Farm of the Future

Farm of the Future

GLONASS

• Russia’s global satellite navigation system

• 24 satellites in three orbits

• Five satellites visible

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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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GPS System Segments

• Consists of three distinct segments:o Space Segmento Control Segmento User Segment

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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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Control Segment

• The control segment tracks the GPS satellites, updates their orbiting position and calibrates and synchronizes their clocks.

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User Segment

• Anyone with a GPS Receiver

• You in about 15 minutes!!

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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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Distance to Satellite

• Distance = Velocity x Timeo Speed of Light = 186,000 mi/seco Song Analogyo Pseudo Random Code

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2-D Location

• Locations are found by trilaterationo i.e. distances from

the satelliteso Triangulation

implies angle measurement

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3-D Location

• Requires 4 Satellites

• Elevation• Latitude• Longitude• Time

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GPS Satellite Singnal

• GPS satellite’s transmit two radio signals

• L1 and L2

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L1

• 1575.42 MHz• Two codes Course Acqusition (C/A)

and Precision (P)

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L2

• 1227.6 MHz• P code only (a.k.a Y-code)• Encrypted• Anti-spoofing

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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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Ionospheric and Atmospheric Delays

• Signal slows down• Satellite Elevation• Density of

Ionosphere Affected by Sun

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Satellite and Receiver Clock Errors

• Time Drift from Satellite• Inaccurate Receiver Clock

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Multipath Errors

• Reflected Satellite Signalo Watero Buildings

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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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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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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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GPS Methods

• Autonomous Navigationo Stand-Alone Receivero Hiker or Soldiero Position Accuracy = 15-100 yards

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GPS Methods

• Differentially Correctedo AKA DGPSo Receiver and Additional Antennaeo Ship or Sprayero Position Accuracy 1.5-15 feet

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GPS Methods

• Differential Phase Positiono AKA- RTK Real-Time Kinematico Position Accuracy = <1 inch

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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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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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Correction Signal Providers

• U.S. Coast Guard• Local FM• Satellite

o WAASo OMNISTAR

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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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Accuracy

• Watch Out For Statistics

• RMS – Root Mean Square• CEP – Circular Error Probable