GPS Guidance Basics
Auto guidance, a technology that pilots farm machinery via GPS satellites, could help farmers boost productivity and expand their farm operations. Benefits of GPS-based guidance include: reduced skips and overlaps, ability to use in conditions of poor visibility, keeps implements in the same traffic patterns year-to-year (controlled traffic), extends hours of operation, low-skilled tractor drivers, increased yield, energy and time savings, increased application accuracy, and enhanced operation safety.
There are two major categories at which GPS-based guidance is being offered: Navigation aids and Auto-guidance.
Relatively inexpensive navigation aids known as parallel tracking devices or, more commonly, Lightbars, are being used by operators to visualize their position with respect to previous passes and to recognize the need to make steering adjustments. The Lightbars are replacement for foam marker and are suitable for fertilizer and pesticide applications.
Positional accuracy depends on the quality of the DGPS receiver supplying data to it and the driver’s ability to “follow the lights” Most DGPS receivers used with Lightbars are sub-meter accuracy. Price depends on GPS Receiver and options.
More advanced auto-guidance options possess similar capabilities with an additional option to automatically guide the vehicle. The accuracy of the auto-guidance can be described either from a long-term static test (year-to-year accuracy) or short-term dynamic test (pass-to-pass accuracy). Year-to-year accuracy is important when different field operations are expected to be performed using exactly the same passes (such as controlled traffic, harvesting, etc.). On the other hand, many field operations (such as fertilizer spreading, disking, etc.) can tolerate long-term inconsistency of measurements obtained. Therefore, errors expected within 15-minute time intervals are more commonly reported when characterizing less accurate systems. Based on the quality of differential correction and internal data processing, the guiding systems have been separated into three categories:
Sub-meter accuracy usually means approximately 2-4 foot year-to-year and less than 1 foot pass-to-pass errors. The Differential GPS source could be from Coast Guard beacon, WAAS, OmniSTAR, and John Deere StarFire1. The OmniSTAR requires an annual subscription fee. These systems are relatively inexpensive. An example of a sub-meter system while performing tillage, some types of fertilizer and chemical applications, seeding and harvesting. However, operations requiring highly accurate guidance are not feasible with sub-meter level equipment. These devices can be easily transferred between vehicles, so the same steering system can be used on different vehicles.
Decimeter accuracy approximately 4-8 inches year-to-year and 3-5 inches pass-to-pass errors are feasible with decimeter accuracy systems. This can be achieved using either a local base station or dual frequency receivers with private satellite differential correction services, such as OmniSTAR High Performance (HP) or John Deere StarFire 2 (SF2). With the increased performance, operators can use auto-guidance during most of the conventional field practices excluded above.
Centimeter accuracy can be obtained when a local base station or network of base stations with the Real Time Kinematic (RTK) differential correction is used. Both long-term and short-term errors for these systems have been reported around one inch. Vehicles equipped with this high-level An example of a RTK system equipment can be used to conduct strip tilling, drip tape placement, land leveling and other operations requiring superior performance, as well as virtually any other task. In addition to the ability to accurately determine geographic location, auto-guidance systems usually measure vehicle orientation in space, and compensate for unusual attitude, including roll, pitch and yaw.