Global Positioning System

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Global Positioning System
Technology Overview
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Global Positioning System

• developed by the US Dept. of Defense
• satellite-based
• designed to provide positioning and timing
  information
• 24 hours/day, 7 days/week
• under any weather conditions
• anywhere in the world

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Main Components of GPS

• Space component
• Control component
• User component

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Space component

• 24 satellites in constellation
• orbit every 12 hours at 11,500 miles
• 4 satellites in each of 6 orbital planes
• transmit a uniquely coded radio signal
• equipped with onboard atomic clock

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Control component

• ground-based monitoring and upload stations
• control orbit and timing information

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User component

• military and civilian users
• navigation and positioning applications on land,
  sea, in the air and in space

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How does GPS work?

• the receiver picks up the signals from the
  satellites
• uses signal travel time to calculate distance to
  the satellites
• triangulates to determine position of the receiver

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Speed of Light Measurement

• measure how long it takes GPS signal to get to us
• multiply elapsed time by 186,000 miles/sec
• time (sec) x 186,000 miles from satellite to
  receiver

Trimble Navigation
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Triangulation
One measurement narrows down our position to the
surface of a sphere.
We are somewhere on the surface of this sphere.
11,000 miles
Trimble Navigation
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Triangulation
Second measurement narrows it down to the
intersection of two spheres.
11,000 miles
Intersection of two spheres is a circle.
12,000 miles
Trimble Navigation
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Triangulation
Third measurement narrows to just two points.
Intersection of three spheres is only two points.
11,000 miles
12,000 miles
13,000 miles
Trimble Navigation
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Triangulation
Fourth measurement will decide between the two
points.
Fourth measurement will go through only one of
the two points.
11,000 miles
14,000 miles
12,000 miles
13,000 miles
Trimble Navigation
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3 Classes of GPS Receivers
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3 Classes of GPS Receivers
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3 Classes of GPS Receivers
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IMPORTANT NOTE!
Handheld Receivers do NOT allow you to control
the quality of the data you are collecting. Data
quality control is important if you are creating
a GIS database.
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Factors Affecting GPS Accuracy

• satellite constellation geometry (GDOP)

• un-modeled atmospheric delays

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Geometric Dilution of Precision (GDOP)
Relative position of satellites can affect error
6 secs
4 secs
Idealized situation
Trimble Navigation
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Geometric Dilution of Precision (GDOP)
Real situation - fuzzy circles
6 secs
4 secs
uncertainty
uncertainty
Point representing position is really a box
Trimble Navigation
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Geometric Dilution of Precision (GDOP)
Even worse at some angles
Box gets bigger if satellites are closer together
Trimble Navigation
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Geometric Dilution of Precision (GDOP)
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Atmospheric Delays

• Ionosphere (band of charged particles)
• Troposphere (our weather)

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Multipath Error
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Accuracy 15m - typical handheld GPS accuracy
without corrections 3m - typical WAAS accuracy
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• Errors Corrected by WAAS
• Satellite clocks
• Orbit errors
• Ionosphere and troposphere (partially corrected)
• Errors NOT Corrected by WAAS
• Multipath error
• Receiver noise

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Navigation
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Questions?
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This GPS presentation was prepared by Tom Luther,
USDA Forest Service. It includes slides from
Trimble Navigation.
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History of Geodesy
Sphere
Ellipse
Lumpy Potato
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Datum - Reference Model of the Earth

• Ellipsoid Size
• Ellipsoid Shape
• Location of ellipsoid
• (relative to Earth)

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Common Datums
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Coordinate Systems
Latitude / Longitude
NH State Plane (GRANIT) UTM
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Universal Transverse Mercator (UTM)
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When does this matter?

• When navigating to a waypoint, you need to make
  sure that the receiver knows what datum that
  waypoint is in. Datum errors can affect your
  position by several hundred feet!

• The Garmin 76 will always export positions
  using WGS84.

• The datum / coordinate system set on the GPS will
  affect how your position is displayed on the GPS
  receiver.

• Terrain Navigator will show coordinates in the
  datum/coordinate system you set in Preferences.

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This Geodesy presentation was prepared by the
University of New Hampshire Cooperative Extension
and includes images from the US Defense Mapping
Agency, NH GRANIT, and Mentor Software Inc.