Hikers and drivers who use satellite-derived location information may not need to know their latitude and longitude down to the decimeter. But phone company workers digging near fiber-optic cables, drillers working on offshore oil-and-gas rigs, and farmers tracking crops row by row do need that kind of accuracy. Right now, though, they depend on expensive, localized ground-based reference-and-correction systems to get high precision from the 24 satellites of the Global Positioning System (GPS), run by the U.S. Department of Defense.

Select from the choices above to read the entire article.