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Posted at 10:12 p.m. PDT Monday, May 1, 2000

You are here: GPS units more precise

Position system gains accuracy of military's satellite signals

BY GLENNDA CHUI
Mercury News

Trying to find yourself? The government just made it easier.

As of Monday, ordinary folks could use military satellites to determine their location on the face of the planet more precisely than ever before -- narrowing it down from an area the size of a football field to one the size of a tennis court.

The Defense Department, which used to distort the satellite signals before passing them on to civilians, stopped doing that at 5 p.m. Monday.

The immediate beneficiaries will include boaters, hikers, loggers, gold panners, people who forage for mushrooms or keep track of delivery trucks and anyone else who uses an inexpensive receiver to plug into the Global Positioning Satellite system and find out where they are.

``This is a major step forward,'' said Bradford Parkinson, a Stanford University professor of aeronautical engineering who led the Air Force team that invented GPS in the 1970s. ``This is very exciting news.''

The higher level of precision could help rescuers find stranded motorists or missing hikers who have tiny GPS units in their cell phones.

Eventually, experts said, the benefits may trickle down to more sophisticated and expensive systems that are used to navigate cars, monitor earthquake faults and guide airliners to landings in thick fog.

The impact on these uses will be muted, though, because people had already found clever ways to work around the military's deliberate distortion of civilian GPS signals, which was done in the name of national security and technically known as ``dithering.''

For their part, people in the industry hope the changes will accelerate the growth of GPS technology, which is now used by an estimated four million people worldwide. The Commerce Department estimates the GPS market at $8 billion, and it's expected to double in three years.

Historic change

``I think, overall, this is a really major milestone in the history of GPS,'' said Ann Ciganer, vice president for public policy for Trimble Navigation Ltd., a GPS technology company in Sunnyvale. ``This is really a very, very big deal.''

``I think there are going to be a lot of happy consumer users,'' she said. ``All handheld units are going to be far more accurate, so backpackers, campers, search and rescue, everybody who uses GPS to navigate in their cars, the accuracy is going to be increased.''

The GPS network consists of 25 military satellites that orbit 11,000 miles above the ground. They beam radio signals to Earth at two frequencies, one of which is available to civilians.

By precisely measuring the time it takes for signals to arrive from three of these satellites, receivers on the ground can determine their location within about 10 yards. They can add the signal from a fourth satellite to get the altitude, as well.

From the start, the Defense Department worried that hostile forces could hijack these signals and use them against the United States. So they messed with the civilian signals ever so slightly, advancing or delaying their timing by no more than 100 billionths of a second, said Clark Cohen, founder of IntegriNautics, a Menlo Park company that develops ways to use GPS to guide tractors and airplanes.

This was enough to throw off accuracy ten-fold. While military users could pinpoint locations within about 10 yards, civilians could determine them only within 100 yards -- the length of a football field.

Researchers found ways to work around this. By comparing readings from more than one GPS receiver on the ground, they could figure out how much the signals had been ``dithered'' and get rid of the distortion.

Advanced uses

And with enough computing power, they went far beyond the original capabilities of GPS -- using it, for instance, to watch the sides of the San Andreas Fault move past each other with an accuracy of just a few millimeters per year, Parkinson said.

While these fixes are too complicated and expensive to benefit the average GPS user, they became so effective that the military's signal-scrambling effort ``just wasn't working very well anymore,'' said Col. Rick Skinner, who works in the Pentagon office that oversees the GPS program.

After President Clinton announced in 1996 that he wanted the GPS system modernized and made more useful for civilians, the military investigated alternatives to dithering, Skinner said.

The conclusion: It would be more effective to give civilians a more precise signal that could be blocked in regions of military conflict.

Over the next few years, 18 GPS satellites that are being built or waiting for launch will be retrofitted to improve their civilian signals even further, Skinner said. And the government has pledged to add two more civilian frequencies by 2005.

Expanding appeal

U.S. Commerce Secretary William M. Daley said Monday that the move to more accurate GPS ``has the potential to do for GPS what the PC has done for computing, making this powerful information technology far more accessible and affordable to the broad public.''

GPS has already spread into dozens of uses. Some cars now come equipped with navigation systems that partially rely on GPS satellites. For about $40,000, a farmer in the Salinas Valley can buy a system that allows her to plow fields into precise furrows at night. And for about a hundred dollars, anyone can buy a handheld GPS receiver at a discount or sporting goods store.

Miners ``go out gold panning and use them to find their way home,'' said Don Bishop, manager of the sporting goods department at Kmart on Blossom Hill Road in San Jose. ``If you go out in the hills, you can find your way back to your car,'' he said. ``The people who pick mushrooms buy them so they can find their way back.''

Bishop said he's used GPS himself while fishing off the Washington and Alaska coasts.

``In the fog it would bring me back to the same slip. I didn't even have to look,'' Bishop said. ``If they're getting it better than that, they're getting it pretty darned good.''


Contact Glennda Chui at gchui

@sjmercury.com or (408) 920-5453.


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