sealPurdue News

July 2000

New GPS standards won't affect precision farming

WEST LAFAYETTE, Ind. – A recent upgrade of the United States' global positioning system (GPS) will do little to assist farmers in their fields, says a Purdue University expert.

Mark Morgan, assistant professor of agricultural and biological engineering, says farmers' combines and tractors already have devices more accurate than hand-held GPS receivers.

On May 1 the federal government removed the selective availability component of the global positioning system. This component intentionally downgraded the GPS signal to prevent it from being accurate to within 100 meters. By removing the selective availability component, the federal government claims to increase the accuracy of the system from 100 meters, or approximately 300 feet, to 20 meters, or 60 feet.

"From my personal experience, I've learned that GPS is now accurate to within 5 meters most of the time," Morgan says.

The selective availability component of GPS was the military's way of preventing hostile forces from having that kind of accuracy. "One hundred meters may save a building from artillery. But now the military has come up with a way to degrade the system in local areas if they need to, so that hostile countries can't use GPS against American forces," Morgan says.

However, most precision farming equipment used by farmers utilizes Differential GPS (DGPS), which allows the equipment to be accurate within as little as 3 feet. The equipment receives the GPS signal, but it also receives a second differential signal from U.S. Coast Guard transmitters or commercial satellites. Because the transmitters are in fixed, known locations, the equipment is able to "correct" the GPS signal for greater accuracy. "We're able to get accuracy to within 1 meter," Morgan says. "I don't think when the military allowed public use of this system that they thought we'd ever be that accurate. Surveyors are able to use equipment that can be accurate to within a few centimeters, but that is very expensive."

Morgan says farmers may be able to use the improved GPS system for chores that don't require extreme accuracy. "The accuracy for a hand-held receiver is now within the width of a combine header, so you could almost scout for weed patches or other problems as you walk the fields," he says. "That's particularly true for large fields. On fields of 100 acres or less you would still need a differential receiver to accurately pinpoint spots."

Although GPS-aided precision farming has yet to become the revolution in agriculture that some have been forecasting for the past decade, Morgan says that it is changing how – and even when – farmers go about their business.

"When you're spraying a field, you need straight lines," Morgan says. "If the rows wander a bit, you'll have patches where you'll overspray and patches where you won't have sprayed at all. Now, with DGPS, farmers can get those nice straight lines all the way across the field."

Before the introduction of GPS and differential receivers in agriculture, farmers used a variety of methods to ensure they were driving the straight and narrow.

"During aerial spraying, farmers used to have people stand in the field with flagpoles to indicate a straight line, but those people were always covered with whatever was being sprayed," Morgan says. "Sometimes, for more conventional spraying using boom sprayers, farmers use bits of foam placed as markers, but that has problems, too, because sometimes the foam nozzle clogs."

With DGPS systems on their equipment, farmers now use a row of lights on a dashboard display to aid in navigation. Using GPS and differential signals, the lightbar indicates if the farmer is off track. "All you have to do is keep the light in the center of the display and the row will be perfectly straight," Morgan says. "Farmers can even spray at night when there's less wind. Now you don't even need to see the field. You just follow the navigation lights."

Source: Mark Morgan, (765) 494-1167;

Writer: Steve Tally, (765) 494-9809;

Purdue News Service: (765) 494-2096;

Related Web sites:
White House fact sheet on GPS and selective availability
U.S. Department of Commerce fact sheet on GPS and selective availability

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