Growers exploring the automated steering world for the first time are confronted with a range of options that affect steering accuracy and the ability of systems to find the same line in a field from day to day, or from year to year. Here are some basics to help sort through the product specifications.

Pass-to-pass accuracy

The accuracy of steering systems from one field pass to another is determined primarily by the accuracy of the GPS signal arriving at the GPS receiver. The term “pass-to-pass accuracy” acknowledges that, because of ever-changing satellite locations, varying upper atmospheric conditions and myriad other factors, GPS receiver positions drift over time — that is, when measured against the location of the previous field pass. As a result, steering system manufacturers typically quote pass-to-pass accuracies that assume the next pass will take place within 15 minutes.

GPS accuracy

For practical purposes, there are three broad GPS accuracy categories available for automated steering systems: submeter (less than a meter), decimeter (less than a tenth of a meter, typically claimed to be between 2 and 6 in.), and real-time kinematic (RTK), with an accuracy of 1 in. or lower, depending on the system. The least-accurate signals are free; the others require an annual subscription fee and, in many cases, a more expensive GPS receiver and/or the operation of a separate GPS base station.

Submeter signals

DGPS or WAAS. Both the Differential Global Positioning System (DGPS) and the Wide Area Augmentation System (WAAS), which has largely superseded it, were developed by the U.S. government to improve the accuracy of signals from GPS satellites. Various automated steering system manufacturers claim that these signals provide steering accuracies of ±6 to 12 in. from pass to pass, or simply state a “submeter” accuracy. In reality, the accuracy of a WAAS signal can vary by time of day and location, so pinpointing its accuracy is a moving target.

John Deere SF1. John Deere provides this proprietary signal free of charge to customers. According to Deere, it has an accuracy of ±13 in.

Decimeter signals

OmniStar XP/HP. These DGPS signals offer pass-to-pass accuracy of ±4 to 6 in. and ±2 to 4 in., respectively, according to steering system manufacturers. OmniStar officials say the XP signal provides accuracy of about a 3-in. radius 95% of the time over 24 hours. An HP signal provides an accuracy of about a 2-in. radius. Annual subscription fees are $800 and $1,500, respectively.

John Deere SF2. John Deere provides this proprietary signal to customers for an annual subscription fee of $800. It has an accuracy of ±4 in., according to Deere.

RTK. RTK systems combine decimeter GPS signals with real-time location information from an RTK base station. These systems can provide accuracy of ±1 in. or less, although some manufacturers sell systems offering less accuracy.

Repeatability

Of the GPS signal options, only RTK systems offer the ability to determine locations precisely, independent of time-based inaccuracies associated with satellite GPS signals. Many players in the automated steering industry call this “repeatability.” Assuming an RTK base station's location remains unchanged, you can find a precise location (within the accuracy level of the system) anytime in the future.

Matching a system to the job

Sub-inch RTK-based automated steering systems are ideal for strip-till and other planting systems that require both predictable and repeatable accuracies, says Scott Shearer, an agricultural engineer at the University of Kentucky with extensive experience with automated steering systems. Making multiple in-season field passes in row crops with an automated steering system also may require RTK, although higher-accuracy non-RTK systems also may be up to certain jobs, such as spraying.

On the other hand, pulling a chisel plow or spraying a burndown herbicide doesn't require the high degree of accuracy of RTK. For these jobs, a system with submeter or decimeter accuracy may be up to the job. These lower-accuracy systems also may work well for seeding and spraying of solid-seeded crops such as soybeans and wheat.