Q. It pinpoints places so precisely it's like every square meter of the Earth has its own unique address, instantly available and free of charge. Even a hiker out in the wilderness can use it to help keep from getting lost. This workhorse system aids also in air and sea navigation, aerial photography, crop dusting, surveying, guiding an emergency vehicle to its destination, tracking a forest fire, monitoring an earthquake or the shifting of the planet's tectonic plates. Telecommunications companies increasingly rely on it for synchronization of their land-based digital networks. Likely you've already guessed it, haven't you?

A. It's the satellite-driven GPS, for "global positioning system," based on the geometric principle of "trilateration," as in "You're now 11,000 miles from satellite X, 12,000 miles from satellite Y, 13,000 miles from satellite Z, so WHERE ARE YOU?" (There are 24 satellites orbiting at 12,000 miles above the Earth.) Once your GPS receiver maps out the three fixed-distance spheres and finds the points of intersection, then uses a fourth satellite to correct for receiver time "drift," you'll know, says Cal Poly University geospatial engineer Peter Boniface. Millions of motorists already have receivers in their cars (or hand-held units), tracking travel time and distance, current and average speed, estimated time of arrival, adds beyonddiscovery.org. With the GPS computerized "yellow pages," you can drive into a new city and instantly locate the nearest restaurant or gas station.

 
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