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I am riding in the "buddy seat" of a big, red Case farm tractor, a 2001 model with tires almost as tall as me. The cab is air-conditioned, of course, a big bubble of glass. Don Glenn is at the helm, and a 45-foot spraying boom is floating behind us, laying down a mist of herbicide. Out in all directions spreads a field of very young corn, just little sprouts six inches tall.

Don and his brother, Brian, call this field F-11 -- Flower Hill 11, 49.5 acres in north central Alabama, a quarter mile south of the twisting Tennessee River, west of Decatur. It's not quite isolated -- there's a Boeing rocket plant just down the road -- but it's rural enough that there's no cable TV.

We are bouncing through this field, a small square of the land that the Glenn brothers farm, under the gentle guidance of eight or nine satellites that are 11,000 miles overhead. Because Don is the quiet brother, I have a little bit of time to consider the implications of this. One thing is quickly clear: Although the brothers have nearly 40 years of farming experience between them, their daddy is close at hand to advise them, and they still are using some equipment that their granddaddy made by hand, Don and Brian are learning things about this land from those orbiting satellites that they could not know any other way -- things that fly in the face of 3,000 years of farming wisdom. In six years, the Glenn brothers have become so convinced of the value of the Global Positioning System (GPS) satellites as a farming tool that they would no sooner give them up than they would give up their tractors or their combine.

Even more astonishing is that, out here in this field (N 34? 40.497', W 87? 8.933', eight satellites reporting, to an accuracy of 1 foot), we know where on Earth we are more precisely than human beings have at any other time in history. Christopher Columbus, Ferdinand Magellan, Francis Drake, Lewis and Clark -- if they knew where they were within 50 miles during their great explorations, they were lucky. In this tractor cab, I know my location with 250,000 times more accuracy than that -- to less than a foot. The positioning is so precise, in fact, that in the adjacent wheat field, the Glenns use GPS to steer their tractors so that the 19-inch-wide tires follow exactly the same tire tracks each time that they work the field. "Controlled traffic patterns," Don calls it.

The implications of knowing where you are with this kind of accuracy -- of knowing where anything is, where everything is, with this kind of relentless precision -- are only just becoming clear, and they are staggering. Think back to the early days of microprocessors. People imagined all kinds of uses for the new devices, but was anyone talking about putting chips in car transmissions? Or in birthday cards? Or in implantable, automatic cardiac resuscitators? GPS will be like that -- a genuinely transformational technology.

As with so many big-bang technologies, war first brought GPS to the popular imagination: initially in the 1991 Gulf War, and this year in Iraq. The U.S. military uses the constellation of GPS satellites that it put in orbit to deliver bombs, missiles, and even individual artillery shells to targets with the exactitude of a FedEx package. War planners can literally pick which doorway -- front or back? -- to fly a bomb to.