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Some farmers experience interruption of GPS service

A few farmers and agricultural service providers went to the field in late July and August to find their global positioning systems (GPS) were not operating as anticipated.

“Online discussion forums are continuing to have numerous questions about GPS problems. Many farmers' first suspicion is their GPS equipment malfunctioning,” said Terry Griffin, economist with the University of Arkansas Cooperative Extension Service.

This interruption in service occurred after two Wide Angle Augmentation System (WAAS) satellites were turned off. These satellites are used by some GPS equipment as a source of differential correction.

Farmers use the GPS in precision agricultural operations that help them make better management decisions and more precise field applications, Griffin said.

WAAS is a no-cost differential correction signal for the GPS system that consists of geostationary satellites designed by and maintained for the aviation industry by the Federal Aviation Administration.

WAAS correction is also available for civilian use, including agricultural purposes. A few common agricultural uses of the WAAS correction signals include lightbar guidance, variable rate application of inputs and yield monitoring on combines and cotton pickers.

“Although the GPS system remains operational without differential correction, the accuracy may not be enough to conduct the field operations at the desired precision,” Griffin said. “In addition, some agricultural positioning software can be set to not operate without a source of differential correction.”

Two WAAS satellites (identified as PRN 122 and PRN 134) were turned off July 30, according to the FAA Technical Center, Atlantic City, N.J. The old satellites were set to be taken off line earlier in the month, but were allowed to be usable for another two weeks to accommodate users. The two newest WAAS satellites, identified as PRN 135 (NMEA Satellite ID 48) and PRN 138 (NMEA Satellite ID 51), became operational Nov. 9, 2006, and July 13, respectively.

To maintain WAAS-corrected GPS, affected users may need to update their GPS system software, Griffin advised. Those users whose receivers are unable to scan for the new satellites or are otherwise locked onto the old satellites may experience a lack of GPS correction and may become unusable for agricultural purposes.

“Many agricultural GPS equipment manufacturers and providers have posted information on their respective Web sites about firmware updates for specific models,” Griffin said. “This would mean a GPS receiver firmware update may be needed by growers who are readying for fall applications using lightbars for guidance and yield data collection during harvest.”

Farmers and agricultural service providers relying upon other sources of differential correction such as Coast Guard beacon, subscription satellite correction, or base station (RTK) systems for their GPS correction signals, were unaffected by the change in WAAS satellites.

For more information on WAAS, log on to the FAA Web site at For more information on your equipment, check with your GPS equipment manufacturer or service provider.

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