The Federal Aviation Administration mothballed two global positioining system satellites at the end of July, which could interrupt service for many farmers and agricultural service providers, says Terry Griffin, an assistant professor and economist with the University of Arkansas Cooperative Extension Service in Little Rock.

The FAA is switching the service over to new satellites.

Griffin says the affected users will need to update their GPS system software to maintain service.

The FAA provides a no-cost differential correction signal for the GPS system called Wide Angle Augmentation System, or WAAS. The WAAS system was designed by and maintained for the aviation industry to correct GPS satellite signals, making them more accurate.

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

Two WAAS satellites out of a constellation of three regular satellites were turned off around July 30, according to the FAA Technical Center in Atlantic City, N.J.

Since one new WAAS satellite has recently become operational, North America users will still have access to two satellites for receiving the differential corrections.

"However, 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," Griffin says. "Many of the GPS equipment manufacturers and providers have posted

information on their Web site about firmware updates and models that will no longer be useable due to the decommissioning of satellites.

"This would mean a DGPS receiver firmware update is needed by growers who are readying for mid-season applications using lightbars for guidance and yield data collection during fall harvest."

"It may be the best time to make updates for the GPS receivers on harvesting equipment along with the GPS receivers used for lightbar guidance now; otherwise the updates will need to be scheduled as part of the harvest preparation," says Dharmendra Saraswat, assistant professor/Extension geospatial engineer. "Farmers and agricultural service providers using only subscription satellite correction or base station (RTK) GPS systems will be 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.