Among the thousands of satellites orbiting Earth are 24 that work together as the key elements of the Global Positioning System (GPS), a navigational tool of unprecedented precision. Circling the globe once every 12 hours at an altitude of more than 12,000 miles, the satellites maintain positions relative to one another that enable handheld or vehicle-mounted units to receive signals from as many as six of them at once, from virtually any point on the planet’s surface at any given time. Using signals from at least four satellites, the receivers can calculate their own location—latitude, longitude, and elevation—to within 100 feet; the more satellites the receiver has within its line of sight, the more precisely location can be determined, in some cases within 20 feet.

Each satellite continuously transmits three vital pieces of information an identification code, data on its own location in space, and a time signal. Four atomic clocks on each satellite keep track of time to within 3 billionths of a second, a level of accuracy that enables a receiver’s microprocessor to precisely measure the distance to each satellite from which it receives signals. The microprocessor then applies a sophisticated form of triangulation to determine its own position.

Known officially as NAVSTAR (Navigation Satellite Timing and Ranging), GPS was envisioned as early as the 1950s by Ivan Getting, who subsequently championed its construction. Brad Parkinson, then at the Department of Defense, directed its development for its original military functions. GPS played an important role in Operation Desert Storm, enabling U.S. forces to maneuver during sandstorms and at night; aircraft, ship, tanks, and individual troops made use of more than 9,000 receivers throughout the Gulf region.

Eventually released for civilian use, GPS began finding an increasing number of applications in the 1990s. For example, engineers building the English Channel tunnel used GPS-provided measurements to ensure that the French and British teams digging from opposite ends would meet precisely in the middle. Transportation companies employ GPS receivers to keep track of their fleets, and police cars, fire engines, and ambulances use them to speed their response to emergencies. As receivers become less expensive, GPS is becoming more and more a part of everyday life, helping us all to know quite literally, our place in the world.

HOW GPS WORKS. To calculate a location on or near the earth, a GPS receiver must be able to receive signals from four satellites. Each satellite sends a signal at the speed of light, approximately 186,000 miles per second; the GPS receiver can determine how far it is from a satellite by measuring the time it takes the signal to travel from the satellite to the receiver. (One billionth of a second clock error corresponds to approximately one foot.) This distance represents the radius of an imaginary sphere centered at the satellite and the receiver is located somewhere on its surface. By measuring the distance to a second satellite, the possible receiver locations are reduced to be somewhere on the intersection of the two spheres. To precisely pinpoint its position, GPS must track four satellites. Four satellites are required because the receiver must determine its position (latitude, longitude, altitude) and correct its clock error. By taking multiple “waypoint” readings a hiker can plot the changing elevation of a climb in order to stay on track. In applications where the user receiver knows its own altitude (e.g., a surface ship) and/or has a clock synchronized to the GPS master clocks, signals from fewer satellites are needed.