GPS Declassified

by Richard D. Easton

  Navigational Techniques

  Navigational techniques can be classified as celestial, mechanical, map related, and geographical. Celestial devices, as mentioned previously, use the sun, moon, planets, or stars. They are most commonly used to estimate latitude. The sun and the stars can also be used to locate east and west directions. Since astronomical objects that are not over the poles move from east to west, they also reveal the direction.

  Mechanical devices were developed to improve the accuracy of these measurements. The astrolabe dates back to about 225 BC. Astrolabes were flat metal discs, typically about six to eight inches in diameter, made of brass, with markings representing astronomical features on a moveable faceplate. By rotating the faceplate and a watch-like hand to align with degree markings around the outer ring, users could determine the time of day or night and estimate sunrise, sunset, and the positions of stars.10 Over time, more accurate devices were developed, such as the cross-staff, a simple T-shaped instrument with a moveable crossbar, used to measure the angle of the sun or stars above the horizon, and the octant, a triangular-shaped device with sighting mirrors at the apex and a curved bottom representing an eighth of a circle, or forty-five degrees, for determining latitude.11 Royal Navy captain John Campbell invented the sextant in 1757. It improved upon the octant by increasing the curved scale to one-sixth of a circle, or 60 degrees, enabling measurements of larger angles up to 120 degrees.12 A sky chart shows the relative latitude of the sun over the earth for each day of the year and is useful for determining latitude, if the sun can be observed at noon. The compass appears to have been invented in China around 800 AD. It allowed ships to sail year-round in the Mediterranean and greatly increased trade.

  Lighthouses are useful for sea navigation. However, they are subject to dangerous misinterpretations, as occurred with the SS Atlantic. On March 31, 1873, the ship was headed with 814 passengers and 143 officers and crew toward Halifax, Nova Scotia, to replenish its coal. Lost in the dark, it hit a rock early the next morning. The officers and crew rushed to the deck and were able to free ten lifeboats, but the currents washed the boats away. Twenty people were killed on the deck when the bow on the foremast came loose. Ropes were brought ashore and some people managed to clamber across them to safety. Many married men refused to leave their wives behind and died with them. Five hundred forty-five people died in this catastrophe, which may have been caused by the crew confusing Sambro Light for Devil’s Light, which was farther to the west. None of the 138 women on board the Atlantic survived; undoubtedly, they were handicapped by their heavy clothing. This included one woman disguised as a man who served as a crewmember.13 It was the worst loss of life in a marine disaster prior to the Titanic.

  Lunars, Jovian Moons, and Clocks

  As mentioned earlier, the Board of Longitude was charged with finding a reliable way of estimating longitude. The largest award was £20,000 for a method that could determine longitude to an accuracy of half a degree of a great circle. The astronomical approach used celestial objects such as the Jovian moons or the lunar distances method, also called lunars. Galileo discovered four moons of Jupiter when he viewed the planet with a telescope in 1610. Their orbits range from 1.7 days for Io to 16.7 days for Callisto. Astronomers can predict their orbits and when they will pass behind Jupiter (be eclipsed by Jupiter). With four moons, there are many such eclipses. Observatories established tables showing the time of the eclipses at a well-known longitude such as London. Comparing the time on the published table with the time established by the observer’s local noon gives the difference and the longitude compared to London. Measuring the local time of Jovian eclipses worked well on land; however, the eclipses could be seen only at night when the sky was clear and Jupiter was in sight. There are times in the year when Jupiter is on the opposite side of the sun from the earth.

  Christopher Huygens, a leading seventeenth-century astronomer and mathematician, devised the first pendulum clock in 1656. John Harrison, as memorably recounted by Dava Sobel in Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, built on the work of Huygens and others in devising methods to offset the effects of heat and humidity on clocks. Harrison built increasingly sophisticated clocks in his attempt to win the £20,000 prize. His first clock, H1, was taken on a sea trial in 1736. It performed well, but the Board of Longitude insisted that a transatlantic trial was needed. His last clock, H4, a compact, five-inch-diameter sea watch, was taken by his son on such a trip in 1761.

  The lunar method requires an accurate measurement of the moon’s angle in relation to specific stars. Lunar tables were completed by 1767, the same decade in which Harrison’s H4 received two official sea trials. Both methods were used by navigators between 1770 and 1850. Royal Navy captain James Cook took a replica of Harrison’s H4 on his voyages of discovery and called it “our faithful guide through all the vicissitudes of climates. ”

  The competition between the astronomical and the lunar approach ultimately came down to the price of the marine chronometer, since finding longitude with a chronometer was a faster and easier process and could be done every day provided you had clear weather. Frank Reed, who leads weekend celestial navigation demonstrations at Mystic Seaport, in Connecticut, spoke about lunars during the 2012 conference “After Longitude—Modern Navigation in Context ” at the National Maritime Museum in Greenwich, England. He commented that the best time for “shooting ” lunars is when the moon is half-full (first quarter or third quarter). It is done in daytime.

  Inertial navigation, sometimes called dead reckoning, estimates position based on speed and direction. Thus a mariner on the equator sailing due west two hundred hours at five miles per hour ends up one thousand miles, or fifteen degrees, farther west since the circumference of the earth is about twenty-four thousand miles at the equator. During the first half of the nineteenth century, inertial navigation estimates could be adjusted for currents to make them accurate for about two weeks at a time. This worked well with lunar updates (adjusting for slight variations in the sun’s path) every other week. Thus the longitude estimate from inertial navigation was updated by using the lunar method.

  The chronometer, then costing about fifty pounds, was considered too expensive by American mariners. For example, the Brig Reaper voyaged in 1808 from America to Calcutta to buy coffee using inertial navigation and lunars. By 1850, chronometers became so inexpensive that the use of lunars disappeared.

  The discovery of radio waves in the second half of the nineteenth century quickly became important to navigation. Scottish physicist James Clerk Maxwell mathematically predicted radio waves in 1864, and Heinrich Hertz, a physics professor at Karlsruhe Polytechnic in Germany, proved their existence by engineering instruments to transmit and receive them in 1887. Italian inventor Guglielmo Marconi and Serbian-American inventor Nikola Tesla both devised methods of wireless communication using radio waves. The British tracked the German fleet in the 1916 Battle of Jutland using radio direction-finding techniques. Radio engineers also recognized the feasibility of using radar for navigating ships or airplanes. Radar navigation is the reverse of tracking objects; the same reversal occurred in moving from tracking satellites to using them for navigation (see chapter 3).

  The speed of airplanes accentuated navigational challenges, especially for solo pilots. There was no time for twenty-minute lunar calculations, and if there had been, the plane’s speed prevented calculating a current position. U.S. Navy captain Philip Van Horne Weems pioneered methods for faster airborne navigation using mechanical devices. He invented a special watch for accurately determining Greenwich mean time, improved the sextant, and established a company, Weems & Plath, to market his navigational aids.14 Along the way, he befriended prominent pilots such as Charles Lindbergh and the more controversial “Wrong Way ” Corrigan, who claimed that he flew from New York to Ireland instead of Los Angeles due to misreading his compass. N. W. Emmot, in “The Grand Old Man of Navigation ,” an article b
ased on an interview with Weems, notes that invoice records show that the information and charts Weems sold Corrigan were all about the North Atlantic. Another Navy aviator, Capt. Charles Blair, planned to fly in 1951 across the North Pole in a modified P-51 Mustang. Weems plotted in advance the sun’s altitude for points along the entire flight, allowing Blair to compare his sightings to a graph without performing computations while flying.15 In the 1960s, early position calculations using the space navigation approach utilized in GPS required extensive manual work. Today, miniature computer processors perform such calculations instantaneously on a smartphone or a dedicated GPS receiver. “The current almost universal utilization of GPS owes as much to advances in microprocessors as to progress in the central system itself ,” observed Dr. Alexander H. Flax, who served as chief scientist for the Air Force from 1959 to 1961 and as director of the National Reconnaissance Office from 1965 to 1969.16

  Engineers at the Lorentz Company in Germany fielded the first electronic guidance system for low-visibility aircraft landings.17 In 1940 the Germans sent bombers over Britain at night using two radio waves sent from the same station 180 degrees out of phase (peaks and valleys canceling each other out). When the bombers received no signal, it meant they were in the area where the waves overlapped and were headed toward the target city.18 They also used a director beam, which the bomber would follow and then drop its bomb when it hit a cross beam.

  Loran, for long-range navigation, was a system that developed out of the British Gee system, a land-based pulse radar that transmitted signals from towers in different positions. For a given time differential between the receipt of the signals, the receiver’s position could be specified on a hyperbola. Thus, a receiver requires signals from another pair of stations in order to obtain a position fix. The system was improved when Loran-A was replaced by Loran-C, which operates at a lower frequency and is able to transmit signals over the horizon.

  The next chapter covers the different space-based navigational system proposals in which satellites replaced land-based towers. After World War II, rocket pioneers in the United States and the Soviet Union built on the achievements of Wernher von Braun’s German team, which developed the V-2 rocket. As noted in chapter 1, von Braun led the Army program that launched the first U.S. satellite. Space visionaries saw the potential for satellites to provide worldwide communications and navigation guidance unhindered by the “vicissitudes ” of weather that have affected other techniques through the ages. Many people made navigation proposals; ultimately the Global Positioning System was formulated and revolutionized navigation. The potential problem today is over-dependence on GPS, not finding uses for it.

  3

  Success Has Many Fathers Early Concepts for Satellite Navigation

  Nothing in progression can rest on its original plan. We may as well think of rocking a grown man in the cradle of an infant.

  Edmund Burke, letter to the Sheriffs of Bristol, 1777

  Controversies over priority in scientific ideas are common. There was debate in the seventeenth and eighteenth centuries over the role of Sir Isaac Newton versus Gottfried Wilhelm Leibnitz in inventing calculus. Newton invented it first, but his aversion to publishing resulted in Leibnitz inventing it independently later and Leibnitz’s notations proved to be more useful in practice. It became a partisan issue, with the English asserting Newton’s priority and Germans asserting Leibnitz’s. There was a controversy in the nineteenth century about the roles of British astronomer John Couch Adams and French astronomer Urbain Le Verrier in discovering Neptune. There was also the controversy in the latter part of the nineteenth century over who invented the telephone, especially between Elisha Gray and Alexander G. Bell. There were suggestions in the twentieth century that Rosalind Franklin’s role in discovering DNA’S structure, for which James Watson and Francis Crick won a Nobel Prize, was overlooked because she was a woman. Note, though, that her early death made her ineligible for the Nobel Prize, which is given only to living people. Similarly, a vigorous debate persists over the roles various people played in the origin of GPS.

  Arthur C. Clarke, the author of 2001: A Space Odyssey and a space visionary, wrote in 1945 about communications satellites in geosynchronous orbits. He anticipated a GPS-type system in a 1956 letter:

  My general conclusions are that perhaps in 30 years the orbital relay system may take over all the functions of existing surface networks and provide others quite impossible today. For example, the three stations in the 24-hour orbit could provide not only an interference and censorship-free global TV service for the same power as a single modern transmitter, but could also make possible a position-finding grid whereby anyone on earth could locate himself by means of a couple of dials on an instrument about the size of a watch. (A development of Decca and transistorisation.) It might even make possible world-wide person- to-person radio with automatic dialling. Thus no-one on the planet need ever get lost or become out of touch with the community, unless he wanted to be.1

  Early Satellite Navigation Proposals

  The following list shows eight American pre-GPS satellite navigation proposals.

  U.S. Pre-GPS Space-Based Navigation Proposals

  Angle

  Edward Everett Hale, “The Brick Moon ,” 1870

  Doppler

  Lovell Lawrence Jr., “Navigation by Satellites ,” Missiles and Rockets, 1956

  Transit, George Weiffenbach and William Guier, Applied Physics Laboratory, 1958

  Range Measurement

  Don Williams, Hughes Aircraft, 1959

  SECOR (sequential correlation of range), Army, 1961

  Roy Anderson, GE/NASA, 1963

  Timation (time navigation), Roger Easton, Naval Research Laboratory, 1964 621B, Air Force/Aerospace Corporation, 1964

  The list begins with one far predating the space age. Edward Everett Hale, more famous for his story “A Man without a Country ,” published “The Brick Moon ” serially in the Atlantic Monthly in 1869. Hale proposed using satellites as an aid for measuring longitude, with one satellite over the Greenwich meridian and another one passing over New Orleans, both at altitudes of about four thousand miles.

  The second and third systems used Doppler measurements. A year before the Soviets launched Sputnik, Lovell Lawrence Jr., an early rocket scientist who directed the U.S. Army’s Redstone project, published “Navigation by Satellites ” in the initial issue of Missiles and Rockets. He discussed placing satellites in geosynchronous orbits, but that was beyond existing technological capabilities. Geosynchronous orbits require altitudes high enough that satellites orbit the earth once each day, remaining roughly over one point on the earth. Such orbits require much effort to attain and maintain. A low orbit, six hundred miles in altitude, was more feasible at the time.

  Sputnik’s launch on October 4, 1957, led many people to track it. This was important for predicting Sputnik’s orbit and studying how it decayed over time. Three individuals or groups who tracked Sputnik later proposed satellite-based navigational systems. This was no coincidence and warrants more attention than it often receives. Each recognized that the methods used for tracking could be transformed or inverted for navigation. Space tracking uses one or more ground stations to measure a satellite’s orbit. Satellite navigation systems use one or more satellites to estimate a receiver’s position.

  As noted in chapter 1, three days after the Sputnik launch, William Guier and George Weiffenbach at the Applied Physics Laboratory at Johns Hopkins University listened to the satellite’s twenty-megahertz signal and noted its pitch change over time due to the Doppler shift. They developed a space-based navigation system the following March using this shift. The first satellite launch, Transit 1A, in 1959, was unsuccessful. Transit 1B was successfully launched in 1960. The system used low-altitude satellites, at altitudes of about six hundred miles, and it entered service in 1964. It provided receivers two-dimensional position fixes periodically throughout the day. It was a useful system, especially for Polaris missile
submarines, but over time it became clear that aircraft needed a three-dimensional, continuously available system, which would require a new approach.

  Roy Anderson, a consulting engineer at General Electric’s office in Schenectady, New York, followed Sputnik’s track using radio direction-finder equipment set up in a camping tent. NORAD (North American Aerospace Defense Command) subsequently asked him to track each new satellite for forty-eight hours after launch.2 He recalled the story of tracking Pioneer 4:

  Three tracking stations demonstrated the ability to track Pioneer 4 to the great distance: Jodrell Bank in England with its 150-foot diameter antenna, the Jet Propulsion Laboratory (JPL) at Goldstone Lake, California with its 85-foot antenna, and a temporary setup at the GE Research Laboratory, Schenectady, with an 18-foot diameter parabolic antenna. … There was immense media interest in our effort. Pioneer 4 was the first object to escape Earth’s gravity. We were besieged with phone calls at all hours of the day and night. With our small antenna we were seen as David against Goliath. On the morning of 6 March, the signal was weak and intermittent. Finally, search as we could, we could no longer get a lock on it. In mid-morning, a reporter called and said, “JPL announced that they lost a signal. Do you still have it? ” “No. ” “When did you lose it? ” “I don’t know exactly? ” “Can we say that at 10:25 you said that you lost the signal? ” It was 10:25. “Yeah. ” By 10:27 the whole world was informed that GE had tracked the space probe farther than JPL. I went to a newspaper office and asked them to publish a disclaimer. They were not interested. JPL was not pleased.3