Locator system draws bead on better accuracy; GPS: The federal government has stopped scrambling the signals, rendering devices using the navigation system far more precise.


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.

That scrambling involved throwing random nanoseconds into the precise timing of the signals, says JoAnn Cummings, president of Adventure GPS, an Internet retailer based in Decatur, Ala. In turn, the location of a building, reef or tree would appear to move around within a 50- to 100-meter radius when plotted on a map.

Given the growing use of GPS among civilians -- the White House estimates that about 4 million people globally use the system -- pressure from businesses and private citizens finally persuaded the government to change the policy. No one expected the decision last month.

"We knew it was going to end early, but not this early," said Percy Aquino, who works in sales at Navtech Seminars and GPS Supply.

"A lot of people in the GPS community have been waiting for this," says Derek Reiber, associate editor of GPS World magazine. "Originally, the president signed a directive putting the time line where they were going to turn off [scrambling] by 2006, and they would start looking at it this year."

The real winners are those using GPS for recreation and businesses that cater to them with low-cost units. Manufacturers of cellular telephones, who will be required by the Federal Communications Commission next year to make sure all cell phones are capable of revealing their positions, will benefit from the increased accuracy as well.

In-vehicle navigation may improve the most. For example, under the scrambled system, a driver on Guilford Avenue near Interstate 83 in downtown Baltimore might have appeared in his navigation system to be on I-83 itself. That's far less likely to happen now.

Kornhauser says his job just got easier. For years, he has come up with algorithms -- a series of mathematical equations used to correct for the scrambling in his PC-based Co-Pilot navigation systems. "As it was, the navigation systems rarely made a mistake" because of the use of map-matching algorithms, he says. "It will make fewer mistakes fewer times because it won't be spoofed here and spoofed there."

Ambulance crews, police and firefighters who use GPS should be able to shave valuable minutes off their response times, says Richard Langley, a columnist for GPS World and professor of geodesy and precision navigation at the University of New Brunswick. "Sometimes, in the past, they have gotten a position [through GPS], but been on the wrong side of the freeway," he says.

Even with the change, the GPS signal broadcast to average users needs refining before it's good enough for businesses that demand precision.

In fact, the accuracy of GPS depends upon several factors, including the quality of the receiver, says Langley. For example, atmospheric interference can throw off the timing, and dense tropical forests with high moisture content aren't good for receiving GPS signals at all. In cities, signals bounce off tall buildings, and the urban canyon phenomenon -- think of the narrow streets and skyscrapers of Wall Street -- can block a GPS receiver's view of the sky.

For that reason, precise navigators have been using something called differential GPS for years. DGPS requires an add-on receiver, available for as little as $300, to take measurements from so-called "correction" signals -- one of which is broadcast by the U.S. Coast Guard. If a ship is navigating a busy harbor, the pilot needs DGPS to get accuracy as close as one to three meters. That way, says Cummings, "you don't get an Exxon Valdez."

But things will get better for those who buy cheaper units, too. Langley says the demise of scrambling will give manufacturers an incentive to make all their GPS receivers more precise.

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