Despite the repeal of witchcraft laws, widespread ignorance, misunderstanding, and superstitions about science and technology persisted into the nineteenth century (some may argue that they extend to the present). In a magazine article first published in Charles Dickens’s Household Words and reprinted in the inaugural 1850 edition of Harper’s New Monthly Magazine, Frederick Knight Hunt noted “a superstition not wholly extinct ” that the Royal Observatory in Greenwich was the “abode of sorcerers and astrologers. ”4 In addition to telescopes the observatory featured a room full of chronometers being monitored while alternately subjected to extreme heat and cold—a test laboratory for the Board of Admiralty. Hunt suggested that the tendency to conflate astronomy with astrology was understandable, given that uneducated people might see little difference between predicting the future through horoscopes and producing almanacs foretelling “to a second when and where each planet may be seen in the heavens at any minute for the next three years. ”5
Capt. James Cook (1728–79) was not only a pathbreaking explorer, remembered for leading three famous expeditions around the world and visiting places no European had gone before. He was also an innovative seaman who introduced sauerkraut as a shipboard staple to prevent scurvy and experimented with distillation equipment to provide fresh water. Cook also was an expert navigator, surveyor, and mapmaker, having spent years mastering the technique of calculating positions using celestial observations and astronomical almanacs. Today we would call him an “early adopter ” of new technologies. On his second voyage, from 1772 to 1775, he embraced a new technology that later came to be called the marine chronometer. Cook called it a watch, or watch machine. The timepiece carried aboard Cook’s ship, the Resolution, was a replica of John Harrison’s masterpiece, H4, the product of years spent designing a clock that would remain accurate at sea despite tossing waves and fluctuating temperatures and humidity. Clockmaker Larcum Kendall built the watch, so it acquired the nickname K1. A companion ship, the Adventure, carried three timepieces of a different design built by John Arnold.
The English Board of Longitude commissioned William Wales to join Cook’s expedition as official minder of the Kendall watch, and Captain Cook’s journals contain numerous references to Wales’s observations comparing longitudinal positions predicted by the watch to celestial sightings and to established longi tudes at inhabited islands. Early in the voyage Cook wrote, “Mr. Kendall’s Watch thus far has been found to answer beyond all expectations. ”6 About halfway through the expedition, in January 1774, Cook commented, “Indeed our error can never be great so long as we have so good a guide as Mr. Kendall’s watch. ”7
A Giant Leap
While Captain Cook easily made the transition to using a mechanical timepiece for determining his longitude instead of measuring the distance between the moon and stars, the chronometer augmented celestial navigation; it did not replace it. Even Cook would have difficulty making the leap to our technology nearly two and a half centuries later. As a thought experiment, imagine transporting Cook through time to the bridge of a modern Coast Guard cutter for an introduction to its navigation equipment. His conversation with the commander might go something like this:
Cook: What a remarkable cabin you have here. What is its purpose? It cannot be where you sleep. I see no bed.
Commander: No, sir. This is the helm. We steer the ship from here.
Cook: You have built walls and a roof round your wheel? Where is the wheel?
I don’t see it.
Commander: We don’t use a wheel anymore. We control the rudder with this. It’s called a joystick.
Cook: I am at a loss to understand. How do the boatswains hear your line commands?
Commander: Well, we make announcements by speaking into this microphone and the sound of our voices comes out of speakers—like horns, you might say—located all over the ship. We also have bells, whistles, and lights, but we have no sails. An engine powers the ship, so the boatswains mates have other duties.
Cook: Engine? You mean the steam invention used for pumping water?
Commander: Yes, a descendant of that. They have advanced considerably. We don’t use coal or steam. Diesel fuel—like lamp oil—powers our engine, which turns a ten-foot propeller.
Cook: Pro-pel-ler?
Commander: It’s like a series of oars that turn in a circle—the same principle as Archimedes’s screw. Instead of lifting water, this screw moves us through it.
Cook: (shaking head and peering out the window) You send a lieutenant on deck for your observations?
Commander: Coast Guard cadets still learn celestial navigation, but the fact is this equipment can provide all the navigation information we need.
Cook: What species of instrument is this? It looks like a small map on fine paper held up to a bright window.
Commander: That is our electronic chart display. It’s a video screen—a picture that changes—connected to a computer—a calculating machine—that stores all of our maps and plots our course using GPS—the Global Positioning System. GPS can tell us our latitude, longitude, bearing and speed.
Cook: Indeed! The work of a sextant, a watch, a compass, and a log-line?
You mean this machine can take observations of the moon and stars, calculate the distance, and search the lunar tables for you?
Commander: Not exactly. The information comes from the sky, but not from natural bodies. It comes from machines called satellites that are very, very high overhead—like tiny stars that orbit the earth, same as the moon.
Cook: Your language is quite new to me, and I am confused. It is midday. No stars are visible.
Commander: Well, these are too small to see, even at night.
Cook: What use are invisible stars?
Commander: We don’t need to see them. The satellites transmit electromagnetic frequencies—um, radio signals that our equipment uses to determine our position. Radio signals are very rapid vibrations that our instruments detect with their antennas, which for them are like our ears, but these are not sounds anyone can hear.
Cook: You steer your ship with sounds you cannot hear from stars you cannot see?
Commander: I guess that is one way to describe it.
As this hypothetical conversation illustrates, understanding any form of technology relies on a base of knowledge about scientific advances that preceded it. The knowledge base—or at least an awareness of capabilities, if not a technical understanding of them—increases with each generation. Soon people take advances for granted. The principles of navigation using latitude and longitude have not changed, but Captain Cook would lack the knowledge base to readily grasp the workings of GPS. Meanwhile many casual users of GPS devices are ignorant about navigation methods, yet they readily adopt GPS technology. They take for granted electricity, radio waves, wireless communication, video screens, atomic clocks, spaceflight, satellites, solar power, microcomputers, and software programming. All of these are essential enabling technologies for GPS.
The Visible Parts
Of course, GPS satellites are visible, but because they orbit at an altitude of about 12,550 miles, seeing one requires a telescope. This distance is sometimes expressed as 20,200 kilometers, using the metric system favored by science and most nations, or 10,900 nautical miles, a traditional navigation unit based on the circumference of the earth. (A one-minute arc of latitude or one-minute arc of longitude at the equator is one nautical mile. This equals 1,852 meters, or about 6,076 feet, making a nautical mile 1.15078 times longer than a statute mile.) Regardless of the unit of measure, this distance amounts to roughly half the circumference of the earth, a comparison Captain Cook would have understood, or a bit more than one-and-a-half times the diameter of the planet, a more useful mental image for modern readers accustomed to viewing photos of the earth from space. Illustrations of the GPS constellation, such as the official image posted on the government’s website (gps.gov; fig. 5.1), usually exaggerate the size of the satellites and trace invisible orbital pa
ths but convey its overall scale.
The constellation contains several generations of GPS satellites with somewhat different designs. Each weighs around two tons, their heights range from about 6 feet to 11 feet, and their wingspans range from 17 feet to 116 feet.8 The solar-panel wings charge backup batteries on board the satellites and power their clocks, signal transmitters, and other circuitry. While the height and weight of successive satellite designs have varied, the wingspan has grown steadily, raising the solar power generated from 800 watts to 2,450 watts.9 Like the wingspan, the life span of the satellites keeps increasing. Engineers designed early satellites to last seven and a half years, and the newer ones have a twelve-year design life. However, many have lasted much longer than expected. Among older satellites designed to last seven and a half years, those remaining “healthy ” at this writing include one launched in 1990, one launched in 1992, and six launched between 1993 and 1997.10 The U.S. Coast Guard Navigation Center maintains a website (www.navcen.uscg.gov) where users can see the status of every GPS satellite. Each satellite has a unique space vehicle number, or SVN, but higher numbers are only generally indicative of age; the satellites were not launched in strict chronological order. Although the original system design called for a minimum of twenty-four satellites (including three spares) to accomplish all of its aims, surplus longevity has resulted in about thirty operational satellites since late 2004.11
Fig. 5.1. GPS constellation, circa 2012. (Courtesy National Coordination Office for Position, Navigation and Timing)
The largest GPS satellite’s silhouette is roughly the size of two semitrailers placed end to end. Anyone who has flown in a jetliner and viewed eighteen-wheelers on an interstate from five to seven miles overhead can appreciate how tiny they would appear from more than twelve thousand miles away. Many modern telescopes integrate GPS receivers into their aiming systems to speed input of the viewer’s time and location, but spotting a GPS satellite, which must be in a position to reflect sunlight and is traveling at about 7,000 mph, remains a challenge.12 Astronomers occasionally photograph GPS satellites and post their images online.13 NASA’S website features a tracking page where viewers can select from a list of satellites, including GPS satellites, to view animated, three-dimensional orbital paths.14
Each GPS satellite orbits the earth twice a day, or about every twelve hours. The earth is rotating at the same time, so the surface area covered by an individual satellite’s signal constantly changes. To ensure that receivers can acquire signals from at least four satellites twenty-four hours a day anywhere on the planet, the satellites are divided among six orbital planes for better coverage. Each orbit is angled about fifty-five degrees from the equator, and the six orbits are spaced evenly around the globe, separated by sixty degrees of longitude where they cross the equator. Small rocket thrusters on each satellite allow ground technicians to maneuver new ones into designated “slots ” along an orbital path, keep them aligned during use, and boost “decommissioned ” satellites into higher orbits—the space-age equivalent of putting them out to pasture. Some of these “residuals ” remain available for reactivation, if needed.15 Technicians also keep each satellite’s eight spiky antennas, called a helix array, properly aimed toward the earth to transmit signals.
As the number of satellites has increased and each generation’s capabilities have improved, the system’s ground control segment also has expanded and evolved. Since 1986, the Air Force’s 2nd Space Operations Squadron (2SOPS) has managed the constellation from the master control station at Schriever (formerly Falcon) Air Force Base in Colorado.16 An alternate master control station at Vandenberg Air Force Base in California is “functionally identical ” and capable of assuming indefinite control of the constellation during any downtime at the master station.17 A half-dozen original monitoring sites scattered around the world have grown to sixteen, and there are a dozen command and control antennas.18 Integrating ten National Geospatial-Intelligence Agency ground stations into the control segment (a project begun in 2005 and completed in 2008) increased accuracy as much as 15 percent by placing each satellite under continuous active monitoring by three ground stations, whereas the satellites previously were unmonitored for portions of their orbits.19 Continuous monitoring also enhanced security, offering faster detection of hostile attempts to disrupt the constellation.20 Through this network of receivers and antennas, 2SOPS technicians monitor each satellite’s health, compile and uplink almanac data containing details about each satellite’s orbit (they wobble and vary slightly), and keep all operational clocks synchronized. Crews at the master control station generate an alert whenever any planned or unplanned outage or operational issue might affect signals. The Coast Guard Navigation Center website posts each alert as a Notice Advisory to Navstar Users (NANU), and users can sign up for automatic alerts. In 2007 the control segment’s mainframe computer system, based on 1970s technology, was replaced with a modern information technology architecture. Upgrades to the operational control system continue.21
The Invisible (and Inaudible) Part
Unlike the Internet, with which users must connect to a server to download or upload information, GPS is a “passive ” system in which users do not interact with the satellites or the ground stations. That means an unlimited number of users can simultaneously share the system by “listening ” to broadcast signals, the same way any number of people can tune in to a radio station at the same time. It may be more accurate to say that a GPS receiver listens to multiple “stations ,” because depending on the receiver’s design it may process signals from many satellites, both GPS and GNSS systems, such as Russia’s GLONASS, Europe’s Galileo, and China’s Beidou, as well as signals from various land- and space-based augmentation systems. However, whereas a sound can be heard from around a corner, GPS signals are low-power “line-of-sight ” communications. They can penetrate clouds, glass, and some thin fabrics or plastic but not most solid objects such as mountains, tree foliage, or buildings. This is why GPS literature so often features references to the number of satellites “in view. ”
Radio signals comprise content—music, speech, data, and so forth—and a carrier wave on which the content rides from the transmission source to the receiver. Radio waves are in the air all the time, but humans cannot hear them because their frequency is above the range of human hearing, like a dog whistle but much higher. Radio frequency is measured in cycles per second, also known as hertz (Hz), named for Heinrich Hertz (1857–94), a German physicist credited with discovering radio waves.22 Kilohertz (kHz) and megahertz (MHz) describe frequencies in the thousands or millions of cycles per second, respectively. Humans generally hear sounds between 20 Hz and 20 kHz.23 Dogs can hear sounds at about twice as high a pitch, around 40 kHz, and bats navigate by listening to echoes of sounds they project at frequencies around 120 kHz.24
The Federal Communications Commission (FCC) reserves a band of the radio spectrum between 88 MHz and 108 MHz for FM stations.25 When a radio station operating at 100 MHz broadcasts a tone, say the musical note A above middle C on a piano, which vibrates at 440 Hz, the transmitter alters, or “modulates ,” the 100 MHz carrier frequency by 440 Hz. The listener’s FM radio antenna receives the 100 MHz ±440 Hz signal, and an electronic filter extracts the 440 Hz tone to amplify through a speaker.
GPS signals ride much higher frequencies in a part of the radio spectrum called the L-band. Each satellite broadcasts signals on two channels, L1, at 1575.42 MHz, and L2, at 1227.6 MHz.26 The content of these two broadcasts is digital codes—sequences of binary digits (zeros and ones) that feed information to GPS receivers. The signal structure utilizes techniques developed since the 1940s to spread transmitted information across a broad frequency bandwidth to gain signal-to-noise improvements.27 Hence the name “spread spectrum. ” Spread-spectrum techniques are why FM radio (based on frequency modulation) is generally clearer than AM radio (based on amplitude modulation). GPS signals are quite weak after their twelve-thousand-mile journey, so spre
ad-spectrum techniques help receivers pick them out of the cacophony of radio signals that fill the airwaves.28 Military engineers utilize spread-spectrum techniques to make radio communications less susceptible to enemy interference (radio jamming is essentially noise) and to scramble signals so that enemy eavesdroppers perceive transmissions as noise.29
GPS uses a type of spread-spectrum code called pseudorandom noise (PRN) to digitize information about each satellite and the navigation data that receivers use to determine location. Each satellite broadcasts three PRN codes: the coarse acquisition (C/A) code, the precision (P) code, and the (Y) code, which replaces the P code whenever the military activates anti-spoofing measures designed to defeat intentionally misleading counterfeit signals.30 When a user turns on a GPS device the receiver attempts to “acquire ” or lock onto signals from as many satellites or other sources as its circuitry is designed to handle. Signals from a satellite directly overhead take about six-hundredths of a second to reach the ground. Receivers first detect the C/A code, which modulates L1 and repeats one thousand times every second.31 This quick repetition helps receivers acquire satellite signals faster. The longer P code repeats on a seven-day cycle, making it harder to acquire. Because civilians use the C/A code and the P and Y codes are reserved for military use, many references speak of the two in combination as the P(Y) code. Most civilian receivers access only L1, while military users access both L1 and L2; accessing both helps to correct for atmospheric degradation. Otherwise, the accuracy of the civilian and military signals as they travel through space is the same.32
Each satellite transmits a unique C/A code. This identifier is different from the satellite’s space vehicle number, so it appears separately, as the satellite’s PRN code, on the Coast Guard Navigation Center’s Constellation Status website. Another spread-spectrum technique, called code division multiple access (CDMA) arranges the digital sequences in a way that allows receivers to differentiate between numerous satellites transmitting similar codes on the same frequency.
GPS Declassified Page 10
