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publication 442-501

Precision Farming Tools: GPS NavigationRobert “Bobby” Grisso, Extension engineer, Biological Systems Engineering, Virginia Tech Mark Alley, W. G. Wysor Professor, Crop and Soil Environmental Sciences, Virginia Tech

Gordon Groover, Extension economist, Agricultural and Applied Economics, Virginia Tech

GPS: Its Uses and Potential Are GrowingGlobal positioning systems (GPS) are widely available in the agricultural community. Farm uses include:

• mappingyields(GPS+combineyieldmonitor),• variablerateplanting(GPS+variablerateplanting

system),• variableratelimeandfertilizerapplication(GPS+

variableratecontroller),• fieldmapping for records and insurance purposes

(GPS+mappingsoftware),and• parallelswathing(GPS+navigationtool).

ForareviewoftheprinciplesofGPStolocatespecificfieldpoints,refertothisGPS Tutorial (Trimble Navi-gationLimited,2008).GPSandassociatednavigationsystemsareusedinmanytypesofagriculturalopera-tions.Thesesystemsareusefulparticularlyinapplyingpesticides, lime, and fertilizers and in trackingwideplanters/drills or large grain-harvesting platforms.GPSnavigationtoolscanreplacefoamforsprayersandplanter/drill-diskmarkers formaking parallel swathsacrossafield.Navigationsystemshelpoperatorsreduceskips and overlaps, especially when using methodsthat rely onvisual estimationof swathdistance and/or counting rows. This technology reduces the chance ofmisapplicationofagrochemicalsandhasthepoten-tial to safeguardwaterquality.Also,GPSnavigationcanbeusedtokeepimplementsinthesametrafficpat-ternyear-to-year (controlled traffic), thusminimizingadverseeffectsofimplementtraffic.

UseofGPSnavigationinagrochemicalapplicationwithgroundequipmenthasgrownrapidly,andcommercial

applicatorsarequicklyadoptingthetool.Accordingtoa 2007 survey of thosewho offered custom applica-tion(WhipkerandAkridge,2007),82percentappliedat least someof the fertilizer/chemicals using aGPSnavigation system with a manual-control/lightbar guidance system. Twenty-nine percent said they used a GPS navigation system with an auto-control/auto-steer guidancesystemforatleastsomeoftheircustomappli-cation.Onaverage,forallcustomappliedmaterials,57percentwasappliedwithGPSlightbar,and12percentwas applied with auto-steer GPS. GPS navigation has become standard practice forU.S. aerial applicators.Cropproducersarealsostartingtoadoptthesesystems,because GPS navigation is an excellent way to improve accuracy,speed,anduniformityofapplication.

Why are navigation systems important to field operations? Automatedguidanceofagriculturalvehicles(tractors,combines,sprayers,spreaders)hasbeenmotivatedbyanumberoffactors—mostimportantistorelievetheoperator from continuously making steering adjust-ments while striving to maintain field equipment orimplement performance at an acceptable level. Thisis not surprising, considering themany functions anoperatormustmonitor,perform,and/orcontrolwhileoperating the vehicle.

Therequirementsplacedonfarm-equipmentoperatorshavechangeddrasticallywithincreasesinequipmentsize,power,multipleequipmentfunctions,andspeed—aswellasmonitorsreportingonspecificsystemper-formance.These increasingdemandson theoperatorcanresultinincreasederrorsinfunction,costs,envi-ronmentalproblems,andoperatorfatigue.

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Foam markers, a widely used technology Foammarkersarethemostcommonformofnavigationaidusedduringfertilizerandpesticideapplication.Thefoamisdroppedandusedtoaligntheapplicatordur-ingthereturnpass.Foammarkersutilizeanairpumptopressurizeatankcontainingthefoamingagent.Thepressurizedfluidcausesthefoamingagenttoflowintoan accumulating chamber. The foam collects in thischamber until the accumulated mass overcomes sur-facetension,causingafoamblobtofalltotheground.Most often the foam accumulators are placed at theendsoftheapplicatorboomor,alternately,atthecenteroftheapplicatorwhenboomswerenotutilized,asinthecasewithspinner-diskgranularapplicators.Equip-mentoperatorsusethefoamblobsleftonthefieldsur-faceasanavigationaidtoknowwheretheapplicatorhas passed.

GPS + Navigation AidsRelatively inexpensivenavigationaidsknownaspar-allel-trackingdevicesassisttheoperatorstovisualizetheir position with respect to previous passes and to recognize the need for steering adjustments. Theseaidsarecommerciallyavailable in severalconfigura-tions.Onesystemisalightbar(Figure1),whichcon-sists of a horizontal series of Light EmittingDiodes(LEDs) inaplasticcase12 inches to18 inches long.ThissystemislinkedtoaGPSreceiverandamicro-processor.Thelightbarisusuallypositionedinfrontoftheoperator,soheorshecanseetheaccuracyindica-tordisplaywithouttakingtheireyesoffthefield.Thelightbarcanbemounted insideoroutsideof thecab,andtheoperatorwatchesthe“baroflight.”Ifthelightisonthecenterline,themachineisontarget.Ifabaroflightextendstotheleft,themachineisoffthepathtotheleftandneedstobecorrected.Inlikemanner,ifabaroflightextendstotheright,themachineisofftothe right.

Software allows the operator to specify the sensitiv-ity to and distance between the swaths. Similar GPS navigationsystemshavebeenusedforaerialapplica-tionsincetheearly1990s.Also,theGPSsystemgivesthe current location of the implement and,with pasttraffic patterns, the computer interface provides theoperator directions to maintain proper swath width to matchadjacenttrafficpaths.Ifanoperatorleavesthefieldtorefilltheapplicatororisforcedoutofthefieldduetoweather,uponreturn,theoperatorcanresumeandmaintainaccurateswathwidths,andover-sprayingon previous sprayed areas is eliminated.

More advanced systems have a screen showing the swathofthemachineasitmovesthroughthefield(Fig-ure2).Earlymodelsonlyallowedstraight-lineparallelswaths,butcurrentmodelsareavailableforanycontourtrafficpattern.Areascoveredwithpreviousswathsareindicated on the screen. The advanced navigation sys-tem coupled with a variable rate spreader drive and software has the capacity to generate “as-applied”maps showing previous coverage and the application pattern.Thisprovidesanexcellent recordof thepat-ternandtimingwhileoperating in thefield.Portionsofthefieldthatarenottreated,suchaswetareas,canbemarkedinthecomputerandstoredforlateropera-tionswhenconditionspermitapplication.Allofthisisdonewithouthavingtophysicallymarkthefieldareawithflags.

GPS + Auto-Steer NavigationMore advanced navigation systems (auto-steer sys-tems) possess similar capabilities as the navigation aids and also have the additional option to automatically steer the vehicle. Auto-steeringisaccomplishedwithadevice mounted to the steering column or through the

Figure 1. Lightbar navigation system mounted in cab and in view of forward travel.

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electro-hydraulic steering system. The accuracy level ofthesesystemsisbasedonthequalityofdifferentialcorrection and internal data processing: as the accu-racyimproves,thecorrespondingcostincreases.Thesenavigationsystemsareclassifiedinthreecategories:

Submeter accuracy usually means approximately two feet to four feet year-to-year, and less than onefoot pass-to-pass errors. The differential correctionsourcecouldbe fromaCoastGuardbeacon,WAAS(WideAreaAugmentationSystem),orsatelliteprovid-ers. These systems are relatively inexpensive (about $6,000to$15,000)andcanbeusedwhileperformingtillage,sometypesoffertilizerandchemicalapplica-tions, seeding, and harvesting.These devices can beeasilytransferredbetweenvehicles,sothesamesteer-ingsystemcanbeusedondifferentvehicles.However,operationsrequiringhighlyaccurateguidancearenotfeasiblewithsubmeterlevelequipment.

Decimeter accuracy provides approximately fourinches to eight inches year-to-year and three inches to five inchespass-to-pass errors.This canbe achievedusing either a local base station or dual frequencyreceiverswith private satellite differential correctionsubscription.With the increasedperformance,opera-torscanuseauto-steeringduringmostoftheconven-tional field practices. Prices range from $15,000 to$25,000 plus the satellite subscription (up to $1,500annually).

Centimeter accuracy can be obtained by using a local base stationwith the real-time kinematic (RTK) dif-ferential correction. Both long-term and short-termerrors(ofapproximatelyoneinch)havebeenreportedfor these systems.Vehicles equippedwith this high-level equipment can be used to conduct strip tilling,

drip-tapeplacement,landlevelingandotheroperationsrequiringsuperiorperformance—aswellasvirtuallyanyothertask.Inadditiontotheabilitytoaccuratelydeterminegeographiclocation,auto-guidancesystemsusually measure vehicle orientation in space and com-pensateforunusualattitude,includingroll,pitch,andyaw (see “Additional Features” for definitions). Thepricerangesfrom$40,000to$50,000withnoannualsubscriptionfees.

GPS Navigation vs. Foam MarkersPotentialadvantagesofGPSNavigation(GN)relativetofoammarkersforagriculturalapplicationsinclude:

• GN is more reliable and more accurate than foam markers. Usingfoammarkerscouldcauseabout10percentofthefieldtobeskippedoroverlapped.WiththeGN, theskipandoverlapratedrops toabout5percent. Some tests have shown that with an expe-riencedoperator,theskipandoverlapratecanbeaslowas1.5percent.

• GN allows accuracy at higher speeds. GPS navi-gationcanattaina13percent to20percenthigherspeedthanafoammarker(BuickandWhite,1999).Naturally,anincreaseofspeedisterraindependent.Iffieldconditionslimitspeed,thenaGNbenefitisunlikely.

• GN is a possibility with spinner spreaders (Fig-ure 3). Foammarkers are not generally usedwithspinner spreaders. The spreaders have no boom on whichthefoamequipmentcanbeinstalled.Duetothespreadwidth,afoammarkerinthecenterofthemachinepathisdifficulttoseefromthenextswath,and the driver would still be using a visual estimate forthespreaderswath.

• GN is easy to use.Anybodycan learn touseGPSnavigation systems, regardless of computer skills.Thesystemsrequireonlyalittlepractice—typicallyabout30minutes.

• GN provides effective guidance over growing crops. With solid-seeded crops, foam tends to fallthrough the canopy to the ground where it is almost invisible, contributing to skipping or overlapping.CropheightdoesnotaffectGPS.

Figure 2. Display screens for GPS navigation systems.

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• GN allows operation when visibility is poor. GN works at night, in dust, or in fog. This lengthensworking timeduringcriticalplantingandsprayingperiods.Inmanyareas,nighttimeisbestforspray-ing because of low wind speed. Leaf orientationchanges at night and herbicide may or may not be as effectiveasduringdaylight(checkwithlocalExten-sionspecialists).Nighttimesprayingofinsecticidescan protect sensitive bee colonies.

• GN is less affected by weather. In certain condi-tions such as low humidity, heat, and large fields,foammarkerscanberenderedineffective.Thefoamcanevaporatebeforetheoperatormakesthereturnswath.GNworksatanytemperature,includinglowtemperatureswhenfoamsystemsfreeze.

• GN has lower recurring costs. GN has no moving partsor tubes toclog.Dependingon themanufac-turer, software updates are usually free to systemowners. Foam-marker systems require foam, dye,andtankcleaner.

TheprimaryrecurringcostforGPSnavigationissatel-litedifferentialcorrection.Typically, thesubscriptioncosts$600to$1,500peryearforeachGPSunit.ManyproducersalreadyhaveGPSforyieldmonitoringandmay subscribe for differential correction. For them,GNhas almost no recurring costs. Inmany areas ofVirginia,itispossibletousetheCoastGuardbeaconorWAASforGPSdifferential correction.These ser-vicesarefreeandmaybeadequateforsomeapplica-tions,includingdryfertilizerwithaspinnerspreader.Accuracyforsprayingshouldbewithinsixinches(10

centimeters); the GPS equipment supplier can pro-vide informationregarding thedifferentialcorrectionneedsforaspecificlocation.FormoreonGPSsystems,review Precision Farming Tools: Global Positioning System (GPS),VirginiaCooperativeExtension(VCE)publication442-503.

• GN reduces operator fatigue and eye strain. With thelightbarmounteddirectlyinfront,GNoperatorsdonotneedtolookbackwardorsideways.Theycandriveaccurateswathswhilelookingstraightahead.

• GN has lower setup time. Foammarkers requirethattanksbefilledanddyesbechanged.GNbeginsworkingapproximately30secondsafterthemachineis switched on.

• GN is not affected by wind or boom bounce. Blowing foam or a foam system bouncing overroughgroundattheendofalongboommayintro-ducesubstantialerror.GPSsystemsarenotaffectedby rough terrain or wind.

• GN reduces pesticide use by reducing overlaps. Ifa10percentoverlapisreducedto5percent,pes-ticideusealsoisreducedby5percent.Thesameistrueforfertilizerandseed,sousingGNisbetterfortheenvironmentandgoodforthebottomline.

• GN reduces the need for people to enter already sprayed areas. Theoperatorcanmarkwherespray-ing stopped without dismounting.

Additional FeaturesDecisions about adapting GPS navigation should bebasedonparticularfarmneeds,farmoperationproce-dures, and understanding of positioning errors. Suchissuesasvehicledynamics,trackingoftrailedimple-ments, and field terrain also need to be considered.Properalignmentandinstallationof theGPSnaviga-tion system is required for effective field operation.Poorqualityofthesteering-controlsystems,aslopedterrain,ormisalignedimplementswillcausethefieldperformanceofGPSnavigationtosuffer.

AnimportantfeatureofGPSnavigationsystemsistheabilitytofollowparticulartrafficpatternsandprovideeffective feedback so that the operator or auto-steersystem can appropriately respond. Most systems can effectivelyperformstraight-linepatterns(linearswath-ing),andmanycanfollowcontoursandotherfieldfea-tures(Figure4).

Figure 3. GPS navigation system mounted in a spinner spreader.

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AlthoughmostGPSnavigationsystemsaredesignedspecificallyforthetaskofvehiclesteering,somesys-temshavefeaturestocollectspatialdata(suchasappli-cation maps and yield maps) or to operate variable-rate controllers.Additionalfeaturesspreadthecostof thesystemamongseveraltaskswhileincreasingefficiencyinmultipleareasofthecropproductionprogram.

The documentation provided by a GPS system could be usedtoshowthattheproductorfieldworkwasappliedinspecificlocationsataspecifictime.Setbacksspeci-fied by regulatory agencies and other sensitive areas(suchasornamentalcrops,nurseries,gardens,schools,and residential areas) may be defined as “no-applyzones.”The“as-applied”map includes areas in which application occurred and rates can be recorded. It ispossibletoincludenotonlyposition,buttimeanddateinformation in an application record as well. Suchdocumentationcouldbeusedforaccuraterecordkeep-ing foragrochemical applicationandmachineopera-tionand,potentially, forregulatoryandlegaldefensepurposes.

GPS navigation products are distinguished by the com-pactnessoftheircomponentsanduserinterfaces.Somesystems cause technical challenges during installation andcalibration,whileothersmaybefunctionalinlessthananhour.Userinterfacesrangefromintuitive,col-orful,graphictouch-screendisplaystohard-keymenuswithlimitedgraphicsfeedback.

Figure 4. Examples of field traffic patterns that GPS naviga-tion can follow.

A sloped terrain makes control of vehicle dynamicschallenging.Roll(tiltfromside-to-side),pitch(move-mentfromfront-to-rear),andyaw(rotationaroundthevertical axis) alter the GPS antenna location with the projectedcenterofthevehicle(Figure5).Forexample,whendrivingacrossaslope,thehorizontalpositionoftheGPSantennaisdownhillwithrespectofthecenterofthevehicle,andtheguidancewillbeinerrordownthe slope (Figure 6). To compensate, some systemsincludegyroscopes,accelerometers,oradditionalGPSantennas. Less advanced terrain-compensation mod-ulesdealwithonlyrollandpitchanglesofthevehicle,while others can measure total dynamic attitude in six degreesoffreedomandenablethesystemtocompen-sateforvariableterrain.

Figure 5. Compensation needed based on vehicle orienta-tion and field operation.

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Severalmanufacturershaveadd-onimplementsteeringsystemsforproperimplementtracking.Compensatingforknowngeometryofthetrackingfromimplementscanbeadjustedwiththevehicle’strajectorytokeeptheimplement aligned. Optical and crop-based trackingsystems can be useful when positioning implementswith respect to established rows.

GPS Navigation CostThemost frequentlymentioneddisadvantageofGPSnavigationistheup-frontcost.Costsrangefromabout$1,500forafarmerwhoalreadyownsaGPStomorethan$14,500foracommercialapplicatorwhowantsasystemthatkeeps“as-applied”recordsaswellaspro-videsnavigation.Afullyautomaticnavigationsystemthatsteersatractor,sprayer,orcombine—withopera-torengagementonlyatfieldends—couldrangefrom$6,000to$50,000.

Current swathing aids such as foam-marker systemsrangefrom$500 to$3,000.Speed isalsoan issue infoamsystems.Thelower-costfoamsystemsareslower,buttheyworkadequatelywhenapplicationisdonewithatractor.Commercialapplicatorsoperatingat20milesper hour needmore foam output than some systemscan provide.

Costsandbenefitsvarywidelydependingonthecrop,acreagecovered,swathingaccuracyachieved,andotherfactors.Tomakecost comparisons, consult Investing in GPS Guidance Systems? VCEpublication448-076.http://www.ext.vt.edu/pubs/agecon/448-076/448-076.html.As a rule-of-thumb, a navigation system couldcostsixtimesmorethanafoam-markersystem,whichmeansthatjustificationforGPSnavigationoverfoammarkersmustbecomputedfromthebenefitside.

Estimatesoftheeconomicimpactofsprayerskipsarecomplicatedbecauseinfluenceofweedcontroloncropyield varies by crop. For example, theweed popula-tionandlong-termweedseed-bankeffectshavetobeevaluatedandassessed.For fertilizerand limeappli-cation,howmuchyield is lost if theareareceivesnoapplication?Askip ismuchmorecostly inahigher-value crop, such as vegetables or seed crops, than itwouldbeinabulkcommoditysuchascorn,soybeans,orwheat. If theskipoccurs inaverycleanfield, theyield-losseffectdue toreducedweedcontrolmaybeminimal,butinaheavilyinfestedfield,theyieldmaydroptoalmostzerointheskippedarea.Weedscientistssuggestthatthegreatesteconomiceffectofskipsmaybe thecreationofaseedbank thatwill lead toman-agement problems and greatly increased weed control costsinfutureyears.Similarly,forafieldatpH5.8,theyieldlossduetoskippinganareawithlimeapplicationwill probablybe small during thefirst year, butwillbecome greater in later years.

Bottom Line GPS navigation has many advantages over conven-tional marking devices such as foam markers, andespeciallyoverthevisual-estimationmethodforspin-ner spreaders.With an existing GPS being used foryieldmonitoringorfieldmappingand soil sampling,theGPSnavigationsystemcanincreasetheefficiencyofthefarmoragribusinesswhileminimizingadverseenvironmental impacts associated with overlapping applications. The system can also reduce operator fatigue and anxiety regarding fertilizer andpesticideapplication.Finally,useofthistechnologycandemon-strate to the nonagricultural community that advanced technologyisbeingusedtofarmefficientlyandsafely.Theadvantageof“as-applied”maps,providedbysomesystems,isdocumentationthatapplicationsweremadeat the appropriate location and rate.

AcknowledgmentsThe authorswould like to express their appreciationforthereviewandcommentsmadeby:RandyTaylor,associateprofessor,biosystemsandagriculturalengi-neering, Oklahoma State University; John Cundiff,professor, biological systems engineering, VirginiaTech;DavidMoore,Extensionagent,VCEMiddlesexCountyOffice;andBrianJones,Extensionagent,VCEAugusta, Highland, Rockingham, and RockbridgeCountyOffices.

Figure 6. Antenna positioning error on sloping terrain.

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ReferencesAdamchuk, V.I. 2008. Satellite-based auto-guidance.University of Nebraska–Lincoln, Extension publica-tionEC706.www.ianrpubs.unl.edu/epublic/live/ec706/build/ec706.pdf (accessedonAugust8,2008)

Buick,R.andE.White.1999.“ComparingGPSguid-ancewithfoammarkerguidance.”In:ProceedingsoftheFourthInternationalConferenceonPrecisionAgri-culture. Editors: R.H. Rust and W.E. Larson, ASA/CSSA/SSSA,Madison,Wis.

Groover, G. and R. Grisso. 2008. Investing in GPSguidance systems? VCE publication 448-076. http://pubs.ext.vt.edu/448-076/.

Whipker,L.D.andJ.TAkridge.2007.2007Precisionagriculturalservices:Dealershipsurveyresults.Work-ingpaperno.07-13.DepartmentofAgriculturalEco-nomics, Purdue University. http://www.agriculture.purdue.edu/ssmc/publications/2007_Presicion_Ag.pdf (accessedonAugust8,2008)

Virginia Cooperative Extension Precision Farming Series

Precision farming: A comprehensive approach.2002. VCE publication 442-500. http://pubs.ext.vt.edu/442-500/.

Precision farming tools: Yield monitor. 2003. VCEpublication442-502.http://pubs.ext.vt.edu/442-502/.

Precision farming tools: Global positioning system(GPS).2003.VCEpublication442-503.http://pubs.ext.vt.edu/442-503/.

Precision farming tools: Soil electrical conductiv-ity. 2007. VCE publication 442-508. http://pubs.ext.vt.edu/442-508/.

Interpreting yield maps: “I gotta yield map—nowwhat?2002.VCEpublication442-509.http://pubs.ext.vt.edu/442-509/.

WebsitesGeofocus. 1999. 18 reasons to replace foammarkerswith AgGPS Parallel Swathing. www.linco.com/rea-sons.htm#1 (accessedonAugust8,2008)

TrimbleNavigationLimited.1999.GPSvs.foammark-ers:18reasonstoreplacefoammarkerswithAgGPSParallel Swathing. http://trl.trimble.com/docushare/dsweb/Get/Document-3370/ (accessed on August 8,2008)

TrimbleNavigationLimited.2008.GPStutorial.www.trimble.com/gps/index.shtml (accessed on August 8,2008)