At Schriever Air Force Base in Colorado, Airman First Class Sergio Fernandez monitors  signals between a gps satellite and an antenna on Kwajalein Island in the South Pacific.

When you turn on a GPS receiver, it tunes itself to a radio signal called L1 that comes from any GPS satellite—usually one of four to eight—coasting above the horizon. The American military and other authorized users also receive two encrypted signals, one from L1, another from a frequency called L2. Those extra signals are one of the reasons military users can fix their location more precisely than civilian users can. By measuring the time it takes a signal to reach it, a GPS receiver calculates what is called the pseudo-range to the transmitting satellite. With at least four satellites in view, and hence four pseudo-ranges—the minimum for determining accurate location plus time—a GPS receiver can compute its position using basic trigonometry. It can also calculate velocity by comparing location readings taken at different points in time.

AN INDISPENSABLE SCIENTIFIC TOOL

Scientists have made a wonderland of GPS. The technology is invaluable for researchers who require mainly a record of static position, like archaeologists platting a dig or botanists recording the location of endangered plants. It has also radically transformed the mapping of stationary Earth features, like the array of mineral deposits—remnants of old volcanic eruptions—in Antarctic blue ice. GPS really comes into its own, however, when it is used to measure motion and velocity. The Japanese, for instance, are testing  buoys equipped with sensitive gps  receivers that record the vertical displacement caused by waves and may help provide warning of tsunamis. 


GPS is being used to create a detailed and comprehensive portrait of nearly all Earth’s dynamic surfaces, including the zones where tectonic plates meet, the slippage along earthquake faults, and the movement of glaciers. With GPS, scientists have been able to  gather precise geophysical data  on a wider scale than ever before. Ironically, perhaps the most innovative scientific use of gps  is the effort to decode two kinds of natural error that creep into GPS signals.

One is caused by ionospheric disturbance, and the other, called multipath interference, is caused by the unwanted reflection of GPS signals. Ripplelike variations in the composition of the ionosphere result in a signal distortion called wet delay. Scientists use this distortion to gather information about the electron content of the ionosphere. They also use it as a means of profiling air pressure, temperature, and moisture in the atmosphere. Researchers studying multipath interference have discovered that it provides useful details about oceanic wave height and surface wind speed.

The real value of GPS begins to emerge when you consider a GPS receiver’s ability to compare where it is now with where it was moments or hours or days ago. When you begin to move, a GPS springs to life. It announces your directional bearing, average speed, approximate altitude, the estimated time en route to a named destination, the degree to which you’re adhering to a planned path, and the distance to your destination—in short, it calibrates the dimensions of your dynamism or the dynamism of anything you attach it to, from a delivery truck to an outcropping of the Earth’s crust. A navigator’s task has always been to plot his current position, compare it to his previous day’s position, and deduce from those two points some idea of tomorrow’s position. These are the functions inherent, and almost instantly accessible, in a dynamic tracking system like GPS. It’s no wonder GPS has rapidly made its way into the navigation stations of recreational boats and commercial ships alike, replacing older electronic navigation systems as well as celestial navigation.

But for civilian GPS users, there is a catch. The system is purposely compromised, its accuracy intentionally degraded. GPS was designed, as the responsible federal agencies are careful to remind us, to serve “as a dual-use system with the primary purpose of enhancing the effectiveness of U.S. and allied military forces.” One way to do that is to de-enhance everyone else’s effectiveness—to deny nonmilitary users and foreign adversaries the kind of accuracy that military users enjoy, which in all kinds of targeting weapons is a difference of dozens of feet. This has been done by selectively and intermittently introducing error into the information GPS satellites dispense to receivers lacking access to the military’s encrypted signals—in other words, to the receivers you and I can buy. One of the many ironies of GPS, however, is that a system designed mainly for military use and developed through the Defense Department at a cost of more than $10 billion has been engulfed by the commercial market. The result is that Selective Availability, as GPS’s intentional error is called, will most likely be phased out within the next decade. At the moment, however, the President is committed only to “make an annual determination on continued use of GPS Selective Availability.”

THE WONDERS OF HANDHELD GPS

Even the least expensive GPS receivers come equipped with a remarkable array of features. Information is typically organized screen-by-screen as a series of pages. The first page presents a diagram of the sky overhead, with markers pointing out the gps satellites above the horizon, as well as an indicator that shows the signal strength coming from each satellite.

Once the receiver locks on to the satellite signals, a page appears showing your position in degrees, minutes, and seconds of latitude and longitude. You can specify utm/ups coordinates instead of latitude and longitude, or you can convert land miles to nautical miles or to their metric equivalents. You can even specify whether the magnetic heading refers to true north or magnetic north.

A navigation page complements the position page. Both show approximate altitude, accurate time, ground speed, the bearing of your track, estimated time to a destination, estimated time of arrival, and elapsed trip distance. You can mark any position you come to—a way point—or you can enter way point coordinates in advance from a map. You can call up a list of the nearest way points, a list of all way points, and a list of preprogrammed or recorded routes,  as well as the sunrise and sunset times at that spot.  A map page provides a graphic representation of your track and nearby way points. And if you aimlessly walk into the wilderness with your gps on, it can create a map you can follow back out.

The more positional signals a GPS receives, the more accurate it is. That’s one reason why last January Vice President Gore announced a $400 million initiative that would provide for additional civilian signals on GPS satellites scheduled for launch in the next decade—a clear acknowledgment of the scientific, commercial, and economic importance of nonmilitary GPS. But even at present, there are ways around Selective Availability. Some GPS receivers have been manufactured that can also tune in to the Russian equivalent of GPS, called GLONASS, which operates without signal compromise but lacks the reliability of GPS. The commonest solution is Differential GPS, or DGPS, in which “differential corrections”—indications of the degree of error at one station—are transmitted to GPS receivers via a radio link, greatly enhancing their accuracy regardless of Selective Availability. Even DGPS chips have shrunk to the size of postage stamps.

The U.S. Coast Guard operates a maritime DGPS service available to civilians, and the Federal Aeronautics Administration is implementing a similar system, called Wide Area Augmentation System, which uses satellites as well as ground stations. Once a complementary system called Local Area Augmentation Service is in place at selected airports, the FAA will eventually be able to turn over the task of flight navigation, from takeoff to precision landings, entirely to GPS. The result of this is a bizarre irony, in which some branches of the federal government are working hard to offset error purposely created by another federal agency, the Department of Defense.

Nevada geologist Abel Ramirez checks his location using a GPS receiver and a topographic map (above).

Some receivers can be connected with a laptop for a

digital map display.

GPS, especially Differential GPS, has come as a particular godsend to one group of scientists— geophysicists, the men and women whose profession it is to study the physical and dynamic parameters of planet Earth. Most of us think of the earth as an inherently stable platform: bedrock. But to geophysicists, Earth experiences a wide range of volatility—some of it very slow, some of it occurring at the rate of days or weeks. Tectonic plates grind at each other’s edges, cresting upward, thrusting beneath. The crust is still rebounding from the weight of long-vanished ice sheets. It adjusts locally to the shock of earthquakes and volcanoes. And, as one geophysicist writes, “the torques from the sun, moon, and planets move the rotation axis [of Earth] in space; torques from the atmosphere, ocean, and fluid core move the rotation axis relative to the crust of Earth. Both sources of torques change the rotation rate of Earth.” GPS offers geophysicists an extraordinary leap in the rate of data collection, with a corresponding leap in the understanding of Earth’s motion.

As technology becomes more sophisticated, it seems as though freedom gets defined in more basic terms. GPS offers one version of freedom—knowing where you are. But it may ultimately threaten a more basic kind of freedom—being where you are without anyone else knowing it. Everyone would like to have a Marauder’s Map, but no one wants to appear on the Marauder’s Map without approving it. The value of cell phones embedded with GPS chips is obvious when it comes to emergency services. But the fact that cell-phone service providers are able to track the location of a 911 call means that GPS could track the location of every other kind of call as well. Already GPS is being used to monitor the movement of commercial trucks of every description. This is both a form of insight to the vehicle owners and a form of intrusion to the drivers, who find their movements visible to management in a way they never were before. GPS is also being used in experimental programs to monitor the movements of parolees. There is only a difference of emphasis between tracking a parolee with a GPS and tracking a sales representative with the same tool. GPS assimilates and generates an enormous amount of information, and, as we have learned all too well in the last few decades, even the most innocuous information can be assembled in ways that make it potent and potentially dangerous. Location, movement, and time are not innocuous forms of information.

As technology advances, it abstracts us farther and farther from the earth we live on. We inhabit a world of the senses, a world infinitely full of sensory clues to our location and bearing. Directionality is implicit in our being. The very factors that influence Earth’s rotation—the sun, moon, planets, atmosphere, ocean—influence our sense of orientation, if only we can remember how to know them. In his new book, Passage to Juneau, Jonathan Raban talks about a time before GPS, before sextants, even before compasses. “Sailing with no instruments, the primitive navigator knew his local sea in the same unself-conscious way that a farmer knows his fields. The stars supplied a grand chart of paths across the known ocean, but there was often little need of these since the water itself was as legible as acreage farmed for generations. Color, wind, the flight of birds, and telltale variations of swell gave the sea direction, shape, character.” It is far easier, after all, to navigate by pushing a single button and reading the numbers on yet another of the small gray screens that crowd our lives. GPS may mean many wonderful things, but it may also mean yet another death for the powers of human observation.

And, too, GPS may be a perfect example of technology that reaches the market the moment it becomes unnecessary—at least where ordinary consumers are concerned. Now that the nonaqueous, non-arctic globe is mostly paved, and people are as thick upon the Earth as mold on month-old bread, a device has been invented at last that tells you where you are without having to ask strangers.


For the U.S. Naval Observatory gps specifications and information page, see tycho.usno.navy.mil.

Just about everything you could want to know about gps can be found at the University of Texas gps tutorial, www.utexas.edu/depts/grg/gcraft/notes/gps/gps.html.

To find out which gps satellites are flying overhead at any location, see the Naval Air Warfare Center’s interactive gps satellite prediction utility at sirius.chinalake.navy.mil/satpred.