GPS Error

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GPS Error

• Atmospheric effects
• Multipath
• Satellite geometry
• Measurement noise
• Ephemeris data
• Satellite clock drift
• Selective availability (SA)

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Ionospheric Tropospheric Refraction
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Multipath
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Satellite GeometryGeometric Dilution of
Precision (GDOP)

• GDOP can magnify or lessen other GPS errors
• Wider angles better measurements
• Components of GDOP
• HDOP
• VDOP
• TDOP

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Dilution of Precision (DOP)
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(No Transcript)
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Satellite GeometryPosition Dilution of Precision
(PDOP)

• PDOP refers to horizontal (HDOP) and vertical
  (VDOP) measurements (latitude, longitude and
  altitude)
• PDOP values
• 5-8 acceptable
• 9 poor

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Ephemeris Data

• A list of the satellites positions as a function
  of time
• Each satellite broadcasts its individual
  ephemeris
• Almanac vs. ephemeris

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Selective Availability (SA)

• The accuracy of GPS signals was intentionally
  degraded by the DoD
• SA was the largest component of GPS error
• SA was turned off on May 1, 2000

SA on (100 meter spread)
SA off (3 meter spread)
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Shaw, Michael, Kanwaljit Sandhoo and David
Turner, Modernization of the Global Positioning
System, GPS World Magazine, September 2000
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GPS Error Budget

• Ionosphere..................................5.0
  meters (0.4)
• Troposphere................................0.5
  meters (0.2)
• Ephemeris data..............................2.5
  meters (0)
• Satellite clock drift........................1.5
  meters (0)
• Multipath....................................0.6
  meters (0.6)
• Measurement noise.......... ..........0.3 meters
  (0.3)
• Selective availability.....................30-100
  meters
• Total.............................................
  .... 10 meters

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Differential Correction

• GPS receiver on the ground in a known location
  (base station)
• Acts as a static reference point
• 1. Transmits error correction messages to other
  GPS receivers in the local area (real-time),
• or
• 2. Differential correction can be done on
  computer after GPS data are collected
  (post-processed)

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DGPS Sources

• Local base station (real-time or post-processed)
• Wide Area Augmentation System (WAAS, real- time)
• Nationwide Differential GPS (NDGPS) radiobeacons
  (real-time)
• Omnistar communication satellites (real-time)

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GPS Base Station

• Montana State University
• Equipment
• 12-channel Trimble Pro- XL
• Operating hours
• 6 AM to 10 PM daily

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WAAS Stations in North America
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How accurate is GPS?

• Recreational and mapping grade....................
  .....10-15 m
• C/A code
• Autonomous
• Recreational and mapping grade....................
  .........1-5 m
• C/A code
• With differential correction
• Submeter mapping grade............................
  .10 cm to 1 m
• C/A code carrier
• With differential correction
• Survey grade......................................
  .......................1 cm
• Dual frequency
• Advanced survey methods

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Additional Factors Affecting Accuracy

• Type of receivers used at rover and base
• Manufacturer
• Grade (mapping, sub-meter mapping, survey grade)
• Logging rate at base?
• Synched measurements at base?
• Number of visible satellites (influences PDOP)
• Occupation time at a point

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Geo XT AccuracyAfter differential correction

• GPS Pathfinder postprocessing
• Submeter 1ppm (1mm/km) RMS
• GPS Pathfinder carrier postprocessing
• 30 cm 5ppm
• Real-time
• Submeter (accuracy varies with proximity to base
  station)

Minimum of 4 satellites, maximum PDOP of 6,
minimum SNR of 4, minimum elevation of 15
degrees, baseline
multipath conditions
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Six Main Sources of GPS Error

• Atmospheric effects
• Multipath effects
• Satellite geometry
• Measurement noise
• Ephemeris data
• Satellite clock drift