GPS Options for Pasture
Operations
Wesley M. Porter
(Wesley.Porter@OKState.edu)
Where do I start?
• There are many GPS devices currently on the
market.
• Prices range from ~$100-thousands of dollars
• Many of these devices such as a cell phone are
dual purpose.
• Most of these devices cannot be used as
guidance aids.
Devices: Car Navigation Systems
• Car navigation systems are cheap and very easy
to use but have poor accuracy (±50 ft.).
• They have preloaded maps of defined roads and
use a distance weighted algorithm to give you
correct directions.
• They cannot be used in any off-road condition.
• This is not a device to be used in an agricultural
operation.
Devices: Cellular Phones
• Most if not all of current cell phones have GPS
capabilities.
• Some of the phones, including smart phones,
have the applications to log points and paths.
• This data can be useful for marking and
navigating to points.
• However these phones cannot act as guidance
aids and have too poor accuracy (±50 ft.) even if
they had guidance capabilities.
Devices: Handheld GPS
• Most handheld GPS’s can be purchased from any
sporting goods store for a cheap price.
• These systems can collect, export, and download
points, paths, and boundaries.
• These features can be imported to specialized GIS
software for viewing.
• Even though the units can have features downloaded
to them they are not much use in many agricultural
operations.
Devices: Handheld GPS
• Very good for recreational uses.
• Do not have the software or processor
capabilities to operate as a guidance aid, not to
mention the small screen would be very difficult
to see.
Devices: Field Computers
• Field computers can be linked with an existing GPS
system such as a Trimble, AgLeader, or John Deere
system.
• These computers usually run an agriculture based
software such as Apex or Farmworks.
• The software can be used for logging data and
implement control but not for guidance.
• The software can be used in conjunction with a
guidance system for accurate applications.
Devices: Lightbars
Devices: Lightbars
• Lightbars are an “entry” level guidance system.
• They consist of a DGPS unit, a display, and a
microprocessor/computer.
• Lightbars range from just a row of lights to some with an
LCD display screen.
• A guidance path should be set with the device and an
implement width entered.
• It is important to note that lightbars are not “autosteering” devices, but just a guidance aid. The operator
still has to manually steer the machine.
Devices: Lightbars
• The lightbars work by giving LED lights representing
the desired guidance path.
Devices: Lightbars
• The displays usually give a “off-line” distance
accompanied by the lighted LED’s.
• Many of the Lightbars with LCD screens will show the
actual guidance paths along with the current path of
travel highlighted.
Devices: Auto-Steering Systems
• A step above lightbars is the full auto-steer systems.
• These systems are offered by many companies from
the factory or can be added later to certain tractors.
• This system requires the set up of a guidance path,
but unlike using a lightbar, once the vehicle is aligned
on the desired path a button is pushed to engage
auto-steer.
• Once auto-steer is engaged the vehicle automatically
follows the guidance path without interference from
the operator until the end of the field is reached.
Accuracy and Correction
• Most of the above mentioned devices are not very
accurate.
• The typical accuracy range of the car navigation
systems and cell phones falls within the range of ±50
feet.
• Hand held GPS devices which are WAAS correction
compatible can have horizontal accuracy down to
about ±10 feet.
Accuracy and Correction
• There are many ways to improve the accuracy of GPS
units that are correction compatible.
• Most correction types are Differential correction
(DGPS) including:
– WAAS Correction, used by the aviation industry and has a
repeatable accuracy of about ±10 feet.
– Radio Beacon Correction, used and serviced by the U.S.
Coastguard and Army Corps of Engineer and has a
repeatable accuracy of foot to sub-foot pending on signal
strength.
Correction Types Continued
– The private sector such as John Deere has DGPS
correction that can be purchased for a yearly subscription
fee. The accuracy of the private correction types is very
comparable to the Beacon correction network.
– RTK or Real-Time Kinematic correction is currently the
most accurate correction available with repeatability
ranging in the inch to sub-inch range. There are two ways
to obtain RTK correction, either through a base station, or
cell phone networks.
Back to the Beginning: Where do I Start?
• You need a device with guidance and DGPS
capabilities.
• An entry level guidance system is a good start and
about all that is needed for pasture operations.
• You will have a decent accuracy and repeatibility
level which will be better than most operations
without guidance systems.
Benefits of using a Lightbar
• A primary advantage of using a lightbar is a reduction
in application errors such as overlaps and skips.
• Studies have shown that operators using foam
markers with chemical application will overlap about
5% of the machine width on each pass.
• Lightbars can help reduce overlap to less than 3%
without increasing skipped areas translating to a
reduction in chemical use as much as 2%.
Benefits of using a Lightbar
• Improved machine operation in poor visibility.
• Glare during sunrise and sunset can make it difficult
to see markers and references while the lightbar can
be easily seen in the cab of the tractor.
• Cover crops can make subsequent passes difficult to
see while the lightbar will improve this problem.
• Guidance aids can help in reducing driver fatigue by
having a reference right in front of the driver rather
than in the field somewhere.
Benefits of using a Lightbar
• Allows the operator to focus more on other machine
operations and displays.
• Most lightbar systems will record the path of the
machine and some even have the capabilities of
recording a coverage path indicating whether the
machine was applying the product or not.
Benefits of using a Lightbar
How would I determine where
my next pass should be without
skips or overlaps?
Set-Up and Use of a Lightbar
• To operate a lightbar the operator must input basic
machine/implement parameters such as width and an
operation being performed (not always necessary but is
good to have for records).
• The operator then steers the first pass through the field
creating an A-B line by pushing a button to set “A” at the
beginning of the pass and “B” at the end of the pass.
• The computer records the locations of the points and
then uses the implement width to create subsequent
parallel paths across the field.
Set-Up and Use of a Lightbar
• The operator must be alert to perform the headland
turns and realign the vehicle on the next pass.
• The off-line distance or error is compared to the
actual vehicle location and calculated guidance paths
and is displayed on the screen in conjunction with
the lighted LED’s showing the operator which
direction to steer to.
Set-Up and Use of a Lightbar
• The simplest lightbar displays are made of a single
horizontal row of lights.
• The center light indicates the guidance path.
• There are many variations in displays systems:
– Single row of lights
– Double row of lights
– LCD display screen w/ 2-D representation of the field and
guidance paths.
Set-Up and Use of a Lightbar
• Some of the earliest systems were only able to create
straight parallel swaths.
• However, most of the more modern systems have
the capabilities to guide along curved paths.
• The more modern systems can also follow other
various field patterns such as contour strips,
irregularly shaped fields, and circular patterns for
center-pivot irrigated fields.
Set-Up and Use of a Lightbar
• Guidance paths are independent of any obstacles in
the field, thus the operator must still be alert for
these because the system will not warn you of them
unless you have previously marked them.
• This means if you have an obstacle such as a tree,
fence post, or hole you can drive around it and still
realign on your path.
• Some of the path options have “adaptive” curves
that compensate for driving around an object in a
field.
Set-Up and Use of a Lightbar
• The paths can be set up to follow most irregularities
in a field such as terraces or contours.
• As with any guidance system it is very important for
the operator to remain alert during operation to
prevent damage to the machine or implement.
Set-Up and Use of a Lightbar
Common Lightbars and Relative Prices
Lightbar Name
Outback S-Lite
Cost
~$800
TopCon System 110
Raven RGL Lightbars
$815-$1120
Raven Cruizer
$1,695
Raven Cruizer II
$1,695
Trimble EZ Guide 250
~$1500
Trimble EZ Guide 500
Trimble CFX-750
AgGPS FmX integrated Display
Ag Leader L160 Lightbar
Ag Leader Edge
Ag Leader GPS 1500
$395
$2,195
$995
Conclusions
• There are many different types of GPS devices
available on the market.
• Each of these devices has its own specific use and
performs well at its designed task.
• As long as you know your needs you should not have
a problem selecting the right system.
• The right system can have many benefits for most
any agricultural operation.
Questions???