The Federal Aviation Administration has made changes to the Wide Area Augmentation Service that affects the global positioning system correction service used by farmers and many other users. These changes mean that most GPS units used for agricultural applications, such as tractor guidance systems and combine yield monitors, need software updates to continue to function.

WAAS is a system of satellites and ground stations that provide GPS signal corrections to increase accuracy so that it can be used in precision flight approaches for commercial aircraft. The ground stations throughout North America monitor and measure the GPS signal. The measurements from these reference stations are routed to two master stations that generate and send the corrected messages to geostationary satellites.

These satellites broadcast the corrected messages back to earth, where WAAS-enabled GPS receivers apply the corrections to their computed GPS position. Many farmers use the free WAAS correction service with their farm GPS applications.

In late July 2007, the FAA completed the transition from two older geostationary satellites to two new ones. The new satellites are PRN 135 and PRN 138. Most GPS units on tractors and other farm equipment are configured to receive GPS correction signals from the older satellites, but not the new ones.

The procedure to configure GPS units to receive the new satellite signals is different for the various brands and models. Some manufacturers require the units be returned to the dealer or factory for updating. Some allow users to download the updates from the Internet and provide instructions for the owner to do his or her own updates. Some units have been preprogrammed to search for the strongest correction signals available, so those units will not need updates.

Users quickly can discover whether their unit is using the signals from the new satellites by turning it on and trying it in WAAS-enabled mode. All GPS units display some kind of a digital indication, such as the letters DGPS, when they are using a differential correction service.

The FFA decommissioned both of the older satellites in July, so if a unit displays a differentially corrected GPS position after July 31, 2007, when set to use the WAAS service, it does not require an update. If the GPS unit does not display a differentially corrected position, it needs updating.

Other differential correction services include OmniSTAR, John Deere SF1 or SF2 or the Nationwide Differential GPS (formally named the Coast Guard Beacon System). These differential correction services are not affected by the changes to WAAS.

Most WAAS-enabled hand-held GPS units are preprogrammed to search for available WAAS satellites, and do not require software updates.