Radio Frequencies at the Hearts of Atoms Would Be Used in Most Accurate of Timepieces ,” the New York Times announced on January 21, 1945, a day after Nobel Prize–winning physicist I. I. Rabi proposed the idea during a talk to the American Physical Society.40 Seven months later the world discovered the awesome energy unleashed by splitting atoms when bombs destroyed Hiroshima and Nagasaki, but atomic clocks involve no fission or radiation. In 1949 NBS physicists announced they had synchronized a quartz-crystal oscillator to the natural resonant frequency of ammonia atoms. They did this by shooting high-frequency microwaves of about 24 billion Hz (24 gigahertz [GHz]) through a thirty-foot coiled copper tube filled with ammonia gas, causing each ammonia molecule’s single nitrogen atom to flip its position among three hydrogen atoms, like an inverted pyramid.41 They could not see this happen; they measured the effects. At frequencies too high or low, the microwaves passed through the chamber, hitting a detector that adjusted the frequency until at 23.8 GHz the gas absorbed the microwaves. This ammonia device was accurate to within one second in eight months.42

  Atomic clocks (scientists prefer the term atomic frequency standard, or AFS) got smaller and more accurate over time. Cesium, a mercury-like metal that liquefies just above room temperature, replaced ammonia as the preferred atomic frequency standard.43 Instead of shooting microwaves through a cloud of molecules, second-generation atomic clocks shot a beam of vaporized cesium atoms through a microwave signal and compared the energy they contained to another stream that was diverted around the microwaves by magnets. In 1952 a cesium clock achieved an accuracy of one second in three hundred years—for the first time dividing a second into billionths, creating the nanosecond.44 Cesium clocks later achieved an accuracy rate of one second in twenty-five hundred years, and in 1967 the standard international second was redefined as the resonant frequency of cesium atoms, replacing the astronomical definition based on a fraction of the solar day.45 In the 1960s Hewlett-Packard developed a line of 180-pound cesium clocks portable enough to transport in planes to synchronize timekeeping facilities around the world. It was about this time that Roger Easton at the Naval Research Laboratory envisioned placing those clocks in the air continuously aboard satellites.

  The GPS constellation at this writing uses five cesium and twenty-six rubidium atomic clocks.46 Rubidium is a silvery-white metal that is easy to vaporize. Clocks using rubidium in newer GPS satellites have demonstrated superior performance and longer lives than cesium clocks.47 Although newer clocks are superior, the military conservatively rates all clocks in the constellation to be accurate to within one hundred nanoseconds over any three-second interval.48

  Accuracy on the ground depends on a variety of factors, including atmospheric effects and receiver quality, but the accuracy of the Standard Positioning Service offered to civilians is generally three meters or less—under ten feet—99.99 percent of the time. Various techniques used today, including augmentation systems discussed in later chapters and high-end receivers that measure the carrier wave itself, can improve that accuracy to a few millimeters.49

  Fig. 5.2. Instantaneous navigation math. Mathematicians at the Naval Research Laboratory formulated this solution to the problem of two-, three-, and four-dimensional instantaneous navigation using passive ranging signals from satellites. (Courtesy Naval Research Laboratory)

  Before GPS receivers display latitude, longitude, and altitude figures or a moving dot on a map, computer chips inside them perform an astonishing number of mathematical calculations. For a glimpse at the type of formulas involved, consider figure 5.2, which shows how Naval Research Laboratory mathematicians in 1971 formulated a solution for instantaneous navigation by satellites in four dimensions.

  After locking onto a satellite signal the GPS receiver begins repeating the signal’s digital navigation sequences the way people sing “Row, Row, Row Your Boat ” in the round. It then delays its own sequences until they synchronize with the incoming signal. The amount of delay equals the signal’s travel time from satellite to receiver, which, multiplied by the speed of light, yields the satellite’s distance. With computed distances from three satellites and the accurate time from a fourth satellite’s clock to synchronize the battery-powered quartz clock in the receiver, the receiver’s computer can perform the geometric calculations (variously called trilateration, trilateralization, or triangulation) to determine a three-dimensional position as it moves in real time.

  Society has become habituated to inexpensive portable computing power. Without it, satellite navigation would be unavailable to everyday users. Many technologies work together to make GPS perform in a way that would appear magical to prior generations. It is difficult to decide which one is most essential. But at the core of the system beat the “radio frequencies at the hearts of atoms ”—the atomic clocks—so it seems especially fitting that the GPS constellation of satellites surrounding Earth resembles electrons orbiting an atom.

  6

  Going Public The Roots of Civilian GPS Use

  A little inaccuracy sometimes saves tons of explanation.

  Saki (Hector Hugh Munro), “Clovis on the Alleged Romance of Business ,” The Square Egg, 1924

  Thirty-three thousand feet above the Sea of Japan, a Boeing 747 jumbo jet bored its way through the predawn darkness. In the cabin the lights were low, and most of the 240 passengers dozed. In the cockpit the pilot and copilot bantered with a sister airliner, also bound for Seoul, and hailed air traffic controllers in Tokyo to request permission to climb to thirty-five thousand feet. It was a routine procedure on the homestretch of a long flig
ht. Having used most of its fuel, the lightened aircraft could now fly higher and faster.1

  Minutes after completing the maneuver, the 747 was rocked by an explosion as an air-to-air missile blew a hole in the fuselage and severed the left wing. The enormous aircraft, which had mistakenly entered prohibited Soviet airspace, rolled to its left and began an uncontrolled spiral decent lasting more than twelve minutes before it crashed into the sea, killing all 240 passengers and 29 crewmembers.2 The shoot down of Korean Air Lines Flight KE007 by the Soviet Union, in the early hours of September 1, 1983, became a milestone in Cold War superpower relations as well as a catalyst for the spread of GPS technology from military to civilian use.

  Despite having triple-redundant inertial navigation systems (INS) on board, the pilot and copilot of Flight KE007 lacked what millions of motorists using GPS today take for granted—the positional awareness that comes from seeing a moving icon on a map. The pilots blindly trusted that the automatic pilot was faithfully executing, with INS guidance, the coordinates they had programmed into it.

  Navigation methods have always relied on measuring a traveler’s movement against a fixed reference point, such as the sun, a star, or the magnetic pole. INS uses gyroscopes to maintain that fixed reference point within the airplane itself, calculating latitude, longitude, and altitude electronically and feeding the information into the automatic pilot. At the time of the shoot down, commercial airplanes had been using INS for about a decade with no known simultaneous failure of all three systems.3 The Flight KE007 incident led to speculation about how two experienced pilots could fly a civilian jetliner 360 miles off course over Sakhalin Island, home to one of the Soviet Union’s most sensitive military installations. Explanations ranged from equipment malfunction to pilot error to conspiracy theories that the flight was on a covert spy mission. Numerous books and articles have examined the flight and these theories in detail, but it was not until 1993, after the breakup of the Soviet Union and a decade after the incident, that Russian Federation president Boris Yeltsin turned over the black boxes to investigators. The cockpit voice recording proved conclusively that the pilots were unaware they were off course (undoubtedly the reason Soviet military officials never acknowledged finding the black boxes and withheld the evidence). The most plausible explanation is that the pilots “armed ” the INS but it never fully “engaged. ” This could have happened, because early in the flight the airplane was already several miles farther off course than the maximum distance the autopilot computer was programmed to accept when transitioning to automatic INS.4 That left the airplane on a magnetic compass heading that carried it farther and farther off course. Having earlier sailed over Soviet territory above Kamchatka Peninsula and later failing to respond to radio contact on a frequency they were not monitoring or to notice bullets fired past the craft in the darkness, the pilots appeared to Soviet fighters to be executing an evasive maneuver with their routine “step-climb ,” a move that sealed the flight’s fate.

  A Calculated Response

  In the days following the shoot down, President Ronald Reagan and his advisors drafted a National Security Decision Directive (NSDD 102) outlining the administration’s response, which focused largely on strategies to marshal world opinion against the Soviets.5 It is important to appreciate the dismal state of U.S.-Soviet relations at the time. Five months before, on March 8, Reagan had called the Soviet Union the “focus of evil in the modern world ” in his famous “evil empire ” speech.6 On March 23 he delivered a prime-time address from the Oval Office announcing a program to develop a defense against ICBMs.7 Officially named the Strategic Defense Initiative, or SDI, the program was tarred by critics with the moniker “Star Wars ”—from the 1977 George Lucas film—because it proposed using exotic new technologies to destroy ICBMS during their flight through space.8 By December 1983 the United States was set to begin fielding nuclear-tipped Gryphon cruise missiles and Pershing II intermediate-range ballistic missiles in Western Europe to counter the Soviet’s SS-20 missiles.9 Reagan’s military buildup pushed federal spending in 1983 to its highest level as a percentage of gross domestic product since World War II.10 Thus, it was within a general state of high anxiety that the Soviet regime committed the atrocity and responded to world reaction.

  As administration actions under NSDD 102 gained traction, the U.S. Congress, the International Civil Aviation Association, the International Federation of Air Line Pilot Associations, and the United Nations Security Council expressed condemnation through resolutions (with a Soviet veto blocking formal adoption of the UN resolution). Numerous airlines and entire nations suspended flights to or from the Soviet Union. For weeks, Reagan mentioned the incident in speeches and interviews, using the shoot down to draw a stark contrast between Soviet values and behavior and those of democracies. Beyond apologies, Reagan pushed for reparations to the victims’ families. There were passengers from thirteen countries aboard KE007, including sixty-six Americans.11 One was Congressman Larry McDonald of Georgia, part of a six-person delegation en route to South Korea to mark the thirtieth anniversary of the U.S. defense treaty with that nation. The other five delegates were aboard Korean Air Lines Flight 015, which departed Anchorage only minutes after KE007 but was 350 miles away at the time of the shoot down.12 Reagan also called for new aviation protocols to prevent a repeat of the tragedy in the future. Anticipating that twenty-four GPS satellites would be in orbit and operational by 1988, the Reagan administration announced that it had decided to make the new navigation technology available to civil aviation.

  Today, nearly every GPS historical timeline features Reagan’s announcement, but most accounts either ignore the details or stretch the truth. Examples abound, particularly in poorly sourced online histories of GPS, of statements that Reagan “declassified GPS ” or “gave it to private industry. ”13 Very intelligent people have succumbed to such hyperbole. A high-profile example happened at the 2011 South by Southwest (SXSW) festival in Austin, Texas, during an onstage interview of Tim O’Reilly, the founder and CEO of O’Reilly Media, who is credited with coining the term Web 2.0. While making the point that people should view government as a platform that helps birth new technologies, O’Reilly dubbed Reagan “the father of Foursquare. ”14 His remark drew an audible reaction from the audience and attracted lots of press coverage because of the unlikely coupling of the iconic president, who died in 2004, with the rapidly growing location-based service launched at SXSW in 2009. Foursquare employs GPS to allow its users, via smartphones, to share their location with friends, “check in ” at restaurants, shops, and other hotspots, and take advantage of promotional offers targeted to them by businesses.15 What neither the audience nor the press seemed to notice were the factual errors in O’Reilly’s comments: “When the Navy and the Air Force put up the GPS system, they did not have to make the decision to open it up for civilian use. In fact, there was a lot of debate about that. It was Reagan, who after a U.S. airliner was shot down over North Korea because it strayed over North Korean airspace, said ‘Hey, when you guys finish this GPS thingy, let’s open it for civilian use.’ It was an executive order that he gave ” (emphasis added).

  Recalculating the Facts

  Reagan never issued any public executive order pertaining to GPS. (Some assert that NSDD 102, now largely declassified, contains references to GPS within portions that remain redacted.)16 Furthermore, an Internet search of his speeches and interviews archived at the Ronald Reagan Presidential Library offers no evidence that he ever publicly uttered the phrase “Global Positioning System. ”

  Reagan was vacationing in California when the shoot down occurred. His deputy press secretary, Larry M. Speakes, read a short statement on September 1 during a briefing with reporters at the Sheraton Santa Barbara Hotel. The next day, Reagan cut short his vacation, gave prepared remarks to reporters before boarding Air Force One for Washington, and issued Proclamation 5086, which ordered flags to be flown at half-staff at all federal facilities. After approving NSDD 1
02 on September 5, 1983, Reagan addressed the nation on television that night, playing a tape of the Soviet pilot communicating with ground control and saying (in Russian), “The target is destroyed. ” The president devoted his weekly radio address to the incident on September 17 and worked references into a speech—devoted almost entirely to the arms race—to the United Nations General Assembly on September 26.17 On that very night, the world was closer to nuclear Armageddon than anyone knew. In an underground missile silo near Moscow, a computer screen began flashing an alarm that the United States had launched five ICBMS. Disaster was averted only because a Soviet lieutenant colonel, Stanislav Petrov, correctly concluded that the alarm was an error and chose not to react.18

  In all of Reagan’s public comments about the KE007 shoot down, and in numerous other settings in which he might have strayed onto the subject, he apparently never mentioned GPS. Speakes, it seems, made the only official public reference to GPS, while reading a prepared statement before a press conference September 16. It came at the end of the first paragraph in a brief, two-paragraph text: “World opinion is united in its determination that this awful tragedy must not be repeated. As a contribution to the achievement of this objective, the President has determined that the United States is prepared to make available to civilian aircraft the facilities of its Global Positioning System when it becomes operational in 1988. This system will provide civilian airliners three-dimensional positional information. ”19