The Global Positioning System originally was built to pinpoint the location of troops in the field and guide high-tech weapons systems such as Tomahawk cruise missiles and B-2 bombers. Today, it helps locate lost hikers and steers commuters around detours. The commercial sale of GPS equipment is projected to top $8.5 billion in 2000.

The system, planned and launched over two decades, saw use in the 1991 Gulf War, giving U.S. and allied troops a distinct advantage in the featureless terrain of the desert.

The backbone of the system is a $10 billion "constellation" of 24 satellites in low-Earth orbit. By checking signals from multiple satellites and a network of ground stations, a GPS receiver -- such as a hiker's hand-held device -- can pinpoint the hiker's latitude, longitude and altitude.

Every GPS satellite transmits two different signals. Commercially available GPS systems use the Coarse Acquisition (C/A) code. Precision, or "P-code," GPS is used by the military to plot a location within 20 meters.

The government is looking at ways to make the P-code signal available to the public without compromising its military effectiveness. Another method, called Differential GPS, can narrow the margin of error to 5 to 10 meters.

Among the first users of commercial GPS devices were sailors, private pilots and hikers. Combined with a computerized street map, GPS can provide precise driving instructions. This capability is now available as an expensive option on some new cars.