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).

GPSGlobal Positioning System

Characteristics• Free• Accurate• Reliable• Worldwide• Unlimited user

capacity

60’s TRANSIT – doppler system’95 Fully Operational Condition (FOC)

Space Segment

• 28 Rockwell satellites• 7.5 yr life span

– 6 orbital planes @ 55 degree orbits

– 2 orbits everyday– Very high orbit (20,200 km)– Radius of Earth ~ 8000 mi

(~12,900 )

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 synchronized– Orbit errors– Atmosphere Errors– Selective Availability– Multipath Errors

How Serious Are These Potential Errors?• An error of 1 nanosecond

(0.000000001) 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 Availability– Orbit Errors– Atmosphere 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.

Differential GPS

Differential Correction Signal

“Relative” Solution

Trimble

Sources of Differential Correction

• Satellite Transmitter• Land Based Transmitters

– United States Coast Guard & WAAS

– Commercial and Private Stations

Differential GPS

– Three types of differential correction combined with state of the art GPS receivers:

FM WAAS & Coast Guard Satellite Subscription

Satellite Subscription

Coast Guard Beacon

Satellite Subscription

WAAS

GPS Options for Agriculture

Autonomous 50 - 100m S/AAutonomous 50 - 100m S/A

DGPS .5 - 5mDGPS .5 - 5m

RTK Float 20cmRTK Float 20cm

RTK Fixed 1cmRTK Fixed 1cm

GPS 7 - 10mGPS 7 - 10m

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 field

– Shows higher producing parts of the field where more seeds should be planted to optimize production

Applications in Applications in AgricultureAgriculture

• Grid SamplingGrid Sampling• Variable Rate Applications Variable Rate Applications • Increasing EfficiencyIncreasing Efficiency • Yield Mapping Yield Mapping • SafetySafety

Grid Sampling

• Farmers combine GPS satellite information with ground data to create topical maps of soil composition.

• Used in 2 acre grids

Veris Technologies

Veris Technologies

CombiningInformation

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.

Parallel Guidance Systems:

• Light Bar• Follow parallel

track, A-B line• Straight• Curves• Circles

Autonomous Vehicles:

Safety• Many of the GPS operated

machines are equipped with sensors to alert the operator.– Gives operator information on

equipment in operation– Tells 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.

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 10-20 meters.

• DGPS for agricultural uses can cost $3,000-$5,000 for accuracy of 3 feet or less

GPS in the FutureGPS in the Future

• Advances the use of more precision Advances the use of more precision farming.farming.

• Its guaranteed that GPS will operate Its guaranteed that GPS will operate until 2020.until 2020.– Due to current presidential policy and Due to current presidential policy and

the federal radio navigation planthe federal radio navigation plan

• FAA planning a GPS Wide Area FAA planning a GPS Wide Area Augmentation System (WAAS). Augmentation System (WAAS). – Provides on the spot differential Provides on the spot differential

corrections.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?