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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
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 of GNSS


  • Increased accuracy and reliability due to tracking more satellites



Need 5 satellites to maintain RTK-fix

how does gps work
How Does GPS Work

Satellites broadcast several pieces of information

  • Emphemeris (Orbital) data
  • Timing data
sources of gps error
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 error1
Sources of GPS Error

Atmospheric Interference

sources of gps error2
Sources of GPS Error

Ephemeris (Orbital) Variation

Precision Agriculture

University of Tennessee Extension

sources of gps error3
Sources of GPS Error

Satellite Configuration

Good (low) DOP

Poor (high) DOP

sources of gps error4
Sources of GPS Error

Signal Multipath

counteracting gps errors
Counteracting GPS Errors
  • Land-Based Differential Correction
  • Satellite-Based Differential Correction
  • Real-Time Kinematic Correction
differential gps dgps corrections
Differential GPS (DGPS) Corrections

Satellite-Based Differential GPS Correction



JD StarFire

differential gps corrections








Differential GPS Corrections

Real-Time Kinematic (RTK) GPS Correction

Base Station

Virtual Reference Station

Continuous Operating Reference Station (CORS)

gps accuracy
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 accuracy1
GPS Accuracy

Courtesy of Fulton, Auburn University