Technology

On a May 1996 expedition to Mount Everest, geologist Roger Bilham set up a GPS receiver near Base Camp to monitor shifts in the terrain caused by tectonic forces. Most sources list Everest as 29,028 feet above sea lovel. Using GPS equipment, however, a team of scientists from the Massachusetts Institute of Technology and Yale recently concluded that the summit measures slightly less than 29,000 feet.

Most humans who have ever lived have known roughly where they were, day by day, year by year. Not in abstract terms, of course, but in the terms of experience and familiarity—by neighborhood, not map. For eons, we've known things about ourselves that could be expressed in a statement like "I'm standing on the threshing floor in the village of my birth," or "I'm walking across the mid-morning shadow cast by Notre Dame." Or even "I'm in a part of town I've never seen before." Whether we utter it or not, this awareness of "whereness" is part of the meaning of being human. But for centuries, a dedicated band of mapmakers, navigators, astronomers, inventors, and mathematicians has tried to turn this innate sense of place into a more precise determination of position that is intelligible to anyone, not just to locals. On one level, this is like the difference between knowing you're coming to the corner where you always turn left on your way to the grocery store and knowing the names of the streets that cross at that intersection. On another level, however, the pursuit of pure position is about to lead us into a world that not one of us has ever seen. The agent of change will be gps—the Global Positioning System, which, like so many tools of the modern world, is familiar and misunderstood at the same time. 

Until recently, not a single human-made object has ever known where it was. Even a venerable tool of navigation like a sextant knows nothing more about its location than does the Mona Lisa or the pigments of which she is painted. So imagine a world in which man-made objects know where they are and can communicate that information to other self-locating, communicating objects too. This sounds as strange and surprising as the Marauder’s Map in the Harry Potter novels for children. The Marauder’s Map shows the position and movement of every animate creature at the school of wizardry called Hogwarts. A Marauder’s Map of the world would be even stranger. It would show the position and movement—a history of movements, too, if needed—of man-made objects as well. This would be an ever-changing map of a world filled with artifacts busily announcing something significant about themselves to each other and to anyone else who cared to listen.

That world is nearly here. In August, a company called SiRF Technology, based in Santa Clara, California, announced that it had developed an advanced GPS chip no bigger than a postage stamp. Kanwar Chadha, one of SiRF’s founders, declared, “Our vision is to bring location awareness to virtually everything that moves.” This is a subtle but profound change in the history of GPS technology—a change driven, like everything else these days, by increasing miniaturization and declining prices for sophisticated circuitry. In the past few years, consumers have grown used to the sight of handheld GPS receivers, which have been marketed as individual positioning devices for anglers, hunters, hikers, and cyclists—tools, in other words, for establishing one’s individual bodily location. But what SiRF and other companies like it have in mind is conferring upon objects a communicable sense of place. One day soon, the vast majority of GPS devices will not be stand-alone receivers used by those of us who venture off the beaten path but integral components of everyday objects.

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Some of these objects, especially the big ones, are easy enough to imagine, because they exist now. Boats and ships of every kind already incorporate GPS technology, as do some automobiles made by Toyota, Honda, Lexus, and Cadillac. So do the newest farm implements, like combines that allow farmers to map crop yields in precise detail. But some uses of GPS that are not yet widely available will soon be common in smaller devices. Beginning next October, for instance, the Federal Communications Commission will require cellular-phone service providers to be able to identify the location of a cell-phone caller who dials 911. This means that most cell phones will likely include a tiny GPS chip. So will beepers and watches and handheld digital assistants and, who knows, Game Boy Colors and Tamagotchis and dog collars and probably handguns too.

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HOW GPS WORKS

PS operates on the geometric principle of triangulation: calculating location by measuring the distance to other known points. Measuring the distance from a gps receiver to one satellite places the receiver’s location somewhere on the surface of an imaginary sphere that is centered on the satellite and has a radius equal to that distance. Gauging the distance to a second satellite narrows the location to a circle where two spheres intersect. Factoring in the distance to a third satellite reduces the possibilities to two points, one of which is likely to be too far from Earth to be a logical location for the receiver. 

 A measurement from a fourth satellite helps compensate for potential errors. A gps receiver determines the distance from a satellite by measuring how long it takes a radio signal to reach it. But the satellites are equipped with clocks that are much more precise than the clock in a gps receiver. A fourth satellite provides three more redundant triangulation measurements that are averaged out to adjust for any deviations between the receiver clock and Universal time.

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The spreading of a technology like GPS is easy enough to predict, but it’s much harder to foresee what the effect of that spreading will be. The future always takes a shape no one quite anticipates. Technologies dovetail unexpectedly, strange synergies suddenly prevail, and soon the extraordinary seems almost commonplace. But there’s always a limit to how far we can see into the future of the tools we use, especially into a future where those tools become interlinked. There was a time—only as long ago as Bill Gates’s first book—when the value of computers, whatever their size, was believed to lie mainly in their stand-alone power, not in the networks they might form when linked together. Now we have the Internet and the World Wide Web, whose far-reaching implications are only dimly visible but which have already transformed the way countries all over the world do business.

The development of GPS technologies may follow a similar pattern. It’s already obvious how useful GPS is in discrete applications, for surveying and mapmaking, for tracking commercial vehicles, for maritime and aeronautic navigation, for emergency rescue crews and archaeologists. But there is simply no telling what it will mean when, on a planet full of location-aware objects, a way is found to coordinate all the data they send out. Awareness may be a metaphor when applied to inanimate objects, but the potential of that metaphor is entirely literal and, so far, almost entirely beyond our ken.

In the meantime, for most of us, there is still a more basic question to be answered: Where did GPS technology come from and how does it work?

GPS depends on an array of 27 satellites—24 in regular use, plus spares—flying some 12,000 miles above Earth. They were put there by the Department of Defense, which began the NAVSTAR geographical positioning system program in 1973. A version of GPS was first tested in 1964 when the Navy deployed a five-satellite prototype, called Transit, for submarines. It could take an hour and a half for a Transit satellite to saunter above the horizon and then another 10 or 15 minutes to fix the submarine’s position. The current generation of satellites was built by Rockwell International and Lockheed Martin, and each one orbits the planet in about 12 hours, cutting across the equatorial plane at an angle of roughly 55 degrees. The U.S. Air Force tracks the satellites from Hawaii, Colorado Springs, and the various islands of Ascension in the South Atlantic, Diego Garcia in the Indian Ocean, and Kwajalein in the Pacific. These ground stations provide the satellites with navigational information, which the next generation of satellites will be able to supply to each other. Ordinary users can track this constellation of satellites with one of several Web sites or with an appealing public-domain software program called Home Planet, which can map any satellite you choose, GPS or not, against a projection of the Earth’s surface. Or you can track the satellites with a GPS receiver.

GPS uses 24 satellites in six orbital planes. From any point on Earth, as many as eight satellites are above the horizon at one time.

In the world of GPS, knowing where you are, give or take a few meters, depends on knowing precisely when you are. Just as longitude couldn’t be effectively calculated until 1764, when John Harrison’s chronometer was tested on a voyage to Barbados, so the geographical positioning system couldn’t be created until there was a way to mount highly accurate clocks in stable orbits. The problem with finding longitude in Harrison’s era was making a chronometer that could keep accurate time at one location—Greenwich, England—even while the ship carrying that chronometer was halfway around the globe. The chronometer provides a constant frame of reference for the celestial events that shift as a ship moves eastward or westward.

GPS satellites effortlessly provide a constant frame of reference. Each carries four ultra-precise clocks synchronized to GPS time—which is, essentially, Coordinated Universal Time (UTC) without the leap seconds. The satellite clocks are accurate to within one- millionth of a second of UTC—a very thin sliver of difference—as kept by the U.S. Naval Observatory. The GPS receiver translates the time that the satellites transmit into local time. In fact, as far as most civilian users are concerned, GPS is more accurate for time than it is for position. (And, in most cases, GPS is far more accurate for position than it is for altitude.) In 1764, Greenwich time was available only in Greenwich—on the meridian running through it, if you knew where that was—and in the presence of a properly maintained chronometer, of which there were two. Now, GPS time is available globally to anyone with a receiver.