Scott Rodgers

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Types of GPS Receivers

• Presented By
• Scott Rodgers
• UNC Chapel Hill, Engineering Information Services

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Topics

• Accuracy vs. Precision
• Grades of GPS Receivers
• Field Experiment
• Sources of GPS Error
• Planning a GPS Survey

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Definitions

• Precision
• Repeatability of a measurement
• Accuracy
• The proximity to the TRUE value, or accepted
  value of a measurement

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Example
In general, GPS receivers are precise but not
accurate.
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Grades of GPS Receivers

• Consumer
• Garmin Street Pilot
• Garmin eTrex
• Mapping
• Trimble Pro-XR/XRS
• Trimble GeoXH/XT
• Geodetic/Survey
• Trimble 5800
• Trimble R7/R8

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Consumer GPS Receivers

• Track the L1 code frequency only
• L1 Frequency 1575.42MHz GPS carrier frequency
  which contains the C/A-Code, the encrypted P-Code
  (or Y-Code) and the Navigation Message.
• 12 parallel channel GPS antenna
• Capable of simultaneously tracking 12 satellites.
• Track all visible satellites
• Accuracy
• 15-30 meters

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Mapping Grade GPS Receivers

• Track the L1 code and carrier frequencies
• Some models capable of tracking L2 carrier
  frequency
• Built-in software to resist multi-path error
• User definable PDOP and Elevation Masks.

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Mapping Grade (cont.)

• Differential GPS (DGPS) Capable
• WAAS Wide Area Augmentation System
• CORS Continuously Operating Reference Station
• OmniStar or other Satellite based DGPS source
• 12 parallel channel GPS antenna
• Accuracy
• Real time sub-meter
• Postprocessed 1-50 cm

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Geodetic GPS Receivers

• Track the L1 carrier/code frequencies and the L2
  carrier frequency.
• 24 parallel channel GPS antenna
• WAAS Support
• Built-in software to resist multi-path error
• Accuracy
• 5-30mm Horizontal/Vertical

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Differential GPS (DGPS)

• Definition
• an enhancement to the Global Positioning System
  that uses a network of fixed ground based
  reference stations to broadcast the difference
  between the positions indicated by the satellite
  systems and the known fixed positions.

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Differential GPS Sources

• WAAS Wide Area Augmentation System
• Developed by the FAA
• Worst case accuracy is 7.6m at 95
• This type of system is considered a Satellite
  Based Augmentation System (SBAS)

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Differential GPS Sources
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Differential GPS Sources

• Other SBAS sources
• StarFire developed by John Deere engineers to
  aid in crop yield calculations
• OmniStar Mapping and surveying applications
• EGNOS European Geostationary Navigation Overlay
  Service

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Differential GPS Sources

• CORS Continually Operating Reference Station
• National network of GPS sites that collect and
  distribute GPS data
• Sampling rates vary 1, 5, 10, 15 and 30-seconds
• Data available in Receiver INdependent Exchange
  (RINEX) format
• Some stations also broadcast a Real-time
  Differential correction signal

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Differential GPS Settings

• Auto Power The receiver tracks and locks on to
  the most powerful radio beacon signal.
• Auto Range The receiver tracks and locks on to
  the nearest radio beacon signal.
• Manual The receiver tracks only the frequency
  you specify in the Frequency field.

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Differential GPS Settings

• Web Resources for Radio Beacon Frequencies
• http//www.trimble.com/findbeacon.asp

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Comparison of different GPS receivers and
processing methods

• Trimble Pro-XR
• Terra Sync set to Production
• Terra Sync set to Precision
• Real Time DGPS
• Post Processed DGPS

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Production vs. Precision
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Real-time vs. Post-Processed
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All positions Post-Processed
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GeoXH and GeoXT

• GeoXH
• H-Star Technology
• GeoXT
• Sub-Meter GPS
• Both units
• Sub-Meter GPS
• Post-Process Files
• External DGPS Beacon
• ArcMap or TerraSync
• Microsoft Window Mobile

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GeoXH and GeoXT Uncorrected Positions
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GeoXH and GeoXT Corrected Positions
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Comparison of different GPS receivers and
processing methods

• Trimble 5700 Geodetic Receiver
• GPS Data Collected for 15 minutes
• Comparison of Raw and Post-processed positions

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Geodetic vs. Mapping Grade
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Conclusions?
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GPS Error Sources

• Satellite Geometry
• Measures of satellite geometry are called
  Dilutions of Precision or DOPs
• GDOP Overall Accuracy (3-D, Time)
• PDOP Positional Accuracy (3D)
• HDOP Positional Accuracy (2D)
• VDOP Vertical Accuracy (Height)
• TDOP Time accuracy

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GPS Error Sources

• Multi-path
• The reflection of GPS signals off buildings,
  cars, trees, etc.

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GPS Error Sources

• Atmospheric Effects
• Ionosphere
• Electromagnetic particles slow and distort the
  GPS signal
• Troposphere
• Water vapor slows and distorts the GPS signal

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GPS Error Sources

• Clock Inaccuracies and Rounding Errors
• Relativistic Effects
• Errors in Satellite Orbits

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GPS Error Sources

• Summary
• Ionospheric/Tropospheric Effects /- 5.5m
• Shifts in Satellite Orbits /- 2.5m
• Clock Errors /- 2m
• Multipath Effects /- 1m
• Calculation and Rounding Errors /- 1m

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Methods to Eliminate Error

• DGPS Real-time broadcast solutions
• PDOP Setting receivers so data cannot be
  collected when PDOP is greater than 6.
• SNR Signal to Noise Ratio set to minimum of 4.
• Elevation Mask Setting receivers to track only
  satellites 15 degrees above the horizon.

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Methods to Eliminate Error

• Software and antennas designed to resist
  multi-path interference
• Avoid using high-powered CB radios

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Planning a GPS Survey

• Tools
• Trimbles QuickPlan Utility
• Satellite Visibility
• PDOP Chart
• Site Investigation
• Site Visit
• Getting the lay of the land
• Google Earth

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Planning a GPS Survey15 Degree Elevation Mask
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Planning a GPS Survey5 Degree Elevation Mask
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Planning a GPS Survey15 Degree Elevation Mask
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Planning a GPS Survey5 Degree Elevation Mask
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Five Degrees or Fifteen degrees?
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Questions?