Changes to WAAS GPS Correction Service

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

WAAS is a system of satellites and ground stations that provide GPS signal corrections to increase GPS accuracy so that it can be used in precision flight approaches for commercial aircraft. The ground stations located throughout North America monitor and measure GPS signal. The measurements from these reference stations are routed to two master stations which generate and send the correction messages to geostationary satellites. Those satellites broadcast the correction 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 FAA completed the transition from two older geostationary satellites to two new ones. The new satellites are named 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 of GPS units. Some manufacturers require the units be returned to the dealer or factory for updating whle others allow users to download the updates from the Internet and provide instructions for the owner to do their own updates. Some units have been preprogrammed to search for the strongest correction signals available and therefore will not need updates.

Users can quickly discover whether or not their unit is using the signals from the new satellites simply by turning 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 differently 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.

GPS units programmed to use other differential correction services such as OmniSTAR, John Deere SF1 or SF2, or the Nationwide Differential GPS (formally named the Coast Guard Beacon System).

Most WAAS-enabled handheld GPS units are pro-programmed to search for available WAAS satellites and do not require software updates.