Fixing the Location Fix

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.

That’s about to change. Thales, the French aerospace giant, says it has developed the first GPS-correction service that’s accurate to within 10 centimeters virtually everywhere on the planet-compared to the one-meter precision guaranteed by similar existing services. “This makes something that was once very difficult much easier, because you won’t need expensive equipment,” says Andrew Barrows, president and founder of Nav3D, a Palo Alto, CA-based company that develops graphical location displays. The Thales system will beam out signals that subscribers can use to continuously clean up coordinates on ordinary GPS receivers. “They are saying, throw out all your GPS-correction infrastructure; we’ll just send you the signal,’” Barrows says.

A GPS receiver fixes location on the basis of the travel time of radio signals from at least three of the 24 GPS satellites. The Thales system is a “new milestone” in sophisticated computer algorithms that attack several sources of error in the satellite coordinates, says Angus Cooper, marketing director for Thales GeoSolutions Group. By independently tracking satellites’ locations, Thales double-checks the coordinates reported by the satellites. Then it corrects for atmospheric disturbances that might alter signals’ travel time and for known errors within the clocks aboard each satellite. Finally, the company maintains 85 ground-based reference stations worldwide. The correct coordinates of these fixed stations are known, and they are continually compared against the coordinates reported by GPS. By this summer, Thales expects to beam ultracorrected GPS signals from outposts in Singapore and Aberdeen, Scotland, to paying customers.