Basic Concepts (GPS and GIS) Part 1

IN WHICH you are introduced to facts and concepts relating to the NAVSTAR Global Positioning System and have your first experience using a GPS receiver.


A sports club in Seattle decided to mount a hunting expedition. They employed a guide who came well recommended, and whose own views of his abilities were greater still. Unfortunately, after two days, the group was completely, totally lost. ‘You told me you were the best guide in the State of Washington," fumed the person responsible for hiring the guide. "I am, I am" claimed the man defensively. "But just now I think we’re in Canada."

Stories like the one above should be told now (if at all), before they cease to be plausible. Actually, even at present, given the right equipment and a map of the general area, you could be led blindfolded to any spot in the great out-of-doors and determine exactly where you were. This happy capability is due to some ingenious electronics and a dozen billion dollars1 spent by the U.S. government. I refer to NAVSTAR (Navigation System with Time And Ranging; informally the "Navigation Star")—a constellation of from 24 to 32 satellites orbiting the Earth, broadcasting data that allows users on or near the Earth to determine their spatial positions. The more general term in the United States for such an entity is "Global Positioning System" or "GPS." The Russians have such a navigation system as well, which they call GLONASS (Global Navigation Satellite System). (One might reflect that, for some purposes, the cold war lasted just long enough.) A more  general, recent acronym for such systems is GNSS, standing for Global Navigation Satellite Systems. In the western world, GPS usually implies NAVSTAR, so I will use the two designations interchangeably in this text.

Where Are You?

Geography, and Geographic Information Systems (GISs) particularly, depend on the concept of location. Working with "location" seems to imply that we must organize and index space. How do we do that?

Formally, we usually delineate geographical space in two dimensions on the Earth’s surface with the latitude-longitude graticule, or with some other system based on that graticule.

But informally, and in the vast majority of instances, we organize space in terms of the features in that space. We find a given feature or area based on our knowledge of other features— whether we are driving to Vancouver or walking to the refrigerator. Even planes and ships using radio navigational devices determine their positions relative to the locations of fixed antennae (though some of the radio signals may be converted to graticule coordinates).

Unlike keeping track of time, which was initially computed relative to a single, space-based object (the sun), humans kept track of space—found their way on the ground—by observing what was around them.

Another, somewhat parallel way of looking at this issue is in terms of absolute versus relative coordinates. If I tell you that Lexington, Kentucky is at 38 degrees (38°) north latitude, 84.5° west longitude, I am providing you with absolute coordinates. If I say, rather, that Lexington is 75 miles south of Cincinnati, Ohio and 70 miles east of Louisville, Kentucky, I have given you relative coordinates.

Relative coordinates usually appeal more to our intuitive comprehension of "location" than do absolute coordinates; however, relative coordinates can be quite precise.

To pass spatial information around, humans developed maps to depict mountains and roads, cities and plains, radio stations and sinkholes. Maps aid both the formal and informal approaches that humans use to find objects and paths. Some maps have formal coordinates, but maps without graticule markings are common. All maps appeal to our intuitive sense of spatial relationships. The cartographer usually relies on our ability to use the "cognitive coordinates" in our memory, and our abilities to analyze, to extrapolate, and to "pattern match" the features on the map. It is good that this method works, since, unlike some amazing bird and butterfly species, humans have no demonstrated sense of an absolute coordinate system. But with maps, and another technological innovation, the magnetic compass, we have made considerable progress in locating ourselves.

I do not want to imply that absolute coordinates have not played a significant part in our position-finding activities. They have, particularly in navigation. At sea, or flying over unlit bodies of land at night, captains and pilots used methods that provided absolute coordinates. One’s position, within a few miles, can be found by "shooting the stars" for a short time with devices such as sextants or octants. So the GPS concept—finding an earthly position from bodies in space—is not an entirely new idea. But the ability to do so during the day, almost regardless of weather, with high accuracy and almost instantaneously, makes a major qualitative difference. As a parallel, consider that a human can move by foot or by jet plane. They are both methods of locomotion, but there the similarity ends.

GPS, then, gives people an easy method for both assigning and using absolute coordinates. Now, humans can know their positions (i.e., the coordinates that specify where they are); combined with map and/or GIS data they can know their locations (i.e., where they are with respect to objects around them). I hope that, by the time you’ve completed this text and experimented with a GPS receiver, you will agree that NAVSTAR constitutes an astounding leap forward.


While this is a text on how to use GPS in GIS—and hence is primarily concerned with positional issues, it would not be complete without mentioning what may, for the average person, be the most important facet of GPS: providing Earth with a universal, exceedingly accurate time source. Allowing any person or piece of equipment to know the exact time has tremendous implications for things we depend on every day (like getting information across the Internet, like synchronizing the electric power grid and the telephone network). Further, human knowledge is enhanced by research projects that depend on knowing the exact time in different parts of the world. For example, it is now possible to track seismic waves created by earthquakes, from one side of the earth, through its center, to the other side, since the exact time2 may be known worldwide.3


The subject of this topic is the use of GPS as a method of collecting locational data for Geographic Information Systems (GIS). The appropriateness of this seems obvious, but let’s explore some of the main reasons for making GPS a primary source of data for GIS:

• Availability: In 1995, the U.S. Department of Defense (DoD) declared NAVSTAR to have "final operational capability." Deciphered, this means that the DoD has committed itself to maintaining NAVSTAR’s capability for civilians at a level specified by law, for the foreseeable future, at least in times of peace. Therefore, those with GPS receivers may locate their positions anywhere on the Earth.

• Accuracy: GPS allows the user to know position information with remarkable accuracy. A receiver operating by itself, can let you locate yourself within 10 to 20 meters of the true position. (And later you will learn how to get accuracies of 2 to 5 meters.) At least two factors promote such accuracy:

First, with GPS, we work with primary data sources. Consider one alternative to using GPS to generate spatial data: the digitizer. A digitizer is essentially an electronic drawing table, wherewith an operator traces lines or enters points by "pointing"—with "crosshairs" embedded in a clear plastic "puck"—at features on a map.

One could consider that the ground-based portion of a GPS system and a digitizer are analogous: the Earth’s surface is the digitizing table, and the GPS receiver antenna plays the part of the cross-hairs, tracing along, for example, a road. But data generation with GPS takes place by recording the position on the most fundamental entity available: the Earth itself, rather than a map or photograph of a part of the Earth that was derived through a process involving perhaps several transformations.

Secondly, GPS itself has high inherent accuracy. The precision of a digitizer may be 0.1 millimeters (mm). On a map of scale 1:24,000, this translates into 2.4 meters (m) on the ground. A distance of 2.4 m is comparable to the accuracy one might expect of the properly corrected data from a medium-quality GPS receiver. It would be hard to get this out of the digitizing process. A secondary road on our map might be represented by a line five times as wide as the precision of the digitizer (0.5 mm wide), giving a distance on the ground of 12 m, or about 40 feet.

On larger-scale maps, of course, the precision one might obtain from a digitizer can exceed that obtained from the sort of GPS receiver commonly used to put data into a GIS. On a "200 scale map" (where one inch is equivalent to 200 feet on the ground) 0.1 mm would imply a distance of approximately a quarter of a meter, or less than a foot. While this distance is well within the range of GPS capability, the equipment to obtain such accuracy is expensive and is usually used for surveying, rather than for general GIS spatial analysis and mapmaking activities. In summary, if you are willing to pay for it, at the extremes of accuracy, GPS wins over all other methods. Surveyors know that GPS can provide horizontal, real-world accuracies of less than one centimeter.

• Ease of use: Anyone who can read coordinates and find the corresponding position on a map can use a GPS receiver. A single position so derived is usually accurate to within 10 meters or so. Those who want to collect data accurate enough for a GIS must involve themselves in more complex procedures, but the task is no more difficult than many GIS operations.

• GPS data are inherently three-dimensional: In addition to providing latitude-longitude (or other "horizontal" information), a GPS receiver may also provide altitude information. In fact, unless it does provide altitude information itself, it must be told its altitude in order to know where it is in a horizontal plane. The accuracy of the third dimension of GPS data is not as great, usually, as the horizontal accuracies. As a rule of thumb, variances in the horizontal accuracy should be multiplied by 1.5 (and perhaps as much as 3.0) to get an estimate of the vertical accuracy.


Global: anywhere on Earth. Well, almost anywhere, but not (or not as well):

• inside buildings

• underground

• in very severe precipitation

• under heavy tree canopy

• around strong radio transmissions

• in "urban canyons" amongst tall buildings

• near powerful radio transmitter antennas or anywhere else not having a direct view of a substantial portion of the sky. The radio waves that GPS satellites transmit have very short lengths—about 20 cm. A wave of this length is good for measuring because it follows a very straight path, unlike its longer cousins such as AM and FM band radio waves that may bend considerably. Unfortunately, short waves also do not penetrate matter very well, so the transmitter and the receiver must not have much solid matter between them, or the waves are blocked, as light waves are easily blocked.

Positioning: answering brand-new and age-old human questions. Where are you? How fast are you moving and in what direction? In what direction should you go to get to some other specific location, and how long would it take at your speed to get there? And, most importantly for GIS, where have you been?

System: a collection of components with connections (links) among them. Components and links have characteristics. GPS might be divided up in the following way:4

The Earth

The first major component of GPS is Earth itself: its mass and its surface, and the space immediately above. The mass of the Earth holds the satellites in orbit. From the point of view of physics, each satellite is trying to fly by the Earth at four kilometers per second. The Earth’s gravity pulls on the satellite vertically so it falls. The trajectory of its fall is a track that is parallel to the curve of the Earth’s surface.

The surface of the Earth is studded with little "monuments"— carefully positioned metal or stone markers—whose coordinates are known quite accurately. These lie in the "numerical graticule" which we all agree forms the basis for geographic position. Measurements in the units of the graticule, and based on the positions of the monuments, allow us to determine the position of any object we choose on the surface of the Earth.

Earth-Circling Satellites

The United States GPS design calls for a total of at least 24 and up to 32 solar-powered radio transmitters, forming a constellation such that several are "visible" from any point on Earth at any given time. The first one was launched on February 22, 1978. In mid-1994 all 24 were broadcasting. The minimum "constellation" of 24 includes three "spares." As many as 28 have been up and working at one time.

The newest GPS satellites (designated as Block IIR) are at a "middle altitude" of about 11,000 nautical miles (nm), or roughly 20,400 kilometers (km) or 12,700 statute miles above the Earth’s surface. This puts them above the standard orbital height of the space shuttle, most other satellites, and the enormous amount of space junk that has accumulated. They are also well above Earth’s air, where they are safe from the effects of atmospheric drag. When GPS satellites "die" they are sent to orbits about 600 miles further out.

GPS satellites are below the geostationary satellites, usually used for communications and sending TV, telephone, and other signals back to Earth-based fixed antennas. These satellites are 35, 763 km (or 19,299 nm or 22,223 sm) above the Earth, where they hang over the equator relaying signals from and to ground-based stations.

The NAVSTAR satellites are neither polar nor equatorial, but slice the Earth’s latitudes at about 55°, executing a single revolution every 12 hours. Further, although each satellite is in a 12 hour orbit, an observer on Earth will see it rise and set about 4 minutes earlier each day.5 There are four or five satellites in slots in each of six distinct orbital planes (labeled A, B, C, D, E, and F) set 60 degrees apart. The orbits are almost exactly circular.

The combination of the Earth’s rotational speed and the satellites’ orbits produces a wide variety of tracks across the Earth’s surface. Figure 1—1 is a view of the tracks which occurred during the first two hours after noon on St. Patrick’s Day, 1996. You are looking down on the Earth, directly at the equator and at a (north-south) meridian that passes through Lexington, Kentucky. As you can see, by Figure 1—1, the tracks near the equator tend to be almost north-south. The number of each satellite is shown near its track; the number marks the point where the satellite is at the end of the two-hour period.

GPS satellites move at a speed of 3.87 km/sec (8,653 miles per hour). The Block IIR satellites weigh about 1077 kilograms (somewhat more than a ton) and have a length of about 11.6 meters (about 38 feet) with the solar panels extended. Those panels generate about 1100 watts of power. The radio on board broadcasts with about 40 watts of power. (Compare that with your "clear channel" FM station with 50,000 watts.) The radio frequency used for the civilian GPS signal is called "GPS L1" and is at 1575. 42 mega Hertz (MHz).

GPS satellite tracks looking from space toward the Equator.

Figure 1-1. GPS satellite tracks looking from space toward the Equator. 

Each satellite has on board four atomic clocks (either cesium or rubidium) that keep time to within a billionth of a second or so, allowing users on the ground to determine the current time to within about 40 billionths of a second. Each satellite is worth about $65 million and has a design life of 10 years. Figure 1—2 shows an image of a NAVSTAR satellite.

Ground-Based Stations

While the GPS satellites are free from drag by the atmosphere, their tracks are influenced by the gravitational effects of the moon and sun, and by the solar wind. Further, they are crammed with electronics. Thus, both their tracks and their innards require monitoring. This is accomplished by four ground-based stations near the equator, located on Ascension Island in the South Atlantic, at Diego Garcia in the Indian Ocean, and on Kwajalein Atoll, and in Hawaii, both in the Pacific, plus the master control station (MCS) at Schriever (formerly Falcon) Air Force Base near Colorado Springs, Colorado. A sixth station is planned to begin operation at Cape Canaveral, Florida. Each satellite passes over at least one monitoring station twice a day. Information developed by the monitoring station is transmitted back to the satellite, which in turn rebroadcasts it to GPS receivers.

A NAVSTAR GPS satellite.

Figure 1-2. A NAVSTAR GPS satellite.

Subjects of a satellite’s broadcast are the health of the satellite’s electronics, how the track of the satellite varies from what is expected, the current almanac6 for all the satellites, and other, more esoteric subjects which need not concern us at this point. Other ground-based stations exist, primarily for uploading information to the satellites.


This is the part of the system with which you will become most familiar. In its most basic form, the satellite receiver consists of

• an antenna (whose position the receiver reports),

• electronics to receive the satellite signals,

• a microcomputer to process the data that determines the antenna position, and to record position values,

• controls to provide user input to the receiver, and

• a screen to display information.

More elaborate units have computer memory to store position data points and the velocity of the antenna. This information may be uploaded into a personal computer or workstation, and then installed in GIS software database. Another elaboration on the basic GPS unit is the ability to receive data from and transmit data to other GPS receivers—a technique called "realtime differential GPS" that may be used to considerably increase the accuracy of position finding.

Receiver Manufacturers

In addition to being an engineering marvel and of great benefit to many concerned with spatial issues as complex as national defense or as mundane as re-finding a great fishing spot, GPS is also big business. Dozens of GPS receiver builders exist—from those who manufacture just the GPS "engine," to those who provide a complete unit for the end user. In this text we explain the concepts in general, but use Trimble Navigation, Ltd. equipment since it works well, is quite accurate, has a program of educational discounts, and is likely to be part of educational GPS labs throughout the country.

The United States Department of Defense

The U.S. DoD is charged by law with developing and maintaining NAVSTAR. It was, at first, secret. Five years elapsed from the first satellite launch in 1978 until news of GPS came out in 1983. The story, perhaps apocryphal, is that President Reagan, at the time a Korean airliner strayed into Soviet air space and was shot down, lamented something like "this wouldn’t have happened if the damn GPS had been up." A reporter who overheard wanted to know what GPS was. In the almost two decades since—despite the fact that parts of the system remain highly classified—mere citizens have been cashing in on what Trimble Navigation, Ltd. calls "The Next Utility."

There is little question that the design of GPS would have been different had it been a civilian system "from the ground up." But then, GPS might not have been developed at all. Many issues must be resolved in the coming years. A Presidential Directive issued in March of 1996 designated the U.S. Department of Transportation as the lead civilian agency to work with DoD so that nonmilitary uses can bloom. DoD is learning to play nicely with the civilian world. They and we all hope, of course, that the civil uses of GPS will vastly outpace the military need.

One important matter has been addressed: For years the military deliberately corrupted the GPS signals so that a single GPS unit, operating by itself (i.e., autonomously), could not assure accuracy of better than 100 meters. This policy (known as Selective Availability [SA]) was terminated on 2 May 2000. Now users of autonomous receivers may know their locations within 10 to 20 meters.


Finally, of course, the most important component of the system is you: the "youser," as my eight-year-old spelled it. A large and quickly growing population, users come with a wide variety of needs, applications, and ideas. From tracking ice floes near Alaska to digitizing highways in Ohio. From rescuing sailors to pinpointing toxic dump sites. From urban planning to forest management. From improving crop yields to laying pipelines. Welcome to the exciting world of GPS!

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