Introduction to the Global Positioning System (GPS)

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Introduction to the Global Positioning System
(GPS)
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Introduction to the Global Positioning System
(GPS)
Geodesy GPS for Dummies Session III National
Science Teachers Association 2007 Annual
Conference St. Louis, MO March 31, 2007 Casey
Brennan
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Learning Outcomes

• Participants will be able to describe the basics
  of GPS.
• Participants will be able to describe the method
  GPS uses to calculate your position.
• Participants will be able to list at least three
  ways that GPS is important to your daily life.

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Overview

• Part 1 A brief history of positioning
• Part 2 GPS 101
• Part 3 The amazing new world of precise
  positioning

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Where are we?

• What is positioning and what is surveying?
• Positioning in the USA (zero meridian)

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The Importance of Time

• Time has been the limiting factor for a lot of
  science, including the science of positioning
• Ships and clocks (John Harrison and the 18th
  Century)
• Better accuracy requires better clocks

John Harrison 1693-1776
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The Technological Revolution

• Satellites
• Atomic clocks
• Microchips and calculators
• Radio Beacons and other transitions from astro
  methods to modern methods. Ground based systems

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Very Long Baseline Interferometry

• The precursor to GPS
• Quasars and dishes

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The Launch of GPS

• DOD sponsored project puts satellites into orbit
• First Sat launched in 1978
• 24 Sats by mid 1990s
• 28 Currently in orbit, with more coming
• A fundamental change in how positioning is done
• What GPS has changed?

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The GPS Receiver

• Who has a GPS Receiver?
• What the receiver does
• What the receiver does NOT

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Basic Trilateration

• DRxT
• Rate is Speed of light
• Time is the key! Technology made it possible
• One you have distance, its easy

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y
GPS positioning simplified to two dimensions
x
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How a GPS receiver works

• Find the satellites
• Know where the satellites are
• Figure out DRxT
• Trilaterate
• Repeat, repeat, repeat

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The limitations of GPS

• Must be able to see the satellites
• Requires power
• Multiple sources of error

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Sources of Error in GPS

• Multipathing
• Atmospheric Delays
• PDOP
• Clocks
• Orbits
• Receiver electronics
• Relativity
• Geoid models and other really techie stuff

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PDOP (Position Dilution of Precision) or Why
the distribution of GPS satellites in the sky
affects how well I know where I am (Simplified
to a 2-dimensional model)
x
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PDOP (Position Dilution of Precision) or Why
the distribution of GPS satellites in the sky
affects how well I know where I am (Simplified
to a 2-dimensional model)
x
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PDOP (Position Dilution of Precision) or Why
the distribution of GPS satellites in the sky
affects how well I know where I am (Simplified
to a 2-dimensional model)
Blue/Yellow have good geometry so the (green)
error box around x is small (PDOP is small)
Red/Yellow have bad geometry so the (orange)
error box around x is large (PDOP is large)
x
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Just how accurate can we get?

• Consumer Grade GPS
• Survey Grade GPS
• Use of two receivers instead of just one (CORS)

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Some amazing things to do with GPS

• Earth Tides
• Measuring subsidence
• Track the moving earth
• Post Glacial Rebound
• Geocaching
• Emergency Rescue

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Some amazing things to do with GPS
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Conclusion and discussion

• GPS completely changed positioning forever
• GPS will continue to improve