Understanding GPS-GNSS
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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

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Understanding gps gnss

  • 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

Understanding gps gnss

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

Gps accuracy2

GPS Accuracy

Differential correction services

Differential Correction Services

Gps accuracy3

GPS Accuracy

Understanding gps gnss


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