February 10, 2006 | By: Laura Skillman

Before buying precision agriculture products, farmers should understand some of the differences within the technology. To help, University of Kentucky Agricultural Engineer Tim Stombaugh offers some suggestions.

There can be confusion and disappointment when looking to purchase a GPS system because a customer is told what the systems can do – often what top level systems are capable of – but ends up purchasing a lower level system, he said.

"You’ve got to be real careful and look through the literature," Stombaugh said. 

There are differing accuracy levels with global positioning satellite technology. This technology allows anyone with a GPS receiver to determine their location based on latitude and longitude as well as elevation.

Simple, low-cost GPS systems can be accurate within 3 to 15 feet. However, to use some of the more advanced technology in conjunction with the system, producers will need a higher accuracy. The next level is commonly called submeter accuracy and is accurate within 3 feet. This generally costs in the $1,000 to $4,000 range.

The newest technology, subdecimal systems, can get within 6 inches but is very costly and options are somewhat limited. The most accurate are RTK, real time kinematic, systems which can get within about an inch, but costs generally start at about $25,000 and users have to build their own systems.

Some critical questions to ask when looking to purchase GPS equipment include: What does accuracy mean and is it absolute or relative accuracy?

Absolute accuracy is based on some known reference point such as a fence post. Dots on the receiver associated with a GPS system with good absolute accuracy would cluster around this fence post. Dots of a GPS system with high relative accuracy would have a tight cluster pattern but the cluster may be offset from the post.

To determine which is more important depends on what practices a producer plans to use the system to accomplish. For example, if a producer plans to install subsurface irrigation and then come back in and plant between the irrigation rows, then absolute accuracy is important.

Another factor to consider is pass-to-pass accuracy, or how long between passes the accuracy will remain consistent. A final consideration is static versus dynamic accuracy. Most of the submeter systems have pretty good accuracy, but this is one more aspect of the system farmers need to consider. The finer the detail work, the more important the various accuracy levels, Stombaugh said.

Another technology that is gaining interest is light bar systems with the ultimate goal to reduce overlaps and skips in fields. The light bar system is used along with the GPS technology to guide equipment through the field from pass to pass. If the driver gets off the path, the light bar systems shows the necessary adjustments.

Stombaugh said based on UK research conducted in conjunction with the University of Tennessee, light bar use can reduce overlaps by at least 2 percent. The record-keeping capability of the system is also important, as is its ability to allow drivers to be more accurate when working in fields with lots of ground cover or when lighting is less than optimal.

Features to look for include automatic path find, look ahead capability, update rates, data logging, return to point and compatibility with other GPS equipment.

When buying a light bar, the bottom line is that the GPS system is what drives the accuracy. Also think about if it will adapt to your field patterns and the ability to upgrade in the future for accuracy and automatic steering.

Tilt compensation is another issue when using light bars. It will make passes straighter but has a questionable value unless using an auto steer system. Then it is a must, Stombaugh said.


Tim Stombaugh, (859) 257-3000 ext. 214