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

  • 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


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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


GPS Accuracy


Differential Correction Services


GPS Accuracy


Questions


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