understanding gps-gnss michael buschermohle precision agriculture specialist ut extension michael...
TRANSCRIPT
Understanding GPS-GNSS
Michael Buschermohle
Precision Agriculture Specialist
UT Extension
Global Navigation Satellite System (GNSS)
• Global Positioning System (GPS): 24 Satellites (USA)• GLONASS : 18 Satellites (Russia)• Galileo: 2 Satellites (European Union)• Compass: China• QZSS: Japan• IRNSS: India
Satellite Systems
GNSS = GPS + GLONASS + future additional satellite systems
Advantages of GNSS
Advantages
• Increased accuracy and reliability due to tracking more satellites
GPS
GNSS
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/secTransmission 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
Precision AgricultureUniversity of Tennessee Extension
Sources of GPS Error
Ephemeris (Orbital) Variation
Sources of GPS Error
Satellite Configuration
Poor (high) DOP Good (low) DOP
Signal Multipath
Sources of GPS Error
Counteracting GPS Errors
• Land-Based Differential Correction• Satellite-Based Differential Correction• Real-Time Kinematic Correction
Differential GPS (DGPS) Corrections
Satellite-Based Differential GPS Correction
WAASOmniStarJD StarFire
Differential GPS Corrections
) ) ) ) )) )
Real-Time Kinematic (RTK) GPS Correction
Base StationVirtual Reference StationContinuous 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
Correction Service
Pass-to-Pass Accuracy
Potential Range of Drift
WAAS ± 6 to 13 inches ± 4.7 ft
Sub-meter ± 6 to 13 inches ± 2.3 ft
Decimeter ± 2 to 4 inches ± 1.7 ft
RTK ± 1 inch ± 1 inch
Correction Service Pass-to-Pass Accuracy+ Potential Range of Drift*
WAAS + 6 to 13 inches + 4.7 ft
JD StarFire 1OmniStar XP(Sub-Meter)
+ 6 to 13 inches + 2.3 ft
JD StarFire 2OmniStar HP(Decimeter)
+ 2 to 4 inches + 1.7 ft
RTK + 1 inch + 1 inch+Based on manufacturer literature*Based on research conducted at Auburn University
GPS Accuracy
Differential Correction Services
Correction Service Provider Operating Fee/yearWASS FAA NoVBS
OmniSTAR
$800/yearXP $1000/yearHP $1,500/yearStarFire1 John Deere NoStarFire2 $800/yearRTK TN Tractor $800 - 1200/yearRTX Trimble $1000/yearVRS Multiple Providers Varies
CORSTDOT $300/yearEarl Dudley $1000/year
GPS Accuracy
WASS Sub-Meter Decimeter RTK
X X X X Mapping
X X X X Soil Sampling
X X X X Scouting
X X X X Yield Monitoring
X X X X Lightbars
X X X X VRA
X X Section Control
X Auto-Guidance
X Land Leveling
Questions