The umbrella of real-time kinematic (RTK) correction signals that covers much of the Corn Belt will be larger and more robust by the time the 2011 planting season rolls around. As a result, growers in many geographies will have multiple correction signal options for driving RTK navigation systems.

Going into 2010, most of the heart of the Corn Belt was covered by either radio or cellular systems, or both. But there were coverage gaps. By 2011, a coverage gap in Illinois will have been plugged, and coverage in Ohio, Indiana, South Dakota, Nebraska, Kansas, Missouri and other states will have been beefed up.

The improved coverage is a result of build-outs of both traditional radio-based systems and newer cellular delivery systems, which distribute RTK corrections via the Internet through cellular communications networks. Unlike in recent years, when additions to the cellular delivery system were dominated by new and expanding state department of transportation (DOT) systems, this season’s new cellular entries are systems dedicated to agricultural users.

The new systems include MyWay RTK, Raven’s Slingshot technology, and Trimble’s VRS Now cellular system.

These systems are in addition to CORS systems run by state DOT. (CORS stands for Continuously Operating Reference Station, a system of base stations that provide RTK correction signals to surveyors and the general public, usually at no charge.) Midwestern states with these systems, which typically blanket entire states, include Ohio, Michigan, Indiana, Wisconsin, Minnesota, Iowa and Missouri.

Radio systems expand

Radio-based systems operated by John Deere and Trimble/Case IH dealers continue to grow as well.

John Deere has seen its dealer-operated network expand to more than 300 million acres nationwide, up from 220 million acres in 2009. That assumes coverage in a 12-mile range from each base station.

Recent expansion has been driven in part by Deere’s introduction of its StarFire 450 RTK radios in January 2010, says Ryan Roloff, John Deere RTK network specialist. The StarFire 450 RTK radios, which must be licensed by the FCC, typically broadcast with 70W of power, compared to 4W for the unregulated 900-megahertz (MHz) spectrum used in most Deere networks.

“The 450-MHz radios still have a 12-mile range but can blast through trees and over gently rolling hills,” Roloff says. This capability has allowed dealers to expand networks in parts of the Midwest with more challenging terrain.

Networks operated by Trimble dealers in the U.S. now cover 550 million acres. That’s based at a 12-mile range from base stations, up from 425 million acres a year ago. Coverage is about 250 million acres at an eight-mile range offering sub-inch repeatable accuracy. Radio base station sales have slowed only slightly, even as other RTK options proliferate, notes Chad Pfitzer, RTK/VRS systems specialist at Trimble.

New cellular networks

MyWay RTK (www.mywayrtk.com) will provide correction signals covering every tillable acre in Illinois. Although MyWay hasn’t divulged its list of owners, participants include about 50 agricultural retailers. Their goal is to improve the efficiency of custom applications by reducing confusion that occurs when farmers ask applicators to follow guidance lines generated by a hodgepodge of RTK systems.

“If we all used the same correction source, we would eliminate a lot of the concern about AB data not matching up,” says Don Bierman, MyWay RTK manager. “This would allow us to exactly follow their tracks so we don’t have to bump RTK lines or make up for systems that had problems during the day.”

The long-term goal is to build a system that encourages navigation equipment manufacturers to develop hardware that works together and enables data to flow to and from various systems, Bierman says. “Today the concern is that technology manufacturers will continue to innovate in isolation,” he says. “In the end, you have a product that stands alone and can’t interact.”

MyWay RTK subscription prices have not been set, but are likely to be in the $1,250/unit/yr. range. A subscription would provide access to correction signals throughout the system.

The Slingshot system from Raven Industries (www.ravenslingshot.com), which the company introduced in early 2010, provides cellular-delivered RTK correction signals, plus high-speed communications and data transfer capabilities through the Slingshot Field Hub cellular modem/router.

The Field Hub can receive RTK correction data from many Internet-connected sources, including MyWay RTK, Trimble’s VRS Now and state DOT CORS networks. Slingshot partners also are building RTK correction base stations to supplement, and sometimes compete with, existing cellular RTK networks, says Paul Welbig, Slingshot operations manager.

By the 2011 planting, new systems in the Midwest using Slingshot RTK base stations will be available (or are available now) across Ohio, Indiana and parts of Illinois (owned by Equipment Technologies, www.etsprayers.com), as well as eastern South Dakota and southeastern North Dakota. More robust networks supplemented by new Slingshot base stations also will be available in eastern Nebraska and Kansas and western Missouri.

“We are network agnostic,” Welbig says. “We want to leverage what is already there. The end user at the end of the day doesn’t care where he gets his RTK correction signal as long as it works.”

Annual subscription fees are $1,500/unit for Slingshot dealer-provided RTK corrections and $750/unit for outside correction signals. A three-year subscription for outside correction signals is $1,000. All subscriptions include support for GPS, GLONASS and other Global Navigation Satellite Systems (GNSS) sources. A Slingshot Field Hub retails for $2,000.

Since it was introduced in 2009, Trimble’s VRS Now (www.trimble.com/vrsnow) cellular system has expanded from its Illinois base to include northern and western Indiana, most of Iowa, south-central and southwestern Minnesota, the southeastern corner of South Dakota, all of Nebraska except the Sandhills and peripheral areas, as well as other states.

While MyWay RTK, Slingshot dealers and some CORS systems provide corrections from individual base stations, VRS Now uses proprietary software to generate RTK corrections based on location information across its network of base stations. “Unlike simple single-base systems, a networked solution will not lose accuracy as the distance from any individual base increases,” Pfitzer says.

For ag subscribers using Trimble navigation technology, Trimble VRS Now subscriptions are $1,200/yr. for GPS corrections and $1,500/yr. for full GNSS corrections, including GPS and GLONASS.

For non-Trimble users of the VRS Now network, prices vary by region. In northern and southern Illinois, for example, the subscription is $1,200/yr. for GPS and $1,650/yr. for GNSS correction data. Access is limited to a specific region.

Variable accuracy

Although real-time kinematic (RTK) navigation systems typically advertise sub-inch repeatable accuracy, in most cases the actual accuracy you experience in the field can vary from sub-inch to plus or minus 2 to 3 in. or more, depending on how far you are from the RTK base station.

With the exception of sophisticated network correction systems, “at the end of the day, accuracy is really driven by the distance between where you are and where the RTK base station is,” says Matt Darr, precision agriculture specialist at Iowa State University (ISU). “The closer you are to the base stations, the more accurate you will be.”

The distance-versus-accuracy equation has received more attention since cellular-based RTK delivery systems broke the line-of-sight distance barrier of radio-based systems. “RTK is RTK,” Darr says. “We have seen CORS [cellular-based] networks produce sub-inch accuracy just like radio-based RTK.”

A recent ISU test highlights typical RTK accuracies at various distances from a base station. Accuracy was sub-inch at 1 mile, 1.5 in. at 10 miles and 3.5 in. at 30 miles with GPS+GLONASS equipment. Accuracies are not static and can be reduced by various factors, including trees, hills and other obstructions. These obstacles can reduce the number of common satellites visible by the rover receiver and the base station, reducing accuracy.

Trimble’s VRS Now cellular-based network solution gets around the distance-accuracy equation. It accomplishes this through software that customizes corrections for the specific location of the tractor based on data collected from several nearby reference stations. Other networked systems, such as the Leica-based Iowa Department of Transportation Real Time Network (IaRTN), use other approaches to generate network-based corrections.

Network-generated corrections assure consistent accuracy regardless of the distance from the nearest base station, says Trimble's Pfitzer. Unlike a single base-station solution, if a VRS base station goes down, the VRS system can compensate and continue providing a correction signal, he adds.

If this occurs when using a single base station, a grower could switch to an adjacent base station, which likely would reduce accuracy, Darr says.

Although network RTK corrections offer improved reliability and network redundancy, they don’t necessarily provide improved accuracy compared to a single base-station solution, notes John Fulton, a precision ag specialist at Auburn University. “Our measurements indicate that a single baseline solution is on par horizontally with networked solutions up to a 20-mile range, depending on the operating environment,” he says.

In general, Darr recommends that growers using the IaRTN use RTK corrections from a single base station for GPS-only receivers. If using a GPS+GLONASS receiver, the networked solution can provide more accurate results if the closest base station is more than 10 miles away.

Lower accuracies at extended distances from base stations are well within the needs of most growers, he adds. “Other errors, such as implement drift, can add up to more than an extra inch of RTK accuracy,” Darr says. “It all goes back to where you are farming and what you are trying to accomplish."