PRECISION RTK correction networks will largely blanket the heart of the Corn Belt by the 2010 planting season. This dramatic increase in RTK coverage is made possible by new multipurpose networks operated by state transportation departments (DOTs), as well as private operators.

As a result, the cost of adopting sub-inch steering systems will fall for growers whose farmland hasn't been covered by existing agricultural RTK networks. These base station arrays, operated by AutoFarm, John Deere and Trimble dealers, eliminate the need for growers to own their own base stations. They form a patchwork covering tens of millions of acres across the Midwest, but many areas have been without coverage.

The CORS revolution

DOTs across the U.S. began constructing regional and statewide multipurpose RTK networks around 2000. The departments' primary goal was to improve surveying accuracy and efficiency as they managed road and other transportation construction projects.

But the new networks — generally referred to by the acronym CORS, which stands for Continuously Operating Reference Station — also can be used for mobile applications, such as in agriculture. CORS actually refers to the individual base station, but typically the DOT systems are networked using sophisticated software and are sometimes called Real Time Networks, or RTNs.

Currently, in the Midwest, statewide DOT CORS systems are available in Iowa (new in 2009), Ohio, Michigan and Minnesota. In Wisconsin, a CORS system covers the eastern half of the state, but the DOT expects the system will cover the state by 2011.

By the 2010 planting season, new statewide DOT networks also should be up and running in Missouri and Indiana. Privately owned networks covering eastern Nebraska (co-owned by Leica) and southern Illinois and east-central Iowa (operated by Trimble under its VRS Now brand) will fill most of the remaining holes in the central Corn Belt coverage map.

Separate but equal

CORS networks and dedicated ag RTK systems both rely on RTK base stations to gather and relay correction data to provide sub-inch accuracy. In the case of CORS networks, fixed RTK base stations are placed at intervals of 30 to 45 miles, compared to the six-mile grid typical of dedicated agricultural networks.

CORS and traditional ag-only RTK networks differ in several respects, but performance of well-designed, well-run networks of both types is similar.

“Absolutely, they both provide the same level of accuracy,” says Matthew Darr, a precision ag expert at Iowa State University. “I have no hesitation about the quality and accuracy of CORS RTK.”

Two major equipment differences between the network types stand out. First, ag networks use 450-megahertz (MHz) or 900-MHz radios to relay correction signals directly from towers to RTK receivers. CORS networks use the Internet to carry correction signals to a cellular modem, cell phone or data card.

Second, ag networks are brand-specific, but CORS RTK is brand-neutral. So with CORS networks, you are free to use whatever RTK equipment you choose, as long as it's able to use standardized correction data formats provided by the CORS networks. That covers all major manufacturers of agricultural RTK equipment except John Deere, which uses proprietary protocols not available from CORS networks.

The methods used by CORS networks to generate corrections vary depending on whose equipment is used. CORS systems in the Midwest use technology from Trimble or Leica. Topcon and other companies manufacture similar technology used elsewhere in the U.S., as well as other countries.

DOT CORS networks currently don't charge for correction signals, and most have no plans to do so. Exceptions include the Ohio DOT, which expects to institute an annual fee (with the amount to be determined) sometime in 2010. The Indiana DOT also is considering a fee. Fee or no fee, users must pay for a cellular data plan, which can cost up to $800 a year.

Gearing up

As more CORS networks come online, manufacturers are gearing up with offerings that simplify access. New or existing products from Leica, Topcon and Trimble, for example, offer built-in, snap-in or plug-in modems that enable latest-generation receivers to access CORS networks with a minimum of fuss (see photos).

Other manufacturers are making changes to enable third-party CORS solutions to be used with their RTK receivers. AutoFarm, for example, says its RTK receivers, which also are available through Raven, can be reconfigured from a proprietary RTK protocol to a standardized Radio Technical Commission for Maritime Services (RTCM) protocol available on CORS networks. New A220 guidance receivers to be introduced by Outback also will be CORS-capable, according to Jeff Farrar at Outback.

John Deere is monitoring CORS network developments but is noncommittal about whether it will alter its navigation equipment to enable it to accept CORS RTK corrections. “Our current product is not compatible with CORS correction signals, but we will continue to monitor the CORS networks very closely, as we do with any emerging technology,” says Jason Beuligmann, John Deere Ag Management Solutions RTK specialist.

In addition to offering CORS connectivity products, both Trimble and Leica are investing in CORS networks in the Midwest. (They're also the key technology providers for state CORS DOT networks in the Midwest.) In September, Trimble announced its subscription-based VRS Now network, which covers parts of Illinois and Iowa. Leica is a partner in a CORS network in eastern Nebraska and plans to expand to other areas.

Growing pains

For all the interest that CORS RTK networks are generating among navigation system manufacturers, the list of growers who have used CORS RTK is short. In 2009, the total user base in the Midwest is about 300, up from about 20 in 2008, based on interviews with state CORS system administrators and precision ag consultants.

Much of the user base is concentrated in Iowa and Minnesota, where CORS networks faced technical challenges in 2009 as they were established (in Iowa) and expanded (in Minnesota). Problems in both states, which resulted in RTK correction interruptions at times for some customers, have been ironed out, according to system administrators.

“The Iowa CORS system had a few growing pains in its first year,” says Darr of ISU. “But the network is up and running 99% of the time, and the Iowa DOT has been very responsive in addressing technical problems.”

In Ohio, where ag use of the state CORS system grew from a single user in 2008 to about 40 in 2009, precision ag consultant Tim Norris expects farmers who operate in rougher terrain to benefit from CORS RTK. Norris, part owner of a Trimble-based RTK network in central Ohio, says that CORS RTK has more potential on rough ground because cellular signals don't require line of sight for reception, as do radio signals.

“I think there is a ton of opportunity with CORS RTK,” he says. However, competition from CORS RTK could threaten the expansion of existing ag RTK networks and raise questions about investing in new ones, he adds.

But Chad Pfitzer, an RTK systems specialist at Trimble, disagrees. “Current RTK tower arrays will remain the gold standard for accuracy and reliability,” he says. “Growers who are successfully using RTK should have little reason to switch to VRS or CORS.”

See Related Sidebar:

Sidebar #1: Software to Access CORS Networks

Sidebar #2: RTK Netowrk Options