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Published by University Extension, University of Missouri-System

Precision Agriculture: Global

Positioning System (GPS)
Donald Pfost and William Casady, Extension Agricultural Engineers
Kent Shannon, Associate Director of the Missouri Precision Agriculture Center
University of Missouri-Columbia

Note: Mention of companies does not imply recommen-

dation or endorsement by the University of Missouri over Range determination factors
other companies not mentioned. Each GPS satellite continuously broadcasts two
radio signals on separate L-band frequencies (the
L-band is from 1,000 to 2,000 MHz). The L1 signal
GPS technology (transmitted at 1575.42 MHz) carries two codes, a
Global Positioning System (GPS) receivers provide Coarse/Acquisition (C/A) code and a Precision (P)
a method for determining location anywhere on the code. The L2 signal (transmitted at 1227.60 MHz)
earth. Accurate, automated position tracking with GPS carries only the P code, which is encrypted so only
receivers allows farmers and agricultural service the military and other “authorized” receivers can
providers to automatically record data and apply vari- interpret it. The use of both the L1 and L2 signals
able rates of inputs to smaller areas within larger fields. and their P codes produces what is called the
A GPS receiver can be compared with a simple Precise Positioning Service (PPS) and is available to
AM or FM radio. A GPS receiver “listens” for the sig- the U.S. and allied military, U.S. government agen-
nals that are broadcast from the satellites of the cies and authorized civilian users. The system
United States Department of Defense (DOD) Global available for all civilian use accesses only the L1
Positioning System. Orbiting around the earth at an signal and the C/A code and is known as the
altitude of 12,550 miles, these satellites are in pre- Standard Positioning Service.
dictable locations; hence, we refer to the system of
satellites as the GPS constellation.
Each satellite broadcasts almanac information location on the earth are known, the GPS receiver can
containing the position of all satellites in the constella- calculate its terrestrial position. If information from four
tion. GPS receivers use the almanac to determine the satellites is available, elevation can also be determined.
position of the satellites. Minor variations in the orbits
of the satellites occur due to gravitational forces from Accuracy
the sun and the moon. The DOD continuously moni- The accuracy obtained generally depends on five
tors the satellites and adjusts the almanac information factors: 1) proper installation, 2) the degree of technolo-
to represent the actual orbits of the satellites. gy used in the receiver, 3) the number and location of
The broadcast signals also contain a precisely timed satellites, 4) errors introduced by selective availability
predictable code that a GPS receiver can use to deter- (SA), atmospheric conditions, the troposphere, the
mine how long the signal required to reach the receiver. ionosphere, and multipathing — radio signals bounc-
A microprocessor within a GPS receiver uses these ing off objects in the area, and 5) differential corrections.
delays and the position of the satellite to calculate the
distance to each satellite, and then uses this information Installation
to determine location through triangulation. GPS antennas should be mounted on the centerline
Triangulation is a mathematical method for locating of a combine, tractor or truck and above any part of the
points on a plane in three-dimensional space. If the dis- machinery that might obstruct a line of sight to a satel-
tances to each of three satellites and your approximate lite. If the cab is centered and the top of the cab is above

WQ 452 Printed with soy ink on recycled paper

other portions of the machine, a cab-top mounting may important for most agricultural applications and,
be the best location. However, on a steep side slope, a especially, for guidance with applicators and aircraft.
high mounting point will result in an error in position New technology in GPS receivers has shortened reac-
calculation due to the offset in horizontal position. quisition time. Receivers that can track 8-12 satellites
GPS and DGPS receivers may have separate are less susceptible to acquisition loss.
antennas but usually there will be a combination
antenna so that both are centered at the same location. Satellite constellations
A delay of several seconds often occurs in agri- Using triangulation to calculate position, small
cultural applications such as yield monitoring, spray- errors in distance can cause large errors in position. The
ing and fertilizer application. error in calculating position through triangulation
Example: If the antenna on a sprayer traveling 10 increases when the satellites are close together. The best
mph is mounted 30 ft ahead of the booms and a rate accuracy is produced when the receiver can pick up sig-
change at the controller is effected at the boom two nals from many widely dispersed satellites (Figure 1).
seconds later, the rate change will occur when the
booms reach the location of the antenna where the Selective availability and other errors
change was made. At any other ground speed, the To prevent an enemy from using GPS satellite sig-
rate change at the booms will not occur at the same nalsfor determining locations on earth, the DOD
location as the controller. A time adjustment must “scrambles” the signals sufficiently to introduce an
usually be factored into the system to compensate for error of about 100 meters in an uncorrected location
time delays in sensing or product application. calculation. The term for this is “selective availabili-
Electrical interference can result from electrical ty” (SA). Atmospheric, tropospheric and ionospheric
storms, power lines, 2-way radios, nearby radio trans- conditions, however, also cause distortions or errors
mitters, electric motors, microwave towers, cellular in calculating distance; natural errors due to these
phones, vehicular electrical equipment such as alterna- conditions are not easily or reliably predicted. Hence,
tors and ignition systems on spark-ignition engines, even in the absence of SA, differential corrections will
and other sources. Changing the position of the anten- still be required to accurately calculate position.
na or adding noise suppression kits may reduce inter- Multipathing, the phenomenon that creates dis-
ference problems from alternators and ignition systems. torted television signals is caused by signals that
Follow the instructions for installation of the GPS bounce off of other objects before reaching the anten-
equipment, making sure that all connections are tight. na (Figure 2). Multipathing cannot be corrected by
differential corrections.
Technology
Low-cost receivers receive signals from one satel- Differential corrections
lite at a time and require more time to determine the Stationary GPS receivers are used to calculate the
location than a receiver capable of receiving four sig- total error due to SA, variable atmospheric conditions
nals simultaneously. Usually, seven to 10 satellites are and other factors. The concept is simple. A stationary
in view at any one time and more sophisticated receiver always has a known location; because the actu-
receivers will produce the most accurate location. al positions of the satellite and the receiver are known,
Reacquisition time is the time it takes to get an the true range (distance) is known. The distance calcu-
accurate position fix after a short-time loss of satellite lated by the receiver using the broadcast signals is
signals; this may occur for a variety of reasons, known as the pseudorange, which is generally in error
including traveling near trees or buildings and losing due to the combined sources of all errors. The difference
the ‘line of sight’ to satellites. Reacquisition time is between the true range and the pseudorange is the error

Accuracy of GPS units

Accuracy of GPS units may be stated in a variety of (also known as one sigma) and 2DRMS (also known
statistical terms, which one is used may not be speci- as two sigma). The acronym RMS stands for root
fied. Most statistical definitions of GPS accuracy mean square and is approximately equal to the stan-
assume that position errors are random in nature and dard deviation (SD). If the calculated positions were
follow a normal distribution. One measure of accuracy normally distributed about the true position, then 68
is the Circular Error Probable (CEP). This term applies percent of the computed positions would be within
to horizontal position estimates. A CEP of 1 meter is +/- one standard deviation and 95 percent would be
interpreted to mean that 50 percent of the position esti- within +/- two SD’s of the true position (2DRMS).
mates will be within 1 meter of the actual position; the When comparing the accuracy of GPS units,
other 50 percent can be anywhere in the universe. make sure that the accuracies are specified in the
Two frequently used accuracy terms are the RMS same terms (CEP, RMS or 2 DRMS).

Page 2 WQ 452
 available from the Coast Guard or Army Corps of
Engineers and through commercial sources, which,
for a fee, will provide signals from a satellite or a
land-based tower. Where these sources aren’t avail-
able, or for special applications, a private differential
corrections source can be installed.
Some of the newer DGPS receivers combine the
capability of receiving differential signals from both the
Coast Guard beacons and from a satellite service. Refer
to Table 1 for a comparison of features of Coast Guard
and satellite-based differential corrections sources.
Good Satellite Geometry
Coast Guard signals
The Coast Guard signals are broadcast in the fre-
quency range of 285-325 kHz (just below the usual
AM-radio band) where radio waves travel as ground
waves and are not limited to line-of-sight reception
like FM-radio stations.
The signals are series of pulses similar to those of
the GPS satellites. Referred to as Minimum Shift
Keying modulation, the signal is less sensitive to elec-
trical interference and noise than AM-radios.
Missouri has free access to correction signals
from Coast Guard beacons located near St. Louis (@
322 KHz), Kansas City (@ 305 KHz), Tulsa (@ 299
Poor Satellite Geometry
KHz), Rock Island (@ 311 KHz), Memphis (@ 310
Figure 1. Good satellite geometry (widely and evenly spaced KHz) and Omaha (@298 KHz).
satellites) yields more accurate GPS position estimates. The range of the Coast Guard beacons is approxi-
*Deere & Company 
mately 150 miles in good weather (electrical storms
and is known as the differential correction (Figure 3). cause interference). Accuracy decreases with distance
Differential correction data can be purchased and from the transmitter. This service is expected to
used at a later time in a process known as post pro- become the choice of many agricultural users, espe-
cessing to correct the errors in recorded data. cially in Missouri where several signals are available.
However, the most common approach is to connect a A disadvantage of the Coast Guard differential
differential corrections receiver to a GPS receiver to corrections signal is the rate at which the beacon
provide real-time corrections (Figure 4). transmits or repeats messages. Most Coast Guard
Many units incorporate GPS receivers and differ- sites broadcast at 200 bits per second. At this broad-
ential corrections receivers into the same unit. These cast rate, the age of a satellite’s differential correction
are often referred to as differentially-corrected GPS can be as old as four seconds. For some applications,
(DGPS) receivers. Differential corrections signals are such as guidance, this update rate may be unaccept-
able. For guidance applications, update rates of two
to ten times per second may be required.
Typical Coast Guard beacon receivers have two
channels. One channel receives the differential correc-
tion and the other is searching for the best incoming
single path from signal. This helps to ensure against loss of a DGPS
satellite to receiver signal if at least two beacons are within range.

Satellite-based correction signals

For the user, one of the simplest types of differen-
tial corrections signals is transmitted from a geosta-
tionary satellite. Companies such as Omnistar,
Multipath signal reaches receiver later Accqpoint and Racal provide this service.
and causes errors The typical annual user’s fee ranges from $500 to
$800. The correction signal is available throughout
Figure 2. GPS signals can somtimes “bounce” off of objects in most of North America. The accuracy of high quality
their path and cause the signal to reach the receiver on a dif-
ferent path. This is called multipath error. *Deere & Company 
receivers is generally considered to range from one to

WQ 452 Page 3
 most agricultural applications is about $3,000 to
$5,000 and provides RMS accuracy of at least three
meters with a typical accuracy of one meter, which is
ge
R an sufficient for yield monitoring and grid soil sampling.
ue ge
Tr r an If you need a GPS receiver for guidance (for spray-
do
eu ing, fertilizer application, etc.), the cost may be up to
Ps
$25,000. Such systems provide accuracy down to a few
True Range — Pseudorange = Differential
inches. Since sprayers and fertilizer spreaders can trav-
Correction el fairly quickly, lower quality GPS equipment may not
update position quickly enough to be used for guid-
sum of all errors is ance or control, although GPS systems with high
differential correction update rates and accuracies in the range of one foot or
less are becoming available at lower prices.
The annual subscription cost for some differential
Base Station (known position) correction services varies with the level of service
Figure 3. A stationary receiver (base station) measures its dis- (accuracy). Some providers offer three levels of ser-
tance from each satellite (pseudorange) and then calculates vice, e.g., one provider has a premium service for bet-
the error. This error is called the differential correction. ter than 1 meter accuracy, intermediate service for
*Deere & Company  accuracies in the range of 5 meters and a basic service
for accuracies in the range of 10 meters. Typical
three meters RMS (refer to accuracy table on p. 5). approximate costs may be $600, $250 and $75 per
Interference from man-made sources is minimal. year, respectively, depending on the level of service.
Satellite-based signals may have an advantage for
operation around trees and buildings since the satel-
lite is nearly overhead at most locations and within
Coordinate systems
the DGPS receiver’s line-of-sight. Several coordinate systems are in use for mapping
and may cause problems with compatibility between
Land-based correction signals software systems. Users frequently need to transform
Several commercial land-based correction signal position data into a plane (flat) coordinate system,
services are also available for a fee. Some companies either to merge them with another data set, to plot a
put up their own transmitters to broadcast correction map of the GPS results, or to perform further calcula-
signals; these include SatLoc, Mobile Data and CSI. tions for such parameters as area, distance or direction
Some commercial service providers piggyback (plane coordinate systems are usually easier to work
correction signals onto commercial FM radio station with than geodetic coordinates). When using data and
transmitters. These sub-carriers include Pinpoint maps from several sources, coordinates must be based
Communications, DCI and others. on the same datum. The coordinate system differences,
which are caused by a different reference frame, ellip-
Private GPS receiver and radio transmitter soid and data adjustment, are significant (up to several
GPS users not covered by Coast Guard or com- hundred meters) and cannot be ignored.
mercial sources of differential corrections can install a Several commercially-available software pro-
stationary receiver and transmitter to provide their grams produced by well-known GIS vendors treat the
own differential corrections source. Few users in coordinate shifts incorrectly. The National Geodetic
Missouri will choose to buy and install their own Survey provides software (LEFTI and NADCON) at a
fixed GPS receiver and transmitter since the Midwest nominal charge to compute datum shifts. Boundary
has other choices available. coordinates on older paper copies of soil maps should
be converted to the preferred datum (probably
WGS84) before they are digitized.
Cost vs. accuracy GPS receivers can usually report position informa-
The accuracy attainable with GPS depends partly tion in more than one format. The most common format
on how much you are willing to spend, ranging from is lat/lon (latitude and longitude). Lat/lon coordinates
approximately $100 to $100,000. A low-cost (from are recorded in angular units of degrees, minutes and
$100 to $500) GPS receiver without DGPS capability seconds. One second of latitude is equal to about 30
may be sufficiently accurate for some crop scouting meters. GPS receivers may display lat/lon in degrees
applications, for navigating highways or for locating plus minutes to four decimal places (instead of minutes
your favorite fishing spot on a lake. The RMS hori- and seconds). Most geographic information system
zontal accuracy may be about 50 yards. (GIS) software is capable of using more than one format
The cost for a basic DGPS receiver suitable for and may automatically convert lat/lon coordinates to a

Page 4 WQ 452
 equations will allow calculation of coordinates at
any location. The world is divided into 60 zones
each spanning 6 degrees in longitude and extending
north and south from a latitude of south 84 degrees
to north 84 degrees.

Signal
State plane coordinate system
Differential Correction
In the 1930s, the U.S. Coast and Geodetic Survey
established a plane coordinate system for each of the
48 states. One to five zones were established in each
Moving receiver Base station state with a Lambert Conformal or a Traverse
and transmitter Mercator projection. The specific projection and the
Figure 4. In real-time DGPS, the stationary receiver transmits size of the zone was selected to fit the geometry of
the differential correction to the moving receiver via another the state and to keep distortions at or below one part
radio signal. *Deere & Company  in 10,000.

coordinate system such as Universal Transverse

Mercator (UTM) or State Plane Coordinates (SPC) to cal- Summary
culate distances in meters or feet. Together, the Global Positioning System and GPS
UTM and SPC systems project portions of the receivers provide the means for determining position
earth’s curved surface onto a flat map and report anywhere on the earth. Developed by the U.S. DOD and
locations as actual distances from a reference point in used for many purposes, GPS has also made precision
meters and feet, respectively. Hence, no conversions farming a reality. A typical configuration for on-farm
are necessary to calculate distance or area. agricultural applications includes a GPS receiver and
Commercial software available from several GPS antenna, a differential corrections receiver and antenna,
vendors will compute UTM or state plane coordinates and cables to interface differentially-corrected GPS data
from GPS data. These coordinates are usually based from the receiver to other electronic equipment such as a
on the WGS-84 datum and thus are in the NAD-83 yield monitor or a variable rate controller.
system. If these must be transformed to NAD-27, it is GPS can provide accurate position data when
advisable to do the NAD-83 to NAD-27 transforma- installed and operated properly, but can produce false
tion in geodetic coordinates, and then make the con- readings under poor conditions. Use similar statistical
version to plane coordinates as the final step. measures for comparing the performance characteris-
tics of various receivers. Few, if any, receivers will
Universal Transverse Mercator Coordinates provide accurate position estimates 100 percent of the
The UTM coordinate system is a worldwide time. Even in the absence of intentional dithering of
system originally adopted by the U.S. military in signals known as selective availability (SA), differen-
1947, and since has been widely used by civilian tial corrections receivers are necessary to account for
mapping in many countries. The UTM system is other sources of error to provide the accuracy
consistent throughout the world and one set of required for precision farming.

Table 1. Comparison of Coast Guard and satellite differential correction sources by feature.

Feature Coast Guard beacon Satellite Differential

Accuracy (RMS) <1 m (depends on distance from <0.75 m uniform over service
area beacon station) - depends on service provider

Initial equipment cost lower initial cost higher initial cost

Annual subscription cost none in USA but signal not available $500 to $1000 per year,
in many regions of the US depending on level of service

Interference susceptibility subject to local man-made minimal interference from

noise sources man- made sources

Range 100 to 250 miles large coverage area

- most of US

WQ 452 Page 5
GLOSSARY change in the propagation speed of a signal as it passes
through the ionosphere.
Anywhere fix - The ability of a receiver to start position cal-
L-band - The group of radio frequencies extending from
culations without being given an approximate location
1000 MHz to 2000 MHz. The GPS carrier frequencies
and approximate time.
(1227.6 MHz and 1575.42 MHz) are in the L band.
Bandwidth - The range of frequencies in a signal.
Meter - a metric measure of length equal to 3.28 feet.
C/A code - The standard (Coarse/Acquisition) GPS code.
Multipath error - Errors caused by the interference of a sig-
A sequence of 1023 pseudo-random, binary, biphase
nal that has reached the receiver antenna by two or more
modulations on the GPS carrier at a chip rate of 1.023
different paths. Usually caused by one path being
Mhz. Also known as the “civilian code”.
bounced or reflected.
Carrier - A signal that can be varied from a known refer-
Multi-channel receiver - A GPS receiver that can simultane-
ence by modulation.
ously track more than one satellite signal.
Carrier frequency - The frequency of the unmodulated fun-
Multiplexing channel - A channel of a GPS receiver
damental output of a radio transmitter.
that can be sequenced through a number of satellite
Carrier phase GPS - GPS measurements based on the L1 or signals.
L2 carrier signal.
P-code - The Precise code. A very long sequence of pseudo-
Channel - A channel of a GPS receiver consisting of the random binary, biphase modulations on the GPS carrier
circuitry necessary to receive the signal from single at a chip rate of 10.23 MHz which repeats about every
GPS satellite. 267 days. Each one-week segment of this code is unique
Clock bias - The difference between the clock’s indicated to one GPS satellite and is reset each week.
time and true universal time. Precise Positioning Service (PPS) - The most accurate
Code phase GPS - GPS measurements based on the pseu- dynamic positioning possible with standard GPS, based
do-random code (C/A or P) as opposed to the carrier of on the dual frequency P-code and no SA.
that code. Pseudo random code - A signal with random noise-like
Control segment - A world-wide network of GPS monitor properties. It is a very complicated but repeating pattern
and control stations that ensure the accuracy of satellite of 1’s and 0’s.
positions and their clocks. Pseudorange - A distance measurement based on the corre-
Differential positioning - Accurate measurement of the rela- lation of a satellite transmitted code and the local receiv-
tive positions of two receivers tracking the same GPS er’s reference code, that has not been corrected for errors
signals. in synchronization between the transmitter ’s clock and
Dilution of precision - The multiplicative factor that modi- the receiver’s clock.
fies ranging error. It is caused solely by the geometry Satellite constellation - The arrangement in space of a set of
between the user and his set of satellites. Known as DOP satellites.
or GDOP. Selective Availability (SA) - A policy adopted by the
Dithering - The introduction of digital noise. This is the pro- Department of Defense to introduce some intentional
cess the DOD uses to add inaccuracy to GPS signals to clock noise into the GPS satellite signals thereby degrad-
induce Selective Availability. ing their accuracy for civilian users.
Ephemeris - The predictions of current satellite position that Standard Positioning Service - (SPS) - The normal civilian
are transmitted to the user in the data message. positioning accuracy obtained by using the single fre-
Geometric Dilution of Precision (GDOP) - See Dilution of quency C/A code.
Precision. Static positioning - Location determination when the receiv-
Ionosphere - The band of charged particles 80 to 120 miles er’s antenna is presumed to be stationary on the earth.
above the earth’s surface. Ionospheric refraction - The This allows the use of various averaging techniques that
improve accuracy by factors of over 1,000.

REFERENCES
GPS: A Guide to the Next Utility. 1993. Trimble Navigation The Precision-Farming Guide for Agriculturalists. 1997. John
Ltd., P.O. Box 3642, Sunnyvale, CA 94088-3642. Deere Publishing , Dept. 374, John Deere Road, Moline, IL
Differential GPS Explained. 1996. Trimble Navigation Ltd., 61265-8098. Phone 1-800-522-7448.
P.O. Box 3642, Sunnyvale, CA 94088-3642. The State of Site-Specific Management or Agriculture. 1997.
Published by: the American Society of Agronomy, Inc.; the
*Credits: Reproduced by permission of Deere & Company, Crop Science Society of American, Inc.; and the Soil Science
Society of American, Inc.
Published with funds provided to the Missouri Department of Natural
Resources from the Environmental Protection Agency, Region VII. Target Farming: A Practical Guide to Precision Farming
To learn more about water quality and other natural resource issues, Concepts and Technology. 1996. Written and published by
contact the Technical Assistance Program at Missouri Department Ron Johnson, A.Sc.T., 511 Haslam Cres., Saskatoon, Sask
of Natural Resources, P.O. Box 176, Jefferson City, MO 65102.
Toll free 1-800-361-4827. S7S 1E7 Canada, Phone & Fax: 306-665-1610
■ Issued in furtherance of Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Department
of Agriculture. Ronald J. Turner, Director, Cooperative Extension, University of Missouri and Lincoln University, Columbia, MO 65211.
■ University Outreach and Extension does not discriminate on the basis of race, color, national origin, sex, religion, age, disability or status
as a Vietnam era veteran in employment or programs. ■ If you have special needs as addressed by the Americans with Disabilities Act and
need this publication in an alternative format, write ADA Officer, Extension and Agricultural Information, 1-98 Agriculture Building, Columbia,
MO 65211, or call (573) 882-7216. Reasonable efforts will be made to accommodate your special needs.

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