If you have been experiencing difficulties picking up WAAS (Wide Area Augmentation Service) signal with your GPS unit, don’t panic. WAAS has recently undergone some changes. These changes will affect many agricultural and non-agricultural GPS receivers that utilize the free differential correction signal. The good news is that in most instances the problem can be remedied with a firmware upgrade.

Today, WAAS signal has become integral to many agricultural applications, including yield monitoring, guidance systems, field sampling and variable rate applications.

WAAS employs a system of about 29 reference stations spread across the United States. Each station monitors data from the GPS satellites and feeds into a master station (one on each coast). The master stations are responsible for generating the correction message.

This signal is uplinked to geostationary satellites located over the equator. The correction is then broadcast from the satellite on one of the same frequencies as GPS. This means no additional equipment is required to receive the signal. You simply need a WAAS-enabled GPS receiver. Most new receivers come with WAAS capabilities.

An upgrade may be needed
In July of this year, two of the WAAS satellites were decommissioned. The Atlantic Ocean Region – West (AOR-W) represented by PRN 122 and the Pacific Ocean Region (POR) satellite, represented by PRN 134 were removed from service as the lease on the satellites ended.

In preparation for this, two new satellites, Galaxy XV (PRN 135) and Anik F1R (PRN 138) were launched in 2005. As of mid-July, these were both operational and available for use by WAAS-capable receivers. According to the FAA, PRN 138 is expected to provide improved WAAS performance to the Northeast coast, as well as extending the signal into Northeast Canada.

To use the new WAAS satellites (PRN 135 & PRN 138), many agricultural users are required to upgrade their receivers. The satellites will provide differential correction signals to all parts of the country. However, receivers that are running older versions of firmware may not be capable of tracking the new satellites. In most cases, this problem can be overcome by updating the receiver’s firmware.

To find out how to handle these changes, refer to your GPS manufacturer’s Web site or contact your local dealer. Most GPS manufacturers post guidelines on their Web site as well as any new firmware that may need to be installed. In some instances hardware will need to be sent back to the factory for updating. Some GPS units, such as many handheld devices, are programmed to scan for available signals and should not experience an interruption in differential correction signal.

Some systems unaffected
Manufacturers may recommend determining which satellite works best in your area and then locking onto that satellite. This will avoid brief signal loss that may occur if the GPS switches between WAAS satellites when set to autoselect mode.

Agricultural users who rely on subscription-based differential correction such as OmniSTAR or John Deere’s Starfire will be unaffected by WAAS satellite updates. High accuracy survey grade systems (RTK), employed by many autosteer guidance systems, will also be unaffected as they receive their position corrections from a base station.

For up-to-date information regarding WAAS, refer to the FAA Web site at www.nstb.tc.faa.gov. This gives information on satellite status, as well as daily plots and real time data.

Tasha Wells is the research coordinator for the National Environmentally Sound Production Agriculture Laboratory (NESPAL) in Tifton, Ga. Contact Wells at nwells@tifton.uga.edu.