Precision Agriculture: GPS and Differential Corrections

Precision Agriculture  Precise information on agronomic factors.  Precise selection and placement of crop inputs.  Precise position and travel information.

Global Positioning System (GPS)  Space and land based information system to determine precise location and travel information.

GPS: Space System  Satellites: circle the earth in high orbit every 12 hours.  Constellation: cluster of 24 satellites (with backups) surrounding the earth.

GPS Constellation

Space segment- 24 NAVSTAR satellites in outer space

User Segment- The units that receive satellite signals and make them useful to civilians and the military

Modes of Operation:  Real-time DGPS - the base station may be yours, or a “free” one (Coast Guard, WAAS), or a subscription (satellite).

GPS Global Positioning System Characteristics  Free  Accurate  Reliable  Worldwide  Unlimited user capacity 60’s TRANSIT – doppler system ’95 Fully Operational Condition (FOC)

 28 Rockwell satellites  7.5 yr life span 6 orbital 55 degree orbits6 orbital 55 degree orbits 2 orbits everyday2 orbits everyday Very high orbit (20,200 km)Very high orbit (20,200 km) Radius of Earth ~ 8000 mi (~12,900 )Radius of Earth ~ 8000 mi (~12,900 ) Space Segment

How GPS works… Satellite signals are broadcast at the speed of light (186,000 miles per second) Distance to satellite =11,200 miles 300,000 km/s a 1/1,000,000 sec Error =>300 m pos. Error

GPS: Potential Errors Clocks not exactly synchronizedClocks not exactly synchronized Orbit errorsOrbit errors Atmosphere ErrorsAtmosphere Errors Selective AvailabilitySelective Availability Multipath ErrorsMultipath Errors

How Serious Are These Potential Errors?  An error of 1 nanosecond ( ) can throw receiver position off by 1 foot!

Clock Errors  Atomic clocks are too expensive to put in field receivers.  Receiver clocks are not precisely synchronized with satellite clocks.

Correcting Clock Errors  Four Satellites Allows Correction of Clock Error.

Differential Global Positioning System (DGPS)  Provides a way to correct for many of the remaining potential errors. Selective AvailabilitySelective Availability Orbit ErrorsOrbit Errors Atmosphere ErrorsAtmosphere Errors

Real-time Differential Correction

DGPS: How Does It Work?  Satellite sends code to mobile receiver and to fixed receiver at precisely known location.  Fixed receiver calculates errors in signal.

DGPS: How Does It Work?  Fixed receiver station broadcasts error correction information to mobile receivers.  Mobile receiver calculates more precise location.

“Relative” Solution Trimble

Sources of Differential Correction  Satellite Transmitter  Land Based Transmitters United States Coast Guard & WAASUnited States Coast Guard & WAAS Commercial and Private StationsCommercial and Private Stations

Differential GPS Three types of differential correction combined with state of the art GPS receivers:Three types of differential correction combined with state of the art GPS receivers: FMWAAS & Coast GuardSatellite Subscription

Coast Guard Beacon

Satellite Subscription WAAS

GPS Options for Agriculture Autonomous m S/A DGPS.5 - 5m RTK Float 20cm RTK Fixed 1cm GPS m

Scouting and Locating

The Influence of Precision Agriculture !

Interfacing with Agricultural Machines  Soil Mapping  Yield Monitoring & Mapping  Weed Mapping  Moisture Mapping  pH Mapping

Interfacing with Agricultural Machines  Control Systems  Vehicle Guidance  Variable Rate Inputs

Yield Mapping  Allows creation of maps for yield rates across a field This allows farmers to precisely apply fertilizers to lower yielding parts of the fieldThis allows farmers to precisely apply fertilizers to lower yielding parts of the field Shows higher producing parts of the field where more seeds should be planted to optimize productionShows higher producing parts of the field where more seeds should be planted to optimize production

Applications in Agriculture  Grid Sampling  Variable Rate Applications  Increasing Efficiency  Yield Mapping  Safety

Grid Sampling  Farmers combine GPS satellite information with ground data to create topical maps of soil composition.  Used in 2 acre grids

Veris Technologies

Combining Information Veris Technologies

Variable Rate Applications GPS allows farmers to analyze sections of fields that need different amounts of applied chemicals.

Increase of Efficiency  Allows precise spraying with no overlapping.  Exact application amounts are applied accordingly to different parts of the field.

Management Research - GPS Locations

Guidance Systems:

Autonomous Vehicles:

Safety  Many of the GPS operated machines are equipped with sensors to alert the operator. Gives operator information on equipment in operationGives operator information on equipment in operation Tells the operators location in the fieldTells the operators location in the field  showing possibly dangerous locations

Importance of GPS  Give precise direction for custom sprayers.  Allow accurate maps to be kept for annual comparison of insect infestation, soil composition, and yield rates. Can make more efficient decisions based on the annual data.Can make more efficient decisions based on the annual data.

Positive Aspects of GPS Systems  Allow precise application  Give farmer exact maps of fields  Make farming easier  Help to improve overall efficiency in agricultural operations

Negative Points of GPS  Accuracy can be offset by reflection of signals due to interference from other signals, buildings, or atmospheric elements.  Without DGPS in place accuracy may only be within meters.  DGPS for agricultural uses can cost $3,000-$5,000 for accuracy of 3 feet or less

GPS in the Future  Advances the use of more precision farming.  Its guaranteed that GPS will operate until Due to current presidential policy and the federal radio navigation planDue to current presidential policy and the federal radio navigation plan  FAA planning a GPS Wide Area Augmentation System (WAAS). Provides on the spot differential corrections.Provides on the spot differential corrections.

Summary  GPS can provide approximate locations in the field.  DGPS can provide precise (within 1 meter) locations in the field.  Control systems are available to work with GPS/DGPS receivers to record data or control inputs.

Do you know where you are going?