The federal government gave hikers, boaters, truckers and other navigators a surprise present last week when it made their Global Positioning System receivers 10 times more accurate than they were the week before.
On May 1, the military stopped scrambling the GPS signals broadcast from a constellation of 27 satellites orbiting the Earth from a little more than 12,000 miles away. The Defense Department, which had been adding fuzz to the signals to keep enemies from using them in operations against U.S. forces, now says it can rescramble the signals on a regional basis if war breaks out.
Before the switch, GPS devices, which help sailors find their locations and ambulance crews locate accident scenes, could usually pinpoint their location within 325 feet -- a variation larger than a football field.
With the scrambling gone, GPS units will be accurate to between 48 and 60 feet.
People who sell and use GPS devices are ecstatic.
"This was one of those things that was a no-brainer," says Alain L. Kornhauser, a professor at Princeton University and CEO of TravRoute, which makes GPS-based navigation systems used in cars, trucks, boats and other vehicles. "This is a realization that we've been making a mistake. ... The Cold War is over. Why not let the public that has paid for this get the full benefit of it?"
Neal Lane, President Clinton's science adviser, put a slightly different spin on the decision: "It's rare that someone can press a button and make something you own instantly more valuable."
Popular with outdoorsmen and used for navigation, surveying, construction and tracking of truck fleets, GPS has been helping people find their way for more than a decade. Born in 1973 as Navtech GPS, the system went into operation in the late 1980s once the 24 required satellites were launched into orbit.
Those satellites, equipped with accurate atomic clocks, broadcast a series of precisely timed signals. A GPS device reads the signals from four satellites, then uses mathematical equations to compute location. The satellites have been sending signals on two frequencies.
The first frequency, which can be read only by military GPS receivers, is precise but encrypted. It's used for tracking missiles, coordinating forces and guiding munitions, GPS experts say. The second, for civilian use, had been degraded through a technique called "selective availability" by the Department of Defense.
