Global Positioning System (GPS) is a technology devised by the U.S. military in the 1970s that can pinpoint a location anywhere in the world. The system depends on 24 satellites orbiting at an altitude of about 11,000 miles and five earth stations. A GPS receiver can tell its user its precise longitude, latitude and altitude. The military originally used GPS for navigation, but since the early 1980s commercial use has expanded to locating people and tracking objects. The U.S. GPS Industry Council estimates that worldwide sales of GPS related devices will soar to $3 billion in the year 2000.

-- Richard Drezen

To understand the concept of GPS, think of a triangle. The triangle consists of satellites, GPS receivers and central earth stations. Out of the 24 satellites, at least three are needed to coordinate signals to determine longitude, latitude and altitude. This represents the first point of the triangle. The second point of the triangle is the actual GPS receiver itself, whether it is hand-held or a part of an on-board automobile, ship or airplane navigation system. The last point is the central station which monitors the accuracy of the satellite signals.

Each satellite utilizes an atomic clock that helps synchronize the accuracy of the radio signals being beamed down. GPS receivers lock on to signals beamed from the satellites, measuring the distance based on the time it takes for the signal to arrive. The accuracy of the cheaper GPS receivers can range from 60 to 225 feet; higher-end models (also known as differential GPS) can zero in on locations within about an inch.

GPS was used successfully by the military during the 1991 Persian Gulf War and in 1995 when searchers found a pilot whose plane was shot down over Bosnia. Non-military applications include commercial fishing fleets that identify key fishing sites with GPS, allowing them to return repeatedly to the precise location. GPS also enabled French and English diggers of the "Chunnel" under the English Channel to link up with great accuracy. Consumer uses include in-car direction systems and hand-held location devices used while hiking, skiing or boating.

GPS SATELLITES

Name: Navstar

Manufacturer: Rockwell International

Altitude: 10,900 nautical miles

Weight: 1,900 pounds (in orbit)

Size: 17 feet with solar panels extended

Orbital period: 12 hours

Orbital plane: 55 degrees to equatorial plane

Planned lifespan: 7.5 years

Number in orbit: 24

Number built: 39

SOURCES: Aerospace Corp., Consumer Electronics Association, Garmin Corp., Rand McNally, Trimble Navigation, Ltd.