Longevity also describes Minitrack, which spawned several generations of ground tracking systems. By the fall of 1957, when Sputnik I reached orbit, Minitrack ground stations had been constructed at numerous sites around the globe, including at Blossom Point, Maryland; Fort Stewart, Georgia; Havana, Cuba; Quito, Ecuador; Lima, Peru; and Antofagasta and Santiago, Chile.109 This north-south line of stations, located roughly along the seventy-fifth meridian of longitude and with overlapping, fan-shaped reception patterns extending skyward, created an “electronic fence ” to capture each overhead pass of the satellite. Tracking stations transmitted all data by teletype to Washington DC for processing by an IBM 704 mainframe computer, which calculated and plotted each orbit.110
An astute reader may perceive here what scientists and engineers in those early days first comprehended—that if you could pinpoint a single radio-emitting satellite in orbit using multiple receivers on the ground, it should be possible by inverting the process to pinpoint a single receiver’s location using multiple orbiting satellites—the essence of satellite navigation. However, the first operational satellite navigation system emerged from a different tracking method. A few days after Sputnik captured the world’s attention, two physicists at Johns Hopkins University’s Applied Physics Laboratory, George Weiffenbach and William Guier, calculated and eventually were able to predict Sputnik’s orbits by analyzing the Doppler shift of its radio signal as the satellite circled the earth.111 Doppler shift is the apparent change in pitch that occurs as a sound source moves by a listener. Trains, emergency vehicle sirens, race cars, and jets planes have made this effect so common that people today rarely give it a second thought. Frank McClure, chairman of the Applied Physics Laboratory (APL) Research Center, reviewed Guier and Weiffenbach’s findings and challenged them (coincidently, the same day Vanguard I was launched) to “invert the solution ,” that is, to see if they could calculate a receiving station’s position using a known satellite orbit.112 They succeeded, and later in 1958 the APL’S Richard Kershner led a federally funded project to build a system of radio-emitting satellites and worldwide tracking stations for Doppler measurements.113 In 1964 the completed network became the Naval Navigation Satellite System, commonly called Transit. The military made it available for commercial use in 1967 (a pattern repeated later with GPS), and it operated until 1996, helping ships and submarines plot their position to within about five hundred feet anywhere in the world in any weather.114
The advent of satellites in orbit and dreams of sending humans into space led to the creation of NASA, the National Aeronautics and Space Administration, in 1958. Project Vanguard and most of the NRL people who worked on it were absorbed into the new civilian agency.115 As satellites increased in size, number, and complexity, NASA reconfigured the network of Minitrack ground stations to keep pace. New polar and geosynchronous orbits (satellites traveling the same speed as the earth’s rotation to remain at a fixed location above the equator) prompted the building of new sites and the closing of others, and Minitrack was renamed the Spacecraft Tracking and Data Acquisition Network.116 It was followed by the Manned Spaceflight Network, also land-based, and both were replaced in the mid-1980s by the satellite-based Tracking and Data Relay Satellite System.117
Fig. 1.3. Frank T. McClure (center), director of the Research Center at the Johns Hopkins University Applied Physics Laboratory, chats with physicists William H. Guier (left) and George C. Weiffenbach. (Courtesy Johns Hopkins University Applied Physics Laboratory)
Roger Easton, not wanting to uproot his young family, decided to remain at the Naval Research Laboratory, where he turned his attention to one of the few space projects retained by the laboratory—tracking spy satellites.118 The challenge of building tiny transmitters to fit inside satellites and accurate ground receivers to track them was replaced by the problem of tracking silent satellites designed to evade detection. As head of the Space Surveillance Branch, Easton led development of the Naval Space Surveillance System (sometimes shortened to NAVSPASUR, a moniker he never liked because it confused the system with its command structure of the same name).119 The system still operates today as part of the larger Space Surveillance Network directed by the U.S. Strategic Command in Omaha, Nebraska.120 Space Surveillance employs many of the same techniques used in Minitrack. A series of six ground receivers spaced across the southern United States from San Diego, California, to Fort Stewart, Georgia, forms a radar “fence ,” but the signals they track are generated not from satellites but by three large ground transmitters in Alabama, Texas, and Arizona. As satellites pass over the fence, continuous wave signals beamed into space by the transmitters bounce off the satellites and are picked up by the ground stations. A central computer at the program’s headquarters in Dahlgren, Virginia, performs the high-powered computing required for quickly calculating orbits. When the system detected a large unknown satellite in late 1958, it created some excitement at the Pentagon, but the object turned out to be Vanguard’s third-stage booster trailing behind the satellite.121 The system’s precision was sufficient to detect even the strap that had secured the Vanguard satellite to the rocket.122 With improvements over the years, the system gained the capacity to detect basketball-size objects in orbit out to a range of more than seventeen thousand miles, and it is the oldest system for tracking the multitude of space debris now in orbit.123
The Space Surveillance system’s continuous wave radar uses transmitters and receivers placed hundreds of miles apart. To measure the wave accurately, the receiver and transmitter must be precisely synchronized. Easton and his associates first tried transmitting the time code over the horizon, but extraneous noise introduced errors. Next, they carried an atomic clock by vehicle between the stations. That worked better but created a time-consuming task that had to be done continuously. In mid-1964 it occurred to Easton to put the atomic clock in a satellite, where it could transfer precise time to the transmitter and receiver simultaneously.124 From there, using satellites with atomic clocks to transmit time precisely enough for navigational purposes, especially for the speeds associated with aircraft—something Transit could not do—seemed to Easton a logical next step. But the idea was not immediately embraced. “We were subjected to criticism because it was an idea looking for an application and not the other way around ,” Easton recalled in a November 2000 speech accepting a Distinguished Service Award at the Thirty-Second Annual Precise Time and Time Interval Meeting.125
If military leaders were not ready to launch atomic clocks into space, the clocks themselves were not quite ready for the mission either. Much work went into improving the accuracy of atomic clocks and “hardening ” them to withstand cosmic radiation. Technological advances were needed in other fields, such as integrated circuits, which boosted the speed and shrank to a portable size the receivers used to process satellite signals. Complex computer simulations searched for the ideal altitude and arrangement of satellites to keep at least four in view at all times everywhere, while using the fewest possible to reduce cost. Engineers differed over the types of signals that would best transfer information from the satellites without being jammed. Funding was a perennial issue.
Gradually, advocates of advanced satellite navigation became more numerous, particularly in the Navy and the Air Force, and eventually at the Pentagon, which in 1968 decided the military could afford only one such system. A competition ensued, not unlike the one to launch the satellite for the International Geophysical Year, but this contest remained mostly unnoticed by the public, shrouded behind a cloak of military secrecy. That original competition persists today in differing narratives about the origin of GPS.
Subsequent chapters examine the competing claims, drawing on contemporaneous accounts published in technical journals, pertinent government documents that have since been declassified, and interviews with key individuals. Before looking at the competing designs that provided the building blocks of GPS, a short detour is in order. This was not the first time an advance in accurate timekee
ping produced a revolution in navigation. For that story, it is necessary to go back a few centuries.
2
Weather Permitting A Brief History of Navigation
Sleep did not fall upon his eyelids as he watched the constellations—the Pleiades, the late-setting Bootes, and the Great Bear, which men call the Wain, always turning in one place, keeping watch over Orion—the only star that never takes a bath in Ocean.
Homer, The Odyssey, chapter 5
The word navigation comes from navis, the Latin word for ship, and agare, the Latin word that means to move forward.1 Accurately knowing your position at sea can make the difference between life and death. A prime historical example involves Sir Ernest Shackleton (1874–1922), who led several expeditions to Antarctica. As World War I broke out in August 1914, he sailed from Britain on the Imperial Trans-Antarctic Expedition. Its objective was to make the first traverse of Antarctica. His ship, the Endurance, became trapped in pack ice in the Weddell Sea, well south of the Antarctic Circle, and ultimately sank in October 1915. After months on the ice floe, he traveled with his twenty-eight men in three lifeboats to the inhospitable Elephant Island. From there, Shackleton made an epic voyage to South Georgia to summon help in the most seaworthy lifeboat, the James Caird. His 850-mile route was through an area called the Furious Fifties, the zone between fifty and sixty degrees south latitude, where little land area interrupts the water circulating around the South Pole. Missing South Georgia meant certain death since no other inhabited islands were within reach. But taking accurate sightings of the sun is difficult on the rolling deck of a small craft in wind-tossed seas. When they approached South Georgia, his navigator, Frank Worlsey, told Shackleton that he “could not be sure of our position within ten miles. ”2 Given their exhaustion, the lifeboat’s poor condition, and the danger of missing the island completely or being swept away, they headed for the nearest point of land on the uninhabited west coast of South Georgia. Landing successfully, they faced two difficult alternatives. The first was to sail around the island. Shackleton rejected that due to the poor condition of the James Caird. The second was climbing through the uncharted mountains and wastelands of South Georgia to the whaling station of Stromness. That had never been done, but Shackleton and the strongest two of his companions accomplished this feat, which even modern climbers with the best gear available find difficult.
Navigational Challenges
Prior to the balloon’s development in the latter part of the eighteenth century, navigational challenges were mainly two-dimensional and could be separated into land versus sea travel. These two modes present similar but not identical navigational challenges. The navigator wants to safely travel from the current position, point A, to the destination, point B. Often, there are hazards to be avoided or interim points to be reached for resupply. Maps or sea charts provide useful information, but two-dimensional maps distort the three-dimensional earth. And the earth is not a perfect sphere, which adds to the mapmaking and navigational challenge. An eighteenth-century French expedition to South America showed that Sir Isaac Newton was correct in his hypothesis that the earth is fatter at the equator than it is at the poles.3 Gravitational variations exist in both the earth and the moon. One of the challenges in Apollo 11’s landing was a lack of understanding of the moon’s gravitational variations. This was intensively studied between Apollo 11 and 12, allowing Apollo 12 to make a pinpoint landing near the unmanned exploratory craft Surveyor 3.
Land travel has an advantage over sea travel in that there are more physical landmarks to help ascertain your position. Many landmarks are man-made, such as roads or cities. And repeated astronomical observations can be taken from a stationary land position, whereas an unanchored ship is always moving. However, navigating across a featureless desert is much like sea travel. Alexander the Great is believed to have visited the oasis of Siwa in Egypt. Danish scholar Torben B. Larsen gives a sense of how the ancients viewed the risks: “The overland journey was, according to the historian Callisthenes, a dangerous one. Alexander’s party exhausted its water supply, but divine intervention produced a sudden downpour. A sandstorm caused them to lose their way, but divine intervention, Callisthenes says, sent two crows to lead them safely to Siwa. ”4 Even today, mariners seeing nonmigratory birds know that land is nearby.
Every point on the earth can be specified by latitude and longitude. Latitudes are circles running east-west parallel to the equator, which is zero degrees. The North Pole is ninety degrees north; the South Pole is ninety degrees south. Longitudes run north-south, perpendicular to latitudes, but the starting point is arbitrary since there are no unique positions such as the equator or the poles. Given the British Empire’s dominance in the nineteenth century, the British made the arbitrary decision to place the prime (zero) meridian through Greenwich in England. Using the previously defined navigational challenge as moving safely from point A to point B, the estimate of your current position on a map has some uncertainty, and the destination, point B, also has been measured with some uncertainty. During the Apollo 8 mission to lunar orbit, NASA found that the estimated position of the Command Module differed significantly depending on which ground stations were used. Managers at Johnson Space Center in Houston discovered errors in the coordinates of three remote island tracking stations. “The anomalous measurements from Canary, Hawaii and Guam were consistent with geodetic errors [in the positions of these stations] of up to 300 metres ,” recalled Pat Norris, a former Apollo navigation manager.5 The distance that makes up a degree of latitude is constant whereas the distance for a degree of longitude is greatest at the equator and is zero at the poles.
North of the equator, latitude can be measured by the altitude of Polaris, also called the North Star, adjusting for the fact that it is not precisely over the pole. An alternative is measuring the sun’s altitude at local noon, adjusting for the season. The sun lies directly over the equator at local noon on the first day of spring and the first day of fall. If the sun is forty degrees above the horizon at local noon on those days, a ship’s navigator can calculate that he is at the fiftieth meridian. This method has limitations. North of the Arctic Circle, the sun cannot be seen at times during the fall and winter.
Longitude is much more difficult to determine. The British Parliament in 1714 passed the Longitude Act, which authorized a series of rewards for the person who perfected a method of determining it and established a commission, the Board of Longitude, to select the winners. The main competitors were clocks built by John Harrison, a self-taught clockmaker, versus various astronomical solutions.
Geometry has been used for millennia to measure distances. Eratosthenes, a Greek mathematician and founder of the discipline of geography, first calculated the circumference of the earth circa 240 BC. He knew that at noon on the summer solstice in Swene (modern Aswan, Egypt), a city located near the Tropic of Cancer, the sun was directly overhead, whereas in Alexandria the sun appeared at an angle south of the zenith. That angle equaled one-fiftieth of a circle. Eratosthenes reasoned, assuming that Alexandria was due north of Swene (it is actually slightly northwest), the earth’s circumference is fifty times the distance between the two cities.
Harrison’s timepiece, which developed into the marine chronometer, tells a mariner his longitude relative to the embarkation port based on its time compared to local noon (the sun’s highest point in the sky). If the mariner’s clock reads 1:00 p.m. at local noon, he is one-twenty-fourth of a day or fifteen degrees (one-twenty-fourth of a 360-degree circle) west of the embarkation port.
Accurate maps and charts are important for showing hazards. This is important even in the age of GPS. The ferryboat Pride of Canterbury, which travels between Dover, England, and Calais, France, hit the 1917 wreck of the SS Mahratta on January 31, 2008. The submerged wreckage was shown on the electronic sea chart but not at the magnification the Pride of Canterbury was using when the collision occurred.6 Thus GPS by itself is not enough. One must also be aware of software idiosyn
crasies.
Navigational challenges include finding a point along a coast or finding an island. Rear Adm. Daniel Gallery (1901–77) commanded a task force hunting German submarines in World War II. He seized the U505, the first ship captured by the U.S. Navy since the War of 1812, which now resides in Chicago’s Museum of Science and Industry. In a novel written after the Six-Day War in 1967, he described current navigation techniques including Loran, a ground-based system of radio beacons, and the first space-based navigation system, Transit. However, his characters find Tel Aviv by sailing at a bearing where they can be confident that they will hit land north of the city. Then they could sail south and be confident that they would reach it.7 Amelia Earhart, in 1937, failed to find Howland Island. Her navigator, Frank Noonan, wanted to fly to a point between Howland Island and Baker Island, which would have greatly increased their opportunity to check their position, but Earhart flew directly toward Howland Island and disappeared.
One myth is that ancient mariners hugged the coast. Amir Aczel, author of The Riddle of the Compass, points out that “the greatest danger a mariner faces is that of running aground. ”8 Thus, it was critical to master navigation over open waters. In addition, more direct routes are generally quicker. Some methods Aczel discusses are sounding lines for measuring the depth of the sea bottom, knowledge of the shore profile, and knowledge of the winds, current, and the habits of various animals.9 Geographical knowledge was codified over time with maps and sea charts. These gave sailors navigating across great distances interim points to check their accuracy and provided them with information about ports.
GPS Declassified Page 5
