The Federal Aviation Administration (FAA) is mothballing two GPS satellites at the end of July, a move that could interrupt service for many farmers and agricultural service providers, warns Terry Griffin, assistant professor and economist with the University of Arkansas Cooperative Extension Service.

The FAA is switching the service over to new satellites. Griffin said 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 is also 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 (identified as PRN 122 and PRN 134) out of a constellation of three regular satellites (identified as PRNs 122, 134 and 135) will be turned off around July 30, according to the FAA Technical Center, Atlantic City, N.J.

Since one new WAAS satellite (PRN 138) has recently become operational, North America users will still have access to two satellites (PRNs 135 and 138) 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 said.

“Many of the GPS equipment manufacturers and providers have posted information on their Web sites about firmware updates and models that will no longer be useable due to the decommissioning of satellites,” Griffin said. “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,” said 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,” he added.

Griffin said for more information on WAAS, farmers can log on to the FAA Web site at http://www.nstb.tc.faa.gov/. For more information on your equipment, check with your GPS equipment manufacturer or service provider.