Ken OBrien

1
Principles of GPS in agriculture

• Ken OBrien

Beginning Photo Here
2
Principles of GPS in agriculture

• Course Index
• Introduction
• How GPS works
• Yield Monitoring/Mapping
• Guidance/Steering
• Summary
• Exams

Secondary Photo Here
3

• Introduction

The use of Global Positioning System (GPS)
satellites in agriculture has become very
commonplace since July 15, 1995 when the
Department of Defense declared the system had
reached full operational capability. That rapid
adoption was mainly due to two uses for GPS grid
soil sampling and yield mapping. Soil sampling
and yield mapping are still the dominant uses for
GPS in agriculture, but guidance/steering systems
are becoming a widely adopted technology as
well. This module will give a brief overview of
how GPS works and explain some of its uses in
Midwest agriculture.
4

• Introduction

GPS has several uses in Midwest agriculture, but
to fully understand the uses you must first
understand the technology itself.
Objectives

• Define what GPS is and how it works
• Explain how differential correction signals
  work.
• Clarify what causes the wide range of prices for
  GPS receivers
• Describe the basics of how GPS receivers
  communicate with other systems.
• Discuss the application of GPS in agriculture

5

• How GPS Works

The Global Positioning System (GPS)
According to the Department of Defense, GPS is a
satellite constellation that provides highly
accurate position, velocity, and time navigation
information to users. The constellation is
composed of a minimum of 24 satellites in six
orbits above the earth. Each satellite makes a
complete orbit of the earth twice per day. The
satellites transmit radio signals on two
frequencies. The two frequencies are named L1 and
L2. L1 is the less accurate of the two signals
and is the free version, open to anyone on earth.
L2 is a more accurate signal reserved for the US
military.
6
Triangulation

• GPS receivers use a process called triangulation
  to calculate a users position. It is accomplished
  by determining distance from known locations. It
  is best understood using a diagram like the one
  on this page.

1 satellite position could be anywhere along the
outside of the circle
3 satellites position is known to be where all
three circles intersect
2 satellites position is somewhere along the
intersection of the two circles
11,000 miles
7
Triangulation

• The key to triangulation is knowing your distance
  from an object with a known location. So how does
  the GPS receiver know how far it is from a
  satellite that is constantly in motion? The
  answer was in high school geometry class.
• Distance Velocity Time (DVT)
• GPS receivers calculate the time it takes the
  signal to travel from the satellite to the
  receiver. Radio waves travel at the speed of
  sound, so it could take as little as 0.06 seconds
  for the signal to travel from the satellite to
  the receiver. That means the system has to have
  extremely precise clocks, which is another signal
  the GPS satellites broadcast.

8
Triangulation

• Although the demonstration showed using three
  satellites to determine a position, it actually
  takes 4. The fourth satellite serves a few
  functions including adding a 3D position (3
  satellites only provide latitude and longitude,
  no elevation).
• If more satellites are added the accuracy of the
  position can be increased. Although there are 24
  satellites, the most satellites that can
  typically be seen at any one time is 12 because
  the other 12 are located on the other side of
  Earth and their signal is blocked.

9

• Differential Correction Signals

By using just the GPS signal itself most GPS
receivers today can determine a position to
within about 30 feet. The accuracy of the system
is degraded by atmospheric conditions, reflected
signals, and other problems. Remember the length
of time measured by a GPS receiver is only
hundredths of a second, so any interference can
drastically affect position calculation. Differen
tial signals are designed to give GPS receivers a
way to correct for interference. There are a few
types, but they all use the same principles. A
GPS receiver that has a differential receiver as
well is often called a DGPS receiver.
10

• Differential Correction Signals

In order to measure what impact interference has
on the true position (position shift), stationary
ground stations are used. These stations are
located at predetermined lat/long positions and
compare their known position to the measured GPS
position in order to calculate GPS position
shift. Once the position shift has been
calculated the system must get the information to
the GPS receiver so that it can correct the raw
GPS signal and determine its true position with
more accuracy.
11

• Differential Correction Signals

• There are at least 3 methods used to send the
  differential information back to the GPS
  receiver
• Radio towers - cover specific geographies
  (Beacon)
• Satellites (WAAS and private systems)
• Personal systems like Real Time Kinematic (RTK)

In order to receive the differential correction
signal a GPS receiver must have the correct
components. Some GPS receivers cannot receive any
differential signal, while others can receive all
three types listed above.
12
How DGPS Works
DGPS uses a ground station (base) with a known
position. The base station compares the GPS
measured position to the actual known position
and calculates the amount of error. That error is
than sent to the mobile GPS receiver either
directly from the ground station (red line, ie.
Beacon or RTK) or through another set of
satellites (blue lines, ie. - WAAS).
DGPS Satellite
GPS Satellite
Mobile GPS
Base Station
13

• Differential Correction Signals

Beacon signals originate from radio towers
maintained by the United State Coast Guard.
Beacon signals were created to provide enhanced
position accuracy for ships traveling in major
bodies of water. In the United States Beacon is a
free signal. Wide Area Augmentation System
(WAAS) is a satellite based differential
correction signal created by the Federal Aviation
Administration (FAA) for use in aviation
navigation. It is also a free signal. Some
private companies have their own satellites used
for DGPS and will sell subscriptions to the
correction signal. These systems are similar to
WAAS, but tend to be slightly more accurate.
OmniSTAR and John Deere are two companies with
their own satellite based DGPS systems.
14

• Differential Correction Signals

Real Time Kinematic (RTK) systems employ the same
principle as the Beacon system, except on a much
smaller scale. RTK uses a fixed position
receiver which can either have a measured
lat/long, or derive a lat/long position by
averaging the GPS estimated position over a long
period of time. The fixed position receiver than
analyzes the accuracy of the GPS signal and
broadcasts the needed correction to mobile
receivers. RTK systems achieve the greatest
accuracy of all of the differential systems
described here.
15
GPS Review

• It takes ___ satellites to get a 3D position? 4
• WAAS and Beacon are forms of _________?
  Differential or DGPS
• _________ Velocity Time. Distance

16

• Match the DGPS source to its description
• Beacon A signal created by the US Coast Guard
  along bodies of water. Delivered through ground
  based radio towers. Free signal
• Wide Area Augmentation system (WAAS) A free
  signal provided by the Federal Aviation
  Administration. Delivered through a system of
  satellites.
• Real Time Kinematic (RTK) A system implemented
  on a small scale. The most accurate of the DGPS
  sources. Delivered through a privately owned and
  operated system.
• OmniStar A privately owned company that
  provides satellite based differential signals to
  subscribers.

17

• GPS Accuracy

For GPS receivers used in agriculture, accuracy
is measured in two different ways. One is static
accuracy and the other is often called
pass-to-pass accuracy. They really measure two
different things, accuracy and consistency. All
GPS receivers have a measurement for both static
and pass-to-pass accuracy. Static accuracy is
long-term, absolute, and repeatable. Static
accuracy is how close the GPS estimated position
is to the true latitude/longitude position. It
measures the ability of the GPS receiver to
accurately estimate the lat/long position time
and time again, over long periods of time. In
the example target assume the red circle in the
center is the true lat/long position and the dots
represents the static accuracy of a common DGPS
receiver. Notice that the positions all fall
close to the true position, but there is some
error.
18

• GPS Accuracy

Pass-to-pass accuracy is short-term, relative,
and not repeatable. When measuring pass-to-pass
accuracy we are concerned with how close the
position is to previous positions fixed by the
GPS receiver, not the true lat/long position.
Pass-to-pass accuracy is used when talking
about using DGPS receivers for guidance or
automatic steering applications. The GPS does not
need to be accurate to the true lat/long
position, it needs to be consistent in estimating
positions relative to one another. Notice in
the second example target that the points are not
as accurate as the static measurement, but they
are much more consistent to one another.
Example of the measurement of static accuracy.
Example of the measurement of pass-to-pass
accuracy.
19

• GPS Accuracy

These three diagrams are designed to help show
the difference between static and pass-to-pass
accuracy.
Good static accuracy, poor pass-to-pass
consistency. The dots are all relatively close to
the center (true position), but they are
scattered on either side of center and not close
to one another.
Poor static accuracy, good pass-to-pass
consistency. The dots are not close to the
center, but are grouped tightly together
Both static and pass-to-pass are very good. All
of the dots are close to the center and are close
together on one side of center.
20

• GPS Accuracy

The values given are approximate as differences
exist between types of receivers that fall into
each category. Use the values for general
comparison only.
21

• GPS Accuracy

Static accuracy is most important when dealing
with items like yield data, soil sampling,
variable rate recommendations, and field
boundaries. All are items where being as close to
the true lat/long position over multiple years is
key. Pass-to-pass consistency is most important
when dealing with GPS guidance/steering systems.
In guidance being very close to the last pass
made through the field is key to prevent overlap
or skips in application. Knowing the true
lat/long position may not be required unless
there is a need to have the application be
repeatable over a long period of time.
22

• GPS Accuracy

Pass-to-pass accuracy is always equal to or
better than static accuracy. Pass-to-pass
consistency relies on the fact that variables
causing GPS interference change infrequently. For
example, if cloud cover was causing signal
interference it would probably remain relatively
stable for 15 minute periods, but could change
drastically over days, months, or years.
Figure 5. Example of two types of GPS accuracy.
Please note, drawing is not to scale. The black
lines represent the true path of a sprayer. The
red lines indicate the path as mapped by GPS. The
static accuracy of the GPS would be 2.5 and 2.0
ft, as it measures the distance between the true
path and the GPS measured path. The pass-to-pass
accuracy would be 0.5 ft (60 feet actual distance
59.5 feet measured distance). Even though there
is always 2.0ft or more static error the two red
lines only wind up 0.5 ft from being the true 60
ft apart.
23
How does GPS accuracy affect cost?
Typically, the higher the accuracy or consistency
a GPS receiver has, the more it will cost. Using
the chart that previously showed approximate
accuracy of different types of receivers makes it
easier to see the price difference. Note, many of
the receivers capable of utilizing RTK are also
capable of receiving at least some of the lower
accuracy correction signals as well. Cost is
also affected by the speed with which the GPS
receiver updates its position, weather
resistance, number of GPS satellites it can
receive, and overall quality of the product.

Costs are as of 10-30-2007
24
How GPS works

• GPS Receiver Components

25

• GPS Receiver Components

• A GPS receiver has two basic components, although
  they are not always in two pieces
• The antenna think of this as the rabbit ears on
  your TV
• The receiver the actual TV

Some GPS receivers will have the antenna and
receiver all housed in one unit (all-in-one
units).
26

• Connecting GPS receivers data format

In order to use GPS, you first have to connect
the receiver to a device capable of handling GPS
data. Most GPS receivers have the ability to
export data in a format known as NMEA 0183. This
format was created by the National Marine
Electronics Association. This format has become
almost a universal standard (at least in the
United States) for GPS data and is the preferred
format for many GPS capable devices. Some devices
use a proprietary data format and are not
compatible with NMEA 0183. The data is sent in
strings (see Fig. 8) which are all preceded by a
symbol. The strings can contain multiple pieces
of data including lat/long, time, elevation,
speed, etc.
GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24
.4,19.7,M,,,,00001F Figure 8. Example of a GPS
data string using the NMEA 018 protocol.
27

• Connecting GPS receivers cabling

There are many different cabling methods used in
agriculture, let alone all GPS applications. The
most common cabling used in agriculture for GPS
connections is the DB9 connector.
28

• Connecting GPS receivers settings

• When connecting a GPS receiver to a GPS capable
  device using NMEA 0183 there are some common
  settings you can change. These settings include
• Com port This is the communication port the GPS
  is connected to. There can be numerous com ports
  on one GPS capable device, sometimes guessing is
  the only way to determine the correct port being
  used by the GPS receiver.
• BAUD rate A setting which determines at what
  speed data is sent. Often this can be changed
  both in the GPS receiver and the GPS device. The
  two devices need to have to have matching
  settings for best connections
• Data bits The number of bits used to define one
  character. Usually set to 8
• Parity This is an error checking system for
  data transfer. Usually set to none.
• Stop bits This signifies the number of bits
  which represent the end of a data string. Usually
  set to 1.

29
Review questions

• T or F. Static Accuracy is always better than
  pass-to-pass accuracy? False
• _________ accuracy is the most important when
  dealing with guidance systems? Pass-to-pass
• __________ is the almost universal data standard
  used for GPS in the United States? NMEA0183

30

• Yield Monitoring/Mapping

Now that we have learned about what GPS is and
how it works, we will focus on how GPS is used in
Midwest agriculture. The first topic is yield
monitoring/mapping.
Yield Monitoring vs. Yield Mapping
This is a very basic difference. Yield monitoring
is the use of a combine equipped with a yield
monitor, but no GPS for mapping. Yield mapping is
when a yield monitoring system has a GPS receiver
and data logger connected in order to map and
store the yield variation throughout a field.
31

• Components of a Yield Monitor

• A yield monitor is designed to measure the flow
  of grain through a combine, moisture of the
  grain, ground speed of the combine, and width of
  the harvested pass in order to determine the
  yield/acre.
• Almost all combine grain yield monitors have
  similar components, no matter the manufacturer.
  Those components include
• Mass flow sensor
• Grain moisture sensor
• Ground speed sensor
• Header height sensor
• Clean Grain Elevator speed sensor
• Some of these are called sensors when in fact the
  sensors already exist on all modern combines, so
  the yield monitor only reads signal from the
  factory installed sensor (ground speed, elevator
  speed, header height on some models)

32

• Components of a Yield Monitor Mass Flow Sensor

• The mass flow sensor most commonly sits at the
  top of the clean grain elevator and measures the
  impact of force of the flowing grain. The more
  grain flowing through the elevator the harder the
  force against the impact plate.
• Since the speed of the grain would impact its
  inertia and therefore its measured force, the
  monitor also observes elevator speed to be
  certain it stays at a constant rate.
• Parts of a mass flow sensor
• Housing
• Impact plate
• Potentiometer to measure force of grain impact

33

• Components of a Yield Monitor Moisture Sensor

The moisture sensor measures the grain moisture
as it passes the sensor. These can be found in
numerous places on the combine depending upon
manufacturer of the yield monitor and make/model
of the combine, but are most often either on the
side of the clean grain elevator or in the grain
tank.

• Parts of a moisture sensor
• Moisture Sensor
• Housing numerous types

Figure 11. Elevator mount housing from AgLeader.
This housing allows grain to flow in one side, be
augured past the moisture sensor and than dumped
back into the clean grain elevator.
34

• Components of a Yield Monitor Other Sensors

Header Height The header height sensor is
important for yield/acre calculation as it is
what determines if the combine is actively
harvesting or not. If the head is raised the
monitor assumes the combine is not harvesting and
does not count area. It does however continue to
measure grain weight. Ground Speed The ground
speed sensor measures ground speed. This is
needed to determine the area harvested. Speed is
converted into length and the monitor takes
length multiplied by width to get area. Ground
speed is typically measured at the combine
transmission.
35

• Components for Yield Mapping GPS receiver and
  memory card

When added to a yield monitor, a GPS receiver
provides the monitor with position information in
the form of latitude and longitude data. GPS
receivers can also provide ground speed to the
monitor to replace the ground speed coming from
the combine transmission. A memory card is
required to store (log) the collected GPS data.
Most of the newer yield monitors use flash cards
which is the same technology used by digital
cameras. Some older monitors use a type of memory
called SRAM.

• Parts used for Yield Mapping
• GPS Receiver
• Memory card

Figure 13. Flash memory card and adapter used for
data logging. The larger card is called a PC
card, the smaller card is a Compact Flash (CF)
card.
36

• Yield Monitor Calibration

Proper calibration is very important in order to
get accurate data. It doesnt matter if the
monitor is used to make maps from GPS yield data,
or simply used to watch the yield while
harvesting in the field. Different brands of
yield monitors have slightly different
calibration processes, but all focus upon
collecting the correct weight, moisture, and
harvested area of the crop. Once the data has
been collected the monitor can determine yield
per acre.
37

• Yield Monitor Calibration - Weight

Probably the most important, misunderstood, and
difficult item to calibrate in a yield monitor is
weight. Weight is being measured by the mass flow
sensor. The sensor only measures a portion of the
total grain flow. Higher grain flow means more
inertia hitting the impact place and a higher
reading. Calibration is how that higher or lower
reading is converted into weight. Grain weight
is measured as wet or dry. The difference in the
two is what moisture the crop is at when
harvested. Wet weight is the actual weight of
what is being harvested at field moisture. Dry
weight is a calculated weight based upon what the
weight of the crop would be if dried to market
moisture. Wet weight is what is used in the
calibration process as it represents what the
yield monitor is trying to estimate actual
total weight. Dry weight is important as it is
what tells you actual yield per acre. For
example, if corn was harvested at 20 moisture
and the monitor used the wet weight of the grain
it would overestimate the actual yield per acre
since the crop is typically sold at 15
moisture.
38

• Yield Monitor Calibration - Weight

To calibrate a yield monitor for weight, a
quantity of grain is harvested and the yield
monitor estimates the grains weight. That load of
grain is than weighed on a scale that is known to
be accurate, this is now called the actual
weight. The actual weight is programmed into the
yield monitor and it adjusts its calibration to
get as close to the actual weight as
possible. Yield monitors require somewhere
between 1 and 4 calibration loads depending upon
the brand in order to more accurately determine
harvested weight. Most yield monitor
manufacturers recommend calibration loads between
3,000-16,000 lbs for best results.
39

• Yield Monitor Calibration Weight Linear vs.
  Curve

A yield monitor which uses one calibration load
per crop type creates a linear calibration, while
those using multiple calibration loads per crop
create calibration curves. Typically, monitors
using linear calibration methods promote the ease
of calibration of their systems. Monitors using
calibration curves make the claim that they
create more accurate measurements, especially at
flow rates outside the normal average. Which is
better is a personal preference that depends upon
what the individual values most, ease of use or
accuracy.
Figure 14. Example of a linear calibration
Figure 15. Example of a calibration curve
40

• Yield Monitor Calibration - weight

• Weight calibration must be performed for each
  type of crop harvested
• A weight calibration that was accurate may not
  always remain accurate throughout the harvest
  season.
• If accuracy drops below tolerable levels, a new
  calibration must be completed in order to make
  the monitor accurate again.

41

• Yield Monitor Calibration - Moisture

In order to determine the correct yield in dry
bushels the monitor must know the crop moisture.
Moisture is calibrated by taking a representative
sample of grain from the grain tank, measuring
the moisture content of the grain, and entering
that number in the yield monitor. Almost every
monitor uses linear calibration for moisture,
meaning you only need one calibration load per
crop type. Moisture calibration is usually
stored in the monitor as an offset. In other
words if a monitor measured the moisture at 17.4
and it is actually 15.3 than there would be a
-2.1 offset for moisture. When the next moisture
reading was taken, the monitor would subtract
2.1 from the moisture reading to correct it.
42

• Yield Monitor Calibration - Distance

Area Width Distance If you want the monitor
to calculate yield per acre it has to count
acres. That is done by entering the swath width
(number of harvested rows or platform head width)
and measuring distance covered. Distance covered
is read from the speed sensor and must be
calibrated. To perform a distance calibration, a
known distance is measured and marked off, the
combine is driven the distance and the difference
between the estimated distance and the actual
distance is calculated. The actual distance
driven is entered into the monitor and the system
is calibrated. Typically, it is recommended that
you repeat this process until you have a
measurement that is close to correct. Even if a
yield monitor is properly calibrated it can read
incorrect distance if field conditions change.
For instance, if the monitor was calibrated in
dry field conditions, but now is harvesting under
wet field conditions, there could be increased
wheel slippage that alters the measured
distance.
43

• Yield Monitor Calibration Header Height/Stop
  Height

In order to have acres come out correctly
(remember yield is determined using both weight
and area). The monitor must known then to stop
measuring area when in non-crop areas or areas
already harvested. Header height or stop height
measures the position of the combine throat in
order to determine if the operator has the header
in a raised or lowered position. When the header
is down, the monitor counts acres. When the
header is raised the monitor does not count acres
to prevent lowering the yield by measuring too
many acres. Most yield monitors will continue to
measure weight even when the head is raised in
order to be sure to not miss any grain due to the
time it takes the crop to move through the
combine.
44

• Yield Monitor Calibration The Final Product

Yield Weight / Area Yield Bushels /
Acres Once you have gotten everything correctly
calibrated the monitor will show the correct
yield/acre. All monitors have default settings
for grain weight per bushel and market dry
moisture. In some monitors these settings can be
adjusted by the user.
45

• Yield Monitor Calibration Relative data
• I didnt calibrate my yield monitor this year. I
  know it wont show me the correct yield, but it
  will still show me the relative yield difference
  between products and fields.

This statement has been made by many yield
monitor users and is both true and false. It is
true that an improperly calibrated yield monitor
will still detect yield differences. If there is
a true yield difference of 20 bushels/acre an
improperly calibrated yield monitor will most
likely detect a difference, but it may not
accurately determine the yield difference or the
actual yield. It is false because a poorly
calibrated monitor (or a monitor using out-dated
calibrations) may not keep the relativity
consistent. In other words what is actually a 20
bushel/acre yield difference may be measured as
10 or 30 bu/acre in an improperly calibrated
monitor. Grain flow changes from year to year,
causing the calibration (linear or curved) to
change. This change could make a monitor that was
very accurate last year, not accurate this year.

46

• Yield Monitor Calibration Relative data

I didnt calibrate my yield monitor this year. I
know it wont show me the correct yield, but it
will still show me the relative yield difference
between products and fields.
Figure 16 shows an example of two different
linear calibrations created for different harvest
years. In 2006 hybrid A had a flow rate of 20,
while hybrid B had a flow rate of 40. That tells
us that hybrid A yielded 50 bu/ac while hybrid B
yielded 150 bu/ac, a 100 bu/ac difference. Lets
assume due to yield differences, the flow rates
for both hybrids stay the same in 2007, even
though their yield is different. Without a
corrected calibration, they would still show a
100 bu/ac difference. With the corrected
calibration they show approximately a 70 bu/ac
difference (105-35).
20 40
Fig. 16. The effect of different calibrations on
the yield difference shown by a yield monitor
when harvesting two varieties over two years.
47

• Yield Monitor Data

Once data has been collected using a yield
monitor its time to compile the data into
reports and maps.
48

• Yield Monitor Data

• Making maps and reports can happen in numerous
  software programs. Some yield monitors require
  their own software in order to create maps, but
  data from most of the major yield monitors can be
  read into many different software programs.
• All of the programs perform similar steps in
  slightly different ways. They are trying to
• Remove outliers from the data
• Correct position errors in the data
• Create attractive maps for viewing and printing
• Store data
• Allow the data to be analyzed and queried.

49

• Yield Monitor Data - Cleaning

Cleaning yield data is a term used to describe a
process in which the raw yield data from the
monitor is edited. The most common edits applied
are removing outliers in the dataset (yield and
position), adjusting the start/stop and lag
times, and trying to fix position
errors. Removing yield outliers is usually done
either by minimum and maximum values for a crop
yield (ie. corn yield has to be at least 25 bu/ac
and cannot be higher than 400 bu/ac, anything
outside that range should be removed) or by
deleting any data outside a certain number of
standard deviations from the mean. Trying to fix
position errors is just what it sounds like. If
you have a row of points following one another
nicely and the program detects one point way out
of line from the others it can move it back into
place by referencing the other points nearby.
Likewise, if you have established field
boundaries the software can delete any points
that fall outside that boundary.
50

• Yield Monitor Data Cleaning Start/Stop Lag
  Times

One constant issue with yield mapping is that it
takes the crop time to travel through the combine
and reach the mass flow sensor. The monitor has
to adjust for that time in order to place the
correct yield data with the correct GPS location.
The delay caused by the time it takes for the
grain to travel from the header to the mass flow
sensor is called the lag time. It is often around
10-12 seconds, and can be adjusted in most
software packages in order to match the correct
GPS point with its corresponding yield.
Start/Stop time is the period of time when
first starting a new pass or when ending a pass
in which the combine is not running at capacity
and/or is changing speed. This causes some data
errors and is why some yield maps show low
yielding areas at the beginning or ending of
passes. Data collected during the start/stop time
is deleted in the software to remove inaccurate
yield readings. Start/stop time is usually
defaulted to 3-4 seconds in most software
packages
51

• Yield Monitor Data Cleaning Start/Stop Time

The start/stop and lag times can be adjusted when
making maps in most software programs. This
adjustment allows the user to be certain the map
shows accurate yield data and is not shifted.
Fig. 17. The images on the right and left show
the same raw yield data processed with different
start/stop and lag times. The map on the left
looks correct. The lower yielding areas form into
pockets. The map on the right has poor lag time
settings, causing the lower yielding pockets to
be shifted and stretched based upon the direction
of travel while harvesting.
52

• T or F. An improperly calibrated yield monitor
  will still show the difference in yield between
  two varieties, it just will not show the correct
  yield level? False
• T or F. Yield monitors use scales on the combine
  to weight each load and determine yield? False

53

• Chapter 4 GPS Guidance/Steering Systems

The use of GPS for guidance and/or steering was
mentioned during the discussion about GPS
accuracy and consistency. The goal is simple, to
guide or steer an implement to prevent skips and
overlaps.
Guidance vs. Steering
The first systems were designed to guide the
driver along a previous pass in order to prevent
skips and overlap, newer more advanced systems
actually take over the steering of the implement
in order to meet the same goals.
54

• How does it work?

Using current location, speed, and heading data
provided by a DGPS receiver a computer based
controller can determine what direction an
implement needs to be steered in order to stay on
the desired path of travel. That information can
than either be visibly displayed to a driver or
sent to an automatic steering system.
Figure 18. This shows an example of how the
guidance system can predict where the tractor
will drive to based upon its current location,
speed, and heading.
55

• How does it work?

In a guidance only system, once the system has
determined where to drive the driver is given a
visible display to where to steer in order to
stay on path. In an auto-steer or assisted-steer
system, once the driving directions have been
determined, a steering mechanisms performs the
required tasks to stay on path. The steering
system can be wired directly into the steering
mechanism of the implement making it nearly
invisible, or it can be an add-on unit that
physically moves the steering wheel.
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• How does it work?

Often the GPS antenna has to be placed at the
highest point in the implement in order to have a
clear view of the horizon. This is needed to
receive good signal from numerous satellites, but
can cause position shift in fields with
slopes. For instance, when the antenna is placed
on the roof of a tractor or combine it can be 15
feet or more off of the ground. If the field has
no slope, this is not an issue, but in fields
with slopes this can affect position accuracy.
Figure 19. An example of how GPS position can be
shifted due to slope in the field.
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• Fixing slope caused position shift?

When the slope of a field causes position shift
it could drastically affect the apparent accuracy
of the guidance system. Figure 19 shows a good
example of how sloping ground affects the
position accuracy. Some systems now incorporate
gyroscopes in order to measure the pitch, roll,
and yaw of the implement and correct the position
data accordingly.
Figure 19. An example of how GPS position can be
shifted due to slope in the field.
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Static vs. Pass-to-pass accuracy in guidance
As was mentioned before, pass-to-pass accuracy is
typically more important to guidance systems than
is static accuracy, but that does not mean that
static accuracy has no importance. In the
example below of a field being planted, you can
see that on day 1 the static accuracy was shifted
north, but the pass-to-pass accuracy was high
enough that things worked well. At the end of day
1 it rained, so it was two days later before the
farmer returned to plant the remainder of the
field. In that time the position shift had
changed to be south of the true point, which
moved the farmers line far enough that he would
have been planting a row directly on top of
another row.
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What other agricultural uses for GPS exist?

• Soil sampling used to mark sample locations so
  nutrient maps can be produced
• Grid sampling
• Area/Zone sampling
• Variable rate product application
• Uses GPS to determine the application equipments
  location and apply a predetermined amount of
  product according to a prescription map.
• Auto-section control
• Used in planters, sprayer, and basically any
  equipment that applies product to a field.
• Goal is to prevent over application of product
• Land leveling and Drainage
• Using the highly accurate RTK correction signal,
  GPS can be used to help level land, create
  terraces, or build drainage ditches