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."