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Understanding GPS-GNSS. Michael Buschermohle Precision Agriculture Specialist UT Extension. Global Navigation Satellite System (GNSS). Satellite Systems. GNSS = GPS + GLONASS + future additional satellite systems. Global Positioning System (GPS): 24 Satellites ( USA)
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Understanding GPS-GNSS • Michael Buschermohle • Precision Agriculture Specialist • UT Extension
Global Navigation Satellite System (GNSS) Satellite Systems GNSS = GPS + GLONASS + future additional satellite systems Global Positioning System (GPS): 24 Satellites (USA) GLONASS : 18 Satellites (Russia) Galileo: 2 Satellites (European Union) Compass: China QZSS: Japan IRNSS: India
Advantages of GNSS Advantages • Increased accuracy and reliability due to tracking more satellites GNSS GPS Need 5 satellites to maintain RTK-fix
How Does GPS Work Satellites broadcast several pieces of information • Emphemeris (Orbital) data • Timing data
Sources of GPS Error • Clock Errors • Atmospheric Interference • Ephemeris (Orbital) Variation • Satellite Configuration • Signal Multipath
Sources of GPS Error Clock Errors Let’s put some numbers on it Altitude = 12,500mi. Speed of light = 186,000 mi/sec Transmission Time = 0.067 sec What happens if we are off a little? 0.001 sec * 186,000 mi/sec = 186 mi
Sources of GPS Error Atmospheric Interference
Sources of GPS Error Ephemeris (Orbital) Variation Precision Agriculture University of Tennessee Extension
Sources of GPS Error Satellite Configuration Good (low) DOP Poor (high) DOP
Sources of GPS Error Signal Multipath
Counteracting GPS Errors • Land-Based Differential Correction • Satellite-Based Differential Correction • Real-Time Kinematic Correction
Differential GPS (DGPS) Corrections Satellite-Based Differential GPS Correction WAAS OmniStar JD StarFire
) ) ) ) ) ) ) Differential GPS Corrections Real-Time Kinematic (RTK) GPS Correction Base Station Virtual Reference Station Continuous Operating Reference Station (CORS)
GPS Accuracy • Pass-to-Pass Accuracy • Represents the short-term (<15 min.) relative accuracy of a GPS/GNSS receiver. You can think of this as the accuracy between adjacent, parallel passes made within 15 minutes of one another. • GPS/GNSS Drift - Year-To-Year Accuracy • Drift can be defined as GPS/GNSS receiver (guidance system) accuracy over time. Causes of drift are changes in satellite configuration, operating near trees or other obstacles, and satellite data errors.
GPS Accuracy Courtesy of Fulton, Auburn University
