Farm Industry News

Straight-line showdown

Most GPS autosteer systems can deliver row-after-row consistency to within a meter or less. And for most row-crop applications, that's probably all you need. But “sub-meter” won't be good enough if you want to use satellite-based navigation to band apply fertilizer in the seed zone months before you actually plant. For that, you'll need a system that's accurate to within an inch or two.

Which systems are the most accurate? A recent study from the University of Illinois compared the accuracy of eight DGPS agricultural guidance systems. Researchers compared the “static” accuracy of the receiver/signal configurations and quantified the accuracy of the systems “on the go” in parallel-swathing applications.

When the researchers evaluated the pass-to-pass accuracy of the various receiver/signal configurations, they found that the John Deere StarFire receiver and SF2 signal provided the smallest error of any system tested — an average of 1 in. in 61 tests across a range of speeds.

Australian competitor

Not all GPS systems were evaluated in the Illinois test. Beeline Navigation, for example, installs its system on Terragators and Raven variable rate systems. The top-end, $42,000 system can be installed on wheeled equipment to get 1-in. accuracy at normal speeds, with sub-meter accuracy during high-speed spraying up to 20 mph. The Beeline Arro for row-crop farmers costs about $17,500 and handles most spraying, tillage and some planting operations.

The Beeline computer runs advanced software that combines information from a GPS receiver, a base station (via a two-way radio link), wheel angle sensor and an inertial sensor to provide fast and highly accurate updates on the position and orientation of the vehicle. This information is then used to provide visual guidance to the operator, in addition to manipulating steering actuators that have been installed on the vehicle. The system can keep the rig going straight even if the GPS signal is lost for up to 30 sec.

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Three-part system

Deere's AutoTrac system for hands-free steering is currently available only on John Deere Track (T) tractors, but the company plans to make it available for wheeled tractors, combines and self-propelled sprayers in the near future.

Deere sees AutoTrac as a component of a three-part system: the GreenStar display, the mobile processor and the StarFire receiver. As long as a producer plans to stick with Deere equipment, the system allows him to spread the investment over more uses and applications.

Adding AutoTrac to an existing GreenStar system costs $6,000. Starting from scratch buying all the components brings the total cost up into the mid teens.

Deciding whether you need GPS guidance for your equipment and then choosing the best system for your operation requires a complex analysis of price, specific needs of your farm and compatibility with existing farm machinery. In addition to hardware and software, you'll need to decide if you want to pay a monthly subscription fee. Deere and other companies charge a variable monthly rate for their premium signals, but the Wide Area Augmentation System (WAAS) signal is almost as accurate and is available free throughout North America.

Pass-to-pass error of GPS receiversa
GPS receiver Average (inch) Range (inch) 95% probabilityb Two-inch success rate (%)c
(John Deere)
1.0 0.4-3.5 2.0 95
(John Deere)
2.7 1.6-5.1 4.3 38
(John Deere)
3.7 1.2-8.3 8.1 29
3.9 0.8-17.7 7.9 27
6.5 1.2-17.7 13.7 7
3.9 0.8-8.3 7.5 14
(RHS Inc.)
3.0 0.8-8.3 6.8 44
Garmin GPS17N/WAAS
(Garmin Limited)
15.5 3.5-44.9 33.6 0
aAverage results over 61 tests for each receiver/signal configuration. All tests were conducted on a north-south field run.
bIndicates that if a similar test is conducted, there is a 95% chance that pass-to-pass error will be equal to or less than that of the resulting number.
cIndicates the probability of achieving pass-to-pass error equal to or less than 2 in.
How to choose a GPS lightbar
With their lower price and easy installation, the new generation of lightbar systems may be a good compromise if you want to drive straighter without making the leap to full autosteer.

Thanks to the Wide Area Augmentation System (WAAS), you can now tap into a free GPS satellite signal that's comparable in accuracy to most subscription fee services. Just install the equipment on your tractor, make a few adjustments, and you're ready to go. Steer toward the flashing arrow or dot on the display and you're assured of staying on a straight course.

As implements get wider and tractors get faster, driving straight is an increasingly difficult feat to accomplish without GPS. But with more than 10 different brands of lightbar guidance systems available, which one should you choose?

Key performance factors

Researchers at Ohio State University recently tested seven lightbar guidance systems to see which drove the straightest lines. Although the researchers wouldn't reveal the brand names of the products tested, they did say accuracy was fairly consistent for most of them. Which isn't too surprising, considering that all the systems use WAAS.

What's really interesting about the OSU study, though, is what researchers identified as the key accuracy factors to consider when comparing guidance systems. Like links in a chain, each component determines the overall strength of the total system.


First, the obvious factor — the GPS receiver. It needs to be able to pick up the GPS signal and to be reliable enough to not break down. How long does it take the system to acquire a signal? How often does it lose the signal? Will you be able to get service quickly if it breaks down?


The next factor to look at is the guidance algorithm. Your unit's speed, reliability and accuracy depend on this mathematical set of instructions. Although you want to start out with something that works and isn't full of bugs, the system should be readily upgradable to the newest guidance algorithm. Most companies upgrade on a regular basis. Some even let you download the latest update off the Internet.


The most accurate system in the world will be useless if the display (visual interface) is difficult for you to see and use. The OSU researchers noted that operator response differs with various types of guidance systems. Older operators, for instance, tend to prefer lightbar guidance, whereas younger operators prefer 3-D color guidance systems. Also, operators may prefer different locations for installing the guidance display.


Is your driving style closer to that of Mario Andretti or Grandpa Clyde? Driving with a guidance system may let you drive faster, but you've still got to steer. Kids who grew up with video games such as “Night Driver” might think a lightbar on a tractor is a piece of cake. Or they might get overconfident and trade speed for accuracy. Even with a lightbar, driving fast leads to less accuracy. But the more you practice, the straighter you'll be able to drive.

Fine tuning

The sensitivity setting on the lightbar could significantly change the accuracy of your guidance system. Increasing the sensitivity will improve accuracy, but only to a point. Too much sensitivity will make the lights too jumpy, which increases driver strain.

You also need to strike a balance on the DGPS filter setting. In general, using the filter improves guidance accuracy in a straight line but will cause problems in turning and finding the next swath line.

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