GPS puts the Position in Precision Agriculture

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GPS puts the Position in Precision Agriculture
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Global Position Systems

• Examples of this technology use in agricultural
  machinery
• How does GPS work?
• How different types of GPS work.
• What type of GPS is used for different
  applications.

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Examples of GPS use

• Spreading
• Spraying
• Harvesting
• Precision Tractor Guidance
• Sampling

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Spreading
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Spraying
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Harvesting
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Precision Tractor Guidance
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Sampling
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What is GPS?

• A satellite based positioning system
• Three Segments
• Space Segment - 24 satellites
• Control Segment
• User Segment

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Satellite Segment

• NAVSTAR (NAVigation by Satellite Timing and
  Ranging)
• Satellites orbit in 6 orbital plates to provide
  complete coverage
• Satellites orbit at 10,900 miles (2 orbits per
  day)

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GPS Satellite
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GPS Constellation
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User Segment

• Units receive transmission from Satellites
• Cost between 100 and 15000 (depends on
  accuracy)
• Must be line of sight with satellites

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How does it work?

• Position of satellites is known.
• Satellite provides an electronic almanac to the
  receiver
• The speed of the radio signal from each satellite
  is known
• Timing allows the calculation of the distance
  from each satellite.
• Using triangulation, the position of the receiver
  is calculated.

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How measurements are possible

• Each satellite sends information on its location
  and the time code is sent.
• It sends a code that allows the measurement of
  the distance to the satellite.
• If four satellites are visible, a single receiver
  can measure locations to within 13 meters 95 of
  the time.

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GPS
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Triangulation
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Note the importance of accurate clocks
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Dilution of Precision
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Multipath Errors

• Caused by reflected signals
• Changes the distance

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True Range vs. Pseudorange

• Errors in range (distance) result in position
  errors.
• Satellite Clock Error
• Signal Transmission Error
• Error in receiver clock

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Measurements of Accuracy

• RMS (Root Mean Square) or Sigma
• 66 of the time within the value
• 1 Standard Deviation
• 2 RMS or 2 Sigma
• 95 of the time
• 2 Standard Deviations
• CEP Circular Error Probability (50)

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Accuracy Examples
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Differential GPS (DGPS)

• Used to correct for errors
• Uses a ground station at a known location
• Ground station measures error of GPS signal
• Error transmitted to DGPS unit and position is
  corrected.

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Differential GPS (DGPS)
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Types of Different GPS

• Beacon
• Ground Based
• WAAS Wide Area Augmentation System
• Satellite based
• Satellite Services (Omnistar)
• RTK
• Ground Based

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Beacon DGPS

• US Coast Guard
• Near navigable waters
• Free
• Other Locations
• Sub-Meter Accuracy

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GPS Accuracy

• 2 RMS Values
• Assume DOP of lt 4
• Accuracy for Different Types of GPS
• Uncorrected 13m
• Omnistar (Satellite) / Beacon sub-meter to 3m
• WAAS 1-3m
• RTK 2cm

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GPS Output

• Commonly Latitude and Longitude
• Latitude
• North/South position
• Longitude
• East/West Position
• Given in decimal degrees. Ex.-119.727289 36.8227
  58West Lon. North Lat.

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Real-Time Kinematic GPS(RTK)
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Code Phase vs. Carrier Phase

• Sub meter DGPS uses code phase to determine the
  psedorange
• Limited accuracy because of the code phase
  precision
• RTK uses the Doppler effect to determine how much
  out of phase the carrier wave is.

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Carrier-Phase GPS
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Base Station

• Expect a mm of error for every km of distance
  between the mobile and base
• Transmission speed needs to be high enough to
  provide proper correction to the mobile.

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GPS Applications

• Uncorrected None
• WAAS
• New could be used for field scouting
• Beacon/Satellite
• Spraying Spreading
• Harvesting
• RTK
• Precision Guidance

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Common Terms to Know

• GPS
• DOP
• RTK
• DGPS
• RMS/Sigma