GPS Receivers: Basics and Selection

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GPS ReceiversBasics and Selection

• T.S. Stombaugh, J.D. Luck and S.A. Shearer
• Biosystems and Agricultural Engineering
• University of Kentucky

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Global Positioning System

• Space-based radio navigation facility
• System of 24 Rockwell satellite vehicles (plus a
  few obsolete) maintained by the DoD
• Altitude is 12,500 miles
• Radio signals broadcast from satellites allow
  users to calculate position

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Distance to Satellite

• Lightening vs. Thunder
• Radio signals travel at the speed of light
• Lets put some numbers on it
• Altitude 12,500mi
• Speed of light 186,000 mi/s
• Transmission Time 0.067 s
• What happens if we are off a little?
• 0.001 s 186,000 mi/s 186 mi

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Triangulation
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Satellite Position

• Satellites broadcast several pieces of
  information including
• Atomic clock corrections
• Ephemerides - parameters to describe position and
  velocity of each satellite within its orbit

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GPS Errors

• Clock
• Ephemeris
• Satellite Configuration
• Atmospheric interference
• Multipath
• Selective Availability (SA)

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Dilution of Precision
Low DOP
High DOP
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Multipath Error
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Atmospheric Interference
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Selective Availability

• The intentional dithering of timing and ephemeris
  information by the DoD intended to prevent
  hostile use of GPS technology.

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Local Area DGPS

• Establish your own base station
• Radiobeacon (USCG)
• National DGPS (NDGPS)
• Local Area Augmentation System (LAAS)
• RTK Arrays
• RTK Networks

More on these later.
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Correction Signal
Base Station
Roving Receiver
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USCGRadiobeaconCorrection
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Wide Area DGPS Sources

• Commercial satellite providers
• Omnistar
• Racal
• Deere
• Wide Area Augmentation System (WAAS)
• Global DGPS (GDPS)

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Correction Signal
Base Station
Roving Receiver
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RTK DGPS

• Real Time Kinematic or, Carrier Phase DGPS
• Accuracy of Mobile Rovers under 2.0 cm horiz.
  and 5.0 cm vert.
• Accuracy of Static Surveying Units under 1.0 cm
  horiz. and 2.5 cm vert.

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Static Accuracy
1 m
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Terms

• Error difference between measurement and actual
  value
• Accuracy lack of error
• Precision random errors
• Bias systematic errors

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Reporting Methods

• Visualize circles that contain the majority of
  data points
• CEP 50 of measurements
• 95 CEP - 95 of measurements
• 2DRMS 95.4 to 98.2 of measurements
• 1 Sigma 66.7 of measurements
• 2 Sigma 95 of measurements
• Be certain that you compare apples to apples --
  be aware of accuracies reported as CEP or 1 Sigma!

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Static Accuracy 95 CEP
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Dynamic Testing
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Ideal Path
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Test Results
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Filter Evaluation
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Purchase Considerations

• Update rate (1, 5 or 10 Hz)?
• Accuracy?
• Correction signal capabilities (Radiobeacon,
  commercial satellites, or WAAS)?
• Compatibility (RS-232 or CAN data format)?
• Reliability?
• Service after the sale?
• Expected life?

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Interfacing

• User Interface
• On-unit programming
• External programming
• Data Interface Software
• NMEA 0183
• Data Interface Hardware
• RS-232
• CAN

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Satellite Positioning Systems

• GPS (United States)
• GLONASS (Russia)
• EGNOS (European) - GALILEO

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Summary

• Update rate
• Accuracy
• Correction Method
• Compatibility
• Reliability
• Service
• Useful Life