April 2007
Experts Answer Questions about GPS for Potato Growers

What is GPS?

D.J. Mulla: GPS stands for Global Positioning System. This is a network consisting of 24 satellites that broadcast radio signals to earth. Under the right conditions, these signals can be picked up by GPS receivers. The receivers process the signals from multiple satellites to calculate position of the receiver. Signals must be received from at least four satellites to calculate position.
Accuracy using a single GPS receiver often varies between 30 and 60 feet. That’s not good enough for agricultural operations such as herbicide spraying, fertilizing and tillage. Improvements in accuracy can be obtained by using two receivers simultaneously. One is established at a fixed known location and never moves. Readings at this base station are used to correct the location using “differential” processing (hence differential GPS or DGPS), giving an accuracy of from 3 to 15 feet in most cases.

Additional improvements in accuracy can be obtained by using other information in the signal broadcast by the satellites (the carrier wave signal). This is the basis of “real time kinetic” or RTK GPS data, which has an accuracy of from 4 inches to 3 feet. Further improvements using dual frequency RTK receivers can provide location accuracy on the order of half an inch or so.

Jeff Adams: GPS is designed to provide worldwide positioning services with an accuracy ranging from 10 to 15 meters. More precise accuracies are impossible with standard GPS, due to minor timing errors and satellite orbit errors, plus atmospheric conditions that affect the signals and their arrival time on Earth.
There are three primary services for refining GPS to overcome the impact of these errors and achieve position accuracies of less than one meter:
• radio beacons;
• Space Based Augmentation Systems (SBAS) including the United States’ Wide Area Augmentation System (WAAS), Europe’s EGNOS, and Japan’s MSAS;
• privately owned L-band” satellites, including the OmniSTAR system.

Why should growers use GPS on their farms? Should every potato grower use GPS?

Mulla: Growers are increasingly turning to GPS to help determine the patterns in crop yield throughout a field. Using harvesting equipped with a GPS helps show how good the yield was at any location. Farmers are also interested in using GPS for other purposes. These include recording the locations where soil samples were taken so that maps can be prepared showing variations in soil fertility.

Other uses of GPS include the growing interest in auto steer technologies. These free up the farmer from turning the steering wheel, and reduce fatigue. Another benefit is the ability to spray or fertilize without overlapping swaths on successive turns, thereby saving spray product and eliminating missed areas or doubly treated areas.

Adams: GPS enables potato growers to much more accurately map and navigate their fields, which in turn results in cost savings in terms of time, fuel, fertilizer, spray, etc. The grower can plant, cultivate or custom-apply fertilizer, insecticides, herbicides, etc. applying precisely the amount necessary to achieve maximum yields in each part of the field. GPS also enables a grower to navigate back and forth across his field with no skips and minimal overlap achieving enough savings to pay for a GPS receiver in only a year or two.

The only potato grower for whom GPS might not be suitable would be a grower with a very small operation that wouldn’t justify the capital investment. However, it would still be possible for very small growers to share GPS products and thereby achieve a cost-effective GPS investment. To share GPS products, growers should buy products including the Satloc/CSI and Outback lines that can be easily and quickly installed and transferred from one tractor/implement to another.

How can growers get started using GPS?

Mulla: This depends on what a grower is most interested in using GPS for. GPS can be installed on a harvester, a four wheeler or a sprayer. There are also hand-held units that can be carried into the field for weed and pest scouting and linked to a personal data assistant.

Adams: A grower’s first step is to buy a suitable Differential GPS receiver. The grower should investigate whether radio beacon signals will provide him/her with the required degree of accuracy, or whether SBAS and/or OmniSTAR or other privately owned satellite systems may also be necessary. Using this information, the grower can decide whether to buy a Differential GPS receiver that receives only beacon signals, or one that also receives SBAS and/or OmniSTAR signals. During this selection process, the grower should also be looking for a receiver to which other GPS-related products can easily be added, so that his/her GPS system can continue to grow in terms of its applications and benefits. A local base station system should be used for the enhanced accuracy required in planting and cultivating.

What kind of equipment do growers need to buy? Can they use their current tractors?

Adams: In addition to a Differential GPS receiver, a grower can buy mapping software, data-logging software, flow control systems for precise spraying, variable rate control software, lightbar systems for more accurate navigation, graphic display screens, auto-steering systems that enable the grower to drive hands-free and RTK (Real-Time Kinematic) GPS systems that are accurate to within one inch. Most growers should be able to use their current tractors, but they should confirm compatibility with their dealers.

What are the costs associated with implementing and using GPS?

Mulla: The costs vary tremendously depending on the accuracy of the system. The most accurate RTK GPS systems can cost as much as $50,000, while the cheapest GPS systems can be obtained for as little as $300. The most important issue in deciding which system to purchase is the accuracy of the positioning data provided.

Does equipment have to be upgraded often?

Mulla: GPS equipment doesn’t have to be upgraded often, but the software to process the data may change as the application that uses GPS is changed.

Adams: Upgrades shouldn’t be necessary any more often than every five or six years if the grower takes the time before buying to investigate various systems and compare their capabilities with his/her long-term needs and potential applications.

For more information on GPS for agriculture, visit the following Web sites:
• http://precision.agri.umn.edu/links.shtml
• http://gps.faa.gov/GPSbasics/index.htm
• http://gps.faa.gov/FAQ/index.htm
• www.omniSTAR.com
• www.csi-wireless.com
• www.satloc.com”

75 Applewood Dr. Ste. A
P.O. Box 128
Sparta, MI 49345
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