Farm of the Future. GLONASS Russia’s global satellite navigation system 24 satellites in three...
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Farm of the Future
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Farm of the Future
GLONASS
• Russia’s global satellite navigation system
• 24 satellites in three orbits
• Five satellites visible
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Farm of the Future
What is GPS and What Does it Do?
• NAVSTAR GPSo NAVigation System with Time And
Ranging Global Positioning System• GPS is a satellite-based system
that uses a constellation of 24 satellites to give a user an accurate position on Earth.
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Farm of the Future
GPS System Segments
• Consists of three distinct segments:o Space Segmento Control Segmento User Segment
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Farm of the Future
Space Segment
• 24 Satellites • 6 Orbits• 10900 miles above
Earth’s Surface• 12 hour Rotation• 4 Satellites visible
at a 15° angle anytime
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Farm of the Future
Control Segment
• The control segment tracks the GPS satellites, updates their orbiting position and calibrates and synchronizes their clocks.
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Farm of the Future
User Segment
• Anyone with a GPS Receiver
• You in about 15 minutes!!
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Farm of the Future
How Does GPS Work?
• Satellites send information on their location and the time the code signal was sent.
• This code allows the GPS receiver to determine its distance to the satellite.
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Farm of the Future
Distance to Satellite
• Distance = Velocity x Timeo Speed of Light = 186,000 mi/seco Song Analogyo Pseudo Random Code
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Farm of the Future
2-D Location
• Locations are found by trilaterationo i.e. distances from
the satelliteso Triangulation
implies angle measurement
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Farm of the Future
3-D Location
• Requires 4 Satellites
• Elevation• Latitude• Longitude• Time
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Farm of the Future
GPS Satellite Singnal
• GPS satellite’s transmit two radio signals
• L1 and L2
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Farm of the Future
L1
• 1575.42 MHz• Two codes Course Acqusition (C/A)
and Precision (P)
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Farm of the Future
L2
• 1227.6 MHz• P code only (a.k.a Y-code)• Encrypted• Anti-spoofing
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Farm of the Future
Error Sources
• Sources of Error that Degrade GPS Position:o Ionospheric and Atmospheric Delayo Satellite and Receiver Clock Errorso Multipatho Dilution of Precisiono Selective Availability/Anti-Spoofing
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Farm of the Future
Ionospheric and Atmospheric Delays
• Signal slows down• Satellite Elevation• Density of
Ionosphere Affected by Sun
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Farm of the Future
Satellite and Receiver Clock Errors
• Time Drift from Satellite• Inaccurate Receiver Clock
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Farm of the Future
Multipath Errors
• Reflected Satellite Signalo Watero Buildings
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Farm of the Future
Dilution of Precision (DOP)• Measure of the
strength of satellite geometryo VDOP – Vertical
Dilution of Precisiono HDOP – Horizontal
Dilution of Precisiono PDOP – Positional
Dilution of Precisiono GDOP – Geometric
Dilution of Precision
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Farm of the Future
Selective Availabilty/Anti-Spoofing
• Selective Availabilty (S/A)o Denies Civilians full
accuracy of GPSo Dithering with Timeo Ephemeris not realo Turned Off
• Anti-Spoofing (A-S)o Denies Civilians
full accuracy of GPS
o Encrypts part of the Satellite Code
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Farm of the Future
GPS Methods
• There are several different methods of obtaining a position using GPS. The method used depends on the accuracy required by the user.
• Accurateo Hiker or Soldier = Yardso Ship or Sprayer = Feeto Surveyor or Lister = Inch
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Farm of the Future
GPS Methods
• Autonomous Navigationo Stand-Alone Receivero Hiker or Soldiero Position Accuracy = 15-100 yards
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Farm of the Future
GPS Methods
• Differentially Correctedo AKA DGPSo Receiver and Additional Antennaeo Ship or Sprayero Position Accuracy 1.5-15 feet
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Farm of the Future
GPS Methods
• Differential Phase Positiono AKA- RTK Real-Time Kinematico Position Accuracy = <1 inch
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Farm of the Future
Differentially Corrected Positions (DGPS)
• Many of the errors affecting the measurement of satellite range can be completely eliminated or at least significantly reduced using differential measurement techniques.
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Farm of the Future
DGPS
• Uses a ground station at a known location
• Ground station measures error of GPS signal by comparing the known location with the GPS location
• In real-time, the error transmitted to DGPS unit and position is corrected.
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Farm of the Future
Correction Signal Providers
• U.S. Coast Guard• Local FM• Satellite
o WAASo OMNISTAR
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Farm of the Future
Differential Phase GPS
• Achieves relative position accuracies of typically 0.25-2.5 inches.
• A lot of Statistics• Local ground station
sending an FM correction signal.
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Farm of the Future
Accuracy
• Watch Out For Statistics
• RMS – Root Mean Square• CEP – Circular Error Probable