The approximately 29 GPS (Global Positioning System) satellites each circle the globe twice a day — 11,000 miles overhead — to receive/send radio signals to GPS receivers. Six to eight of them are directly visible to a GPS antenna at any time.
To accurately track and steer your moving tractor/sprayer/combine, your GPS receiver needs five or more tracking channels (four are needed for good 3-D position estimates; the other channels scan for satellites just coming into view to replace those leaving radio range). It should have an update rate of 5 Hz to ensure enough data are being sent, and it must be compatible to receive your desired signal(s).
Because these satellite signals produce unavoidable errors — which is understandable when radio signals travel at the speed of light (186,000 mph) 11,000 miles one way — differential correction with a stationary receiver is necessary for repeatable tasks.
To understand your GPS signal choices and ground-based stations, check out the April issue of Farm Industry News and read the story "Automated Steering Basics.”
The real-time kinematic (RTK) signal offers the highest accuracy — within 1 in. — which is repeatable pass after pass and year after year. With this accuracy, growers can now strip-till and band fertilizer in the fall, then come back in the spring and plant (day or night) directly on top of the fertilizer in the middle of the strip — without steering.
When growers begin to till and fertilize in field strips instead of broadcasting—and achieve the same or higher yields—the savings quickly become obvious.
Other growers use RTK (Deere calls its network StarFire RTK, along with its industry-exclusive RTK Extend) to plant on top of buried drip irrigation tape or on beds, as well as plant varied seeding rates or apply varied nutrient rates based on field maps.