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GPS Error. Atmospheric effects Multipath Satellite geometry Measurement noise Ephemeris data Satellite clock drift Selective availability (SA). Ionospheric & Tropospheric Refraction. Multipath. Satellite Geometry Geometric Dilution of Precision (GDOP).

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Gps error l.jpg
GPS Error

  • Atmospheric effects

  • Multipath

  • Satellite geometry

  • Measurement noise

  • Ephemeris data

  • Satellite clock drift

  • Selective availability (SA)




Satellite geometry geometric dilution of precision gdop l.jpg
Satellite GeometryGeometric Dilution of Precision (GDOP)

  • GDOP can magnify or lessen other GPS errors

  • Wider angles better measurements

  • Components of GDOP

    • HDOP

    • VDOP

    • TDOP



Satellite geometry position dilution of precision pdop l.jpg
Satellite GeometryPosition Dilution of Precision (PDOP)

  • PDOP refers to horizontal (HDOP) and vertical (VDOP) measurements (latitude, longitude and altitude)

  • PDOP values

    <=4 excellent

    5-8 acceptable

    >=9 poor


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Ephemeris Data

  • A list of the satellite’s 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 Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • 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 Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • 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 Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • 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 Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • 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 Modernization of the Global Positioning System, GPS World Magazine, September 2000


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How accurate is GPS? Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • 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 Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • 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 Accuracy* Modernization of the Global Positioning System, GPS World Magazine, September 2000After 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 <= 250 km (155 miles) low multipath conditions


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Six Main Sources of GPS Error Modernization of the Global Positioning System, GPS World Magazine, September 2000

  • Atmospheric effects

  • Multipath effects

  • Satellite geometry

  • Measurement noise

  • Ephemeris data

  • Satellite clock drift


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