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GPS Error

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

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

  2. Ionospheric & Tropospheric Refraction

  3. Multipath

  4. Satellite GeometryGeometric Dilution of Precision (GDOP) • GDOP can magnify or lessen other GPS errors • Wider angles better measurements • Components of GDOP • HDOP • VDOP • TDOP

  5. Dilution of Precision (DOP)

  6. 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

  7. Ephemeris Data • A list of the satellite’s positions as a function of time • Each satellite broadcasts its individual ephemeris • Almanac vs. ephemeris

  8. 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)

  9. Shaw, Michael, Kanwaljit Sandhoo and David Turner, Modernization of the Global Positioning System, GPS World Magazine, September 2000

  10. 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

  11. 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)

  12. 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)

  13. GPS Base Station • Montana State University • Equipment • 12-channel Trimble Pro- XL • Operating hours • 6 AM to 10 PM daily

  14. WAAS Stations in North America

  15. 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

  16. 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

  17. Geo XT Accuracy*After 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

  18. Six Main Sources of GPS Error • Atmospheric effects • Multipath effects • Satellite geometry • Measurement noise • Ephemeris data • Satellite clock drift

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