Technology | Winter 2009
Pick the accuracy you need.
By Scott Garvey
When choosing a GPS system these days, there's more to consider than just which brand to buy. You can also pick your accuracy level. But just because pin-point accuracy of plus or minus one inch is achievable, should you spring for this high-cost option?
“It depends on what you're trying to achieve,” says Dwayne Hildebrandt of Outback Guidance, a GPS systems manufacturer.
Basic GPS systems use a WAAS (wide area augmentation signal) — the same one all navigation GPS units use. In agricultural applications, it allows for pass-to-pass accuracy of six to 10 inches. Some manufacturers claim slightly more accuracy, of six to eight inches.
For many, that's good enough, especially if you're moving away from foam markers and into GPS for spraying. “For working into the evening, a conventional foam marker just won't do the job you want,” says Sid Siefken of Trimble Agriculture, another GPS provider. With just the basic WAAS signal you'll notice a big improvement.
But for others who want more accuracy, moving up to a dual frequency system that uses a more precise signal, like that provided by Omnistar, is an option. “(Omnistar) picks up where WAAS leaves off,” says Siefken. This brings accuracy into the two to five inch range. But the extra precision requires an annual subscription fee to access the signal.
If getting it exactly right is what you want, however, RTK (real time kinematics) is the system for you. This allows for one-inch, repeatable accuracy, even year after year. RTK is precise enough to allow seed placement between the previous season's stubble rows.
One other significant advantage RTK offers is three-dimensional mapping ability. Farmers can map elevation on their fields during a harvesting pass, then come back later in the fall with a scraper and precisely contour the field to improve drainage.
