If you're considering making the jump to an automated steering system, one of the variables you will face is GPS signal accuracy. This will be the major factor driving steering accuracy in your fields, so knowing the details of the signal you buy is critical.

After you wade through all the technical details, you will find there are three basic options:

  1. A free signal that will provide pass-to-pass accuracy of about ±13 to 14 in.

  2. An $800 to $1,200 per year “corrected” signal with a pass-to-pass accuracy of between ±1 and 4 in., depending on the service you choose.

  3. A real-time kinematic (RTK) system with accuracy of ±1 in. or better. This requires an RTK base station that will cost $6,500 to $12,000. But if an RTK network exists in your area, you can avoid this one-time expense with an annual subscription that typically costs $1,200 to $2,000 per year per equipped machine.

As you evaluate GPS signal options, there are two terms to be aware of: pass-to-pass accuracy and static (long-term) accuracy. Pass-to-pass accuracy is generally defined as the accuracy of a signal over a 15-minute period (the theoretical time to make a pass in a typical field). Static accuracy is the accuracy of a signal over a longer period, typically 24 hours. It often is stated as a statistical probability — that is, that a specific static accuracy will occur 95% of the time over 24 hours.

Those are the basics, here are the details.

Free GPS signals

DGPS or WAAS

The U.S. government developed both Differential Global Positioning System (DGPS) techniques and the Wide Area Augmentation System (WAAS), which has largely superseded it, to improve the accuracy of signals from GPS satellites.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. Data from OmniStar, a major provider of GPS signal correction, show that WAAS has an average pass-to-pass accuracy of about ±14 in. (measured in successive 15-minute segments in one-second intervals over 24 hours) and a long-term static accuracy of ±24 to 26 in. (95% of the time over 24 hours).

John Deere SF1

John Deere provides this proprietary StarFire signal free of charge to customers. It has an accuracy of ±13 in. pass to pass and a static accuracy of about 30 in., according to John Deere's Web site (www.johndeereag.com).

Subscription signals

OmniStar XP/HP

OmniStar officials say the XP signal provides a pass-to-pass accuracy of about 1 in. and will guide an implement within a 4- to 5-in. radius 95% of the time over 24 hours (static accuracy). An HP signal provides pass-to-pass accuracy of better than 1 in. and a static accuracy of ±2 to 3 in. Annual subscription fees are $800 and $1,500, respectively. The literature from steering system companies typically states that the XP and HP signals have a pass-to-pass accuracy of about ±4 to 6 in. and ±2 to 4 in., respectively.

John Deere SF2

John Deere provides this proprietary signal to customers for an annual subscription fee of $800. It has a pass-to-pass accuracy of ±4 in. and a static accuracy of about 10 in., according to John Deere's Web site.

RTK

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

Signal correction basics

The WAAS GPS signal is considerably more accurate than non-corrected signals from GPS satellites, but it is more variable than corrected signals used by more accurate automated steering systems.

Although OmniStar data show that the WAAS signal has an average ±14-in. pass-to-pass accuracy, at times the signal may provide a pass-to-pass accuracy of ±6 in., says John Pointon of OmniStar. Corrected signals provided by John Deere and OmniStar have less variability because of the signal correction techniques they employ.

Generally speaking, corrected signals are generated by measuring inaccuracies from the GPS satellites caused by charged particles in the ionosphere, as well as errors induced by satellite geometries, orbits and clock inaccuracies. These corrections are broadcast (also via satellite) to the GPS receiver in the tractor or other vehicle being steered.

In the field, a tractor driver using a WAAS-driven steering system will experience this variable accuracy as noticeable shifts in steering as pass-to-pass accuracy changes, Pointon says.

“You can get 6-in. pass-to-pass accuracy at certain times, but at some time it could drift 2 ft.,” as the number of satellites and their geometry change or other factors that cause errors occur, he says. After the 2-ft. shift, pass-to-pass accuracy could improve again for several minutes. “I wouldn't recommend planting with WAAS, but accuracy can be pretty good at times,” Pointon says. “Unfortunately, times with the best accuracy are not predictable.”

Standards in the works

As you compare steering signal options, be aware that there are currently no industry standards for measuring and reporting the accuracy of GPS signals.

The American Society of Agricultural and Biological Engineers has a working group to recommend GPS accuracy measurement standards, and major signal correction providers John Deere and OmniStar say they support setting a standard. But today, pass-to-pass and longer-term static accuracies quoted by industry may or may not be measured the same way.

“When it comes to guidance, error perpendicular to the track you are following [cross-track or pass-to-pass error] is critical,” Pointon says. “The key thing is to get a standard method of measuring this defined so that comparisons can be made.”

“I think that having a standard definition would level the playing field,” adds Curtis Hay, precision guidance team lead for John Deere's Ag Management Solutions group. “The particular metric [measuring method] doesn't matter as much as everybody agreeing on it.”