Which Type of GPS, Guidance System, Should I Select?
By Randy Price
K-State Extension and Research
Department of Biological and Agricultural Engineering
Introduction
In today’s technological world, farmers are faced with many electronic, as well as
agricultural, choices. Because all precision farming activities are now based on global
positioning system (GPS) coordinates, choosing the right GPS for a specific farm has
become extremely important. For most farmers, their GPS signal comes from their
current guidance system, so the choice is simple and usually based on price. Only a few,
however, spend the time and resources to investigate which system fully benefits their
operation. This article attempts to simplify the decision-making process.
Types of GPS
Currently, the three prominent types of GPS and guidance systems are WAAS (Wide
Area Augmentation System), Omnistar®, and RTK (real-time kinematic). (The John
Deere systems - SFI, SFII, and RTK - mimic these systems in both scope and accuracy.)
WAAS, Omnistar®, and RTK are essentially the same except that they employ a variety
of differential correction techniques. (As a reference, a stand-alone GPS without
differential correction is rated for 100 feet or worse resolution.)
WAAS is a free system supplied by the federal government. Most GPS receivers made
after the year 2000 are programmed to receive this signal. Typical accuracies (guidance)
range from 2-15 inches or better, but year-to-year accuracies can vary by as much as 5
feet.
Omnistar® is similar to WAAS, but it uses extra techniques to increase accuracy.
Accuracies range from VBS (sub-meter accuracy) to XP and HP which provide 2-4
inches and 3-5 inches, respectively. (These units also have similar year-to-year
accuracies.) Fees for Omnistar® range from $750-$1500 per year.
RTK systems are regarded as the “Cadillac” of systems because antennae accuracies are
in the +/- 1 inch range. (Actual accuracies may vary as much as 2-4 inches when
factoring in tractor steering and dynamics.) These systems typically cost from $12,000$25,000 and need either a privately-owned base station, correction from an internet
signal, or a subscription to a local tower which costs $750-$1500 per year.
Considerations
Before purchasing any GPS, consider this: GPS units with WAAS correction have greatly
improved in guidance by offering pass-to-pass accuracies in the range of 12-15 inches.
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Year-to-year accuracies can still vary by as much as 3-5 feet, though. These systems
work well for most general farming purposes - general tillage, spraying, and soil
sampling - and the affordable equipment typically ranges from $1000-$5000. Also, many
of these systems offer easy upgrades to the Omnistar® system by trading in the antennae
and inputting a code. Many farmers, however, buy the RTK systems because they
provide the best all-around guidance and GPS readings for all farming operations and can
be used for most farming processes, such as banding fertilizer as set distances from crops.
When choosing a system, note the following:
-
WAAS systems have been reported as working efficiently in some areas, such as
centrally-located Kansas, and not so well in others. Some farmers have reported
these systems working accurately enough to plant corn, while others have had
different results. Potential problems include:
o Location and time of year when system was used.
o Quality of the system manufacturer – word-of-mouth recommendations
are good references for specific regions.
o Sun spots – sun spots are expected to increase to an all-time high by the
year 2012. These spots increase atmospheric noise and radio signal
interference from space, thus disrupting the GPS.
o Satellite jumping: Two satellites are available for the WAAS transmission.
A unit is typically fixed on only one satellite to prevent jumping when
making turns, etc. Satellites were changed, however, in 2007, so units
should be checked to make sure they are upgraded with the newest
software to accommodate the satellite.
-
Omnistar® systems work well, but a 2-4 inch accuracy offset has been reported
when changing directions. This may be caused by power lines and other
obstructions, such as tree lines.
o Some services, like John Deere, allow discounted partial-year
subscriptions to upgrade SFI or WAAS to the higher quality SFII or
Omnistar® signals. This can be a big savings on yearly cost since farming
operations such as combining or general tillage don’t need higher
accuracies.
-
RTK systems work well in most cases. Loss of signal and long re-acquisition
times (5-15 minutes) along tree lines have been reported but are dependent upon
the terrain, location of the unit, and placement of the base tower. New satellites
with high-power transmitters and enhanced L-code channels are being launched to
improve service. Also, new units have accelerometers and IMU’s (inertial mass
units) which continue to guide the vehicle for up to 15 minutes during loss of
signal and reacquisition.
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o Most RTK systems have an accuracy loss of 1-inch or more when greater
than one third of the distance from the base station. Accuracy offsets
increase in proportion to distance.
o Internet RTK correction, which derives its signals from governmentoperated CORS (Continuously Operating Reference Stations) and other
base stations, is a new way to obtain RTK corrections. These systems are
advantageous since only an internet connection through a wireless
provider is needed. Currently, however, these systems are not available in
Kansas and may be delayed because of funding situations, but they do
exist in ear-marked states such as Iowa and Minnesota.
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Most guidance systems, no matter the type, cannot completely control implement
drift in contoured crops, hills, or when making turns in fields. Additional
implement guidance is needed to achieve high accuracy when performing strip
tillage or other farming operations.
The following chart is a guideline for which guidance systems (GPS) are available for
certain farming operations. (Note: the actual accuracy and use of a system is dependent
upon location, application, tractor set-up, cropping system, and the manufacturer of a
specific unit.)
Farming Operation
Soil Sampling
General Tillage
Spraying
Planting Wheat
Planting Corn
Strip Tillage
Second Pass Cropping
Cultivating (2 inch)
Second Pass Banding of
Fertilizer
GPS System
Omnistar®





depends on service
marginal
marginal
WAAS




marginal
RTK









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