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)

Space Segment 28 Rockwell satellites 7.5 yr life span –6 orbital 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

Errors Encountered Satellite ErrorsObservation Errors - Orbit uncertainty - Ionospheric delay - Satellite clock model - Tropospheric delay Receiver ErrorsStation Errors - Receiver clock - Station coordinates - Receiver noise - Multipath

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.

Multipath

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.

“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: FMWAAS & Coast GuardSatellite Subscription

DGPS Hardware and Software Real Time Vs. Post Processed Single Channel Vs. Multichannel

Satellite 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

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

Precision Agriculture Yield Monitoring

Applications in Agriculture Grid SamplingGrid Sampling Variable Rate ApplicationsVariable Rate Applications Increasing EfficiencyIncreasing Efficiency Yield MappingYield 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

Combining Information

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 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.Advances the use of more precision farming. Its guaranteed that GPS will operate until 2020.Its guaranteed that GPS will operate until –Due to current presidential policy and the federal radio navigation plan FAA planning a GPS Wide Area Augmentation System (WAAS).FAA planning a GPS Wide Area Augmentation System (WAAS). –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?

Introduction GPS--Global Positioning System –A satellite-based system of signals that enables specifically designed receivers to calculate precise positions on the surface of the earth.

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

The Effects of GPS in Agriculture Tyler Bruhn, Jr. Cody Carlson Andrew Eberspacher Jon Becker

How Does GPS Work? A GPS receiver “listens” for the signals that are broadcast from 27 GPS satellites operated by the Dept. of Defense. These satellites orbit around the Earth at an altitude of 11,000 miles. It takes 12 hours for each satellite to orbit the earth.

How Does GPS Work? Triangulation –The use of three orbiting satellites to pinpoint a certain position on Earth. –The satellites calculate the time it takes the signal sent to Earth to bounce back to give a position from all three satellites information

DGPS Differential Global Positioning System –Differential GPS (DGPS) is the regular Global Positioning System (GPS) with an additional correction (differential) signal added. This correction signal improves the accuracy of the GPS and can be broadcast over any authorized communication channel

Background Information Brief History –Department of Defense GPS was invented and designed by the U.S. Dept. of Defense for military operations First satellites were launched in 1978 Second generation of GPS satellites were launched in 1989 System became fully operational in 1995

Accuracy of GPS With the military program, “selective availability” GPS was limited to 100 meters accuracy until May 2000 when selective availability was eliminated. Now accuracy is less than 1 meter.

Control segment- Monitoring control stations and a Master station

GPS: User System Receivers: instrument to monitor satellite signals to determine position.

GPS: How Does It Work? Satellite Position –Each satellite in the constellation is in high orbit, 11,000 miles plus, its position constantly monitored and controlled by the Department of Defense.

GPS: How Does It Work? Satellites Transmit Codes –Satellites are synchronized by highly accurate atomic clocks. –Each satellite programmed to transmit a unique “pseudo random code” controlled by the clock.

GPS: How Does It Work? Receivers Compare Codes –Receivers compare satellite codes with codes stored in receiver memory. –Receivers measure time with nanosecond accuracy ( seconds). –Receivers matches code sequence and measure time difference between matching code segments.

GPS: How Does It Work? Time Difference

GPS: How Does It Work? Receiver Calculates Distance –Receiver calculates its distance from the satellite by converting the time it takes the signal to travel at the speed of light from the satellite to the receiver. –Distance = Time x Speed of Light

GPS: How Does It Work? One Satellite

GPS: How Does It Work? Two Satellites

GPS: How Does It Work? Three Satellites

GPS: Review Satellites are in precise locations around the earth. Satellites send out codes synchronized by atomic clocks. Receivers compare codes received to codes in memory.

GPS: Review Time delay converted into distance. Distance from satellites gives position on earth.

How GPS works: (4) A “spread” geometry of satellites in the sky, not too close to the horizon, is best for position accuracy.

Modes of Operation: (1) GPS - one receiver - position within 100 m (with Selective Availability off) DGPS (Differential GPS) accomplished several ways. Two receivers needed, one at a base station (with known location), and one “rover”.

Modes of Operation: (2) DGPS with post processing - data are collected by the base station and the rover at the same time. The rover data are corrected with software afterwards to determine where the rover actually was. The base station may be your own, or not.

Modes of Operation: (3) Real-time DGPS - data are collected by the base station and the rover at the same time. The base station calculates the correction and sends it by radio to the rover receiver. The rover receiver applies the correction to the current position, and records it.

GPS Receiver Features: (1) Number of channels: Each channel can track and receive the signal from one satellite. Eight channels seems like plenty. Carrier tracking: improves the accuracy of the time measurement (and thus position).

GPS Receiver Features: (2) Cold/warm start: indicates the time it takes the receiver to determine its location from start up with no information (cold start), or if they already have a recent satellite almanac, current time, and approximate position (warm start). User interface: data formats, compatible software, technical assistance, etc.

GPS Receiver Features: (2) Power requirements: DC or AC? 12, 24, or 115V? Higher power means more heat means sooner failure (typically) unless cooling equipment (fan) included. Temperature: will the system operate properly in summer temperatures, fall temperatures, winter temperatures?

Coordinate Systems WGS-84 (US) or local? Lat-lon (  N+,  S-,  E+,  W-). UTM (northings and eastings)? Most GIS mapping software will orient maps properly (N at top of paper, W at left). Spreadsheet will not orient maps properly in lat-lon unless +/- used instead of  S,  W.