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G lobal P ositioning S ystem. GeoXT Training January 20, 2003. Training Provided by. Information Center for the Environment University of California, Davis. Instructor: Sky Harrison. SWAP Training Objectives.

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G lobal p ositioning s ystem l.jpg

Global Positioning System

GeoXT Training

January 20, 2003


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Training Provided by

Information Center for the Environment

University of California, Davis

Instructor:

Sky Harrison


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SWAP Training Objectives

Provide sufficient written, hands-on, and field training to fulfill the GPS component of the Drinking Water Source Assessment Program

  • Obtain general knowledge of fundamental GPS concepts

  • Become familiar with the basics of operations of the Trimble GeoXT

  • Become comfortable with the GPS data collection process designed for the SWAP program


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DHS Objectives

  • Collect accurate locations for all public drinking water sources in California

  • Integrate data collection into normal daily work routine

  • Standardize data collection

  • Make it easy!


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What is GPS?

The Global Positioning System (GPS) is a precise worldwide radio-navigation system, and consists of a constellation of satellites and their ground stations, operated and maintained by the US Department of Defense (DoD).


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User

How Does it Work?

The GPS Segments

Space

Colorado Springs

Control


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Space Segment

  • 24 satellites in 6 inclined orbits

  • 4 satellites per orbit - 12 hour revolutions

  • 12,600 miles (20,000 km) altitude


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U S

Air

Force

Control Segment

  • Managed by the U.S. Air Force

  • 4 monitoring and 3 upload stations

  • Daily ephemeris (location) updates

  • Transmits clock and orbit corrections

  • Disable use of satellites

  • Degrade accuracy of the signals


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User Segment

  • U.S. Military

  • Civilian shipping

  • Scientific high accuracy applications

  • Resource managers (GIS data capture)

  • Survey and mapping control


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What is GPS Used For?

Applications

Point Features

Line and Area Features

Navigation

GIS Data Capture


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Nest trees

Survey monuments

Fence corners

Trail markers

Instream structures

Search and rescue

Monitoring devices

Reference points

Cultural sites

Wells

Helispots

Photo points

Hazard sites

Vehicles

Point Features


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Roads

Trails

Fire perimeters

Cultural sites

Campgrounds

Fences

Timber sale units

Stream channels

Vegetation type

Riparian area

Line and Area Features


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Navigation

  • Relocate points of interest with known coordinates

  • Locate mapped features using projected or estimated coordinates


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Spatial Data Recording

  • Method used depends on feature type

    • Point Feature

      • Calculates the average of all positions

    • Line Feature

      • Each position joined to the next in time sequence

      • Calculates the length

    • Area (polygon) Feature

      • Joins each position to the next in time sequence

      • Joins the last position to the first

      • Calculates the area


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GIS Data Capture

  • Tag GPS positions with feature and attribute data

  • Define features of interest prior to field work

  • Collect data in a GIS-compatible format

  • Generate all files needed to drive GIS data loading

  • Allow selective output to GIS based on feature name or attribute values


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Data Dictionaries

  • Data dictionaries are a key in the process of describing the object or objects to be mapped, referred to as “features.” Descriptions of these features can add great value to the positional data collected.

Attribute

PS-Code

Source Number

Date

Value

32S/13E-17K01 M

001

12/13/99

Feature

PWS-4010001


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Trilateration from satellitesto determine position

  • Satellite positions in space are known


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Measuring the distance to a satellite

  • Determination of the “pseudo” range is done by measuring travel time of GPS radio signals traveling at the speed of light.


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distance =

Elapsed time x 186,000 miles

per second

Pseudo Range Trilateration


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x miles

  • The distance (x) from one satellite tells us we're located somewhere on the surface of an imaginary sphere centered on that satellite with a radius of x.


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  • Distance measurements from two satellites limits our location to the intersection of two spheres, which is a circle.


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  • A third measurement narrows our location to just two points.


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  • A fourth measurement determines which point is our true location


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  • Satellite Clock Errors

  • Ephemeris Errors

  • Atmospheric Effects

  • Receiver Errors

  • Operator knowledge and awareness

  • Number of visible satellites

  • Satellite Geometry

  • Occupation time

  • Multipath

GPS Position Accuracy

  • Many factors can affect the accuracy of GPS data. Accuracy can range from 1 centimeter to over 40 meters

Significant Parameters:


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Satellite Geometry

HDOP (Horizontal Dilution Of Precision)

Using satellites from the 4 compass

quadrants will provide a good

Horizontal solution (Low HDOP).

Using satellites from only 1 or 2 quadrants will provide a poor Horizontal solution (HIGH HDOP).


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Satellite Geometry

VDOP (Vertical Dilution Of Precision)

Using satellites well spread out

in the sky will provide a good

Vertical Solution (Low VDOP).

Using only satellites which are

located low on the horizon will

result in a poor Vertical

Solution (HIGH VDOP).


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Satellite Geometry

PDOP (Position Dilution Of Precision)

PDOP is the combination of both the Horizontal

and Vertical components of position error caused

by satellite geometry.

PDOP Values

2-4 = Excellent

4-6 = Good

6-8 = Fair

8-10 = Poor

10-12 = Marginal

above 12 PDOP is

too High Do Not Use


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Selective Availability (S/A)

  • DISCOUNTINUED MAY 1, 2000

  • Intentionally degraded accuracy

  • controlled by the Department of Defense

    • It was the worst source of error

    • Artificial clock and ephemeris (locational) errors were introduced to throw system off

    • Prevented hostile forces from accessing the most accurate GPS capability


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GPS Position Accuracy

  • General Statement of Accuracy:

  • Taking all of the error sources into account, GPS accuracy will be approximately 10 meters for most GPS units. However, any given position may result in accuracy as low as 5 meters or up to 40 meters.

  • ** Selective Availability OFF

  • No Post-processed or Real-time Differential Correction


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Increasing GPS Accuracy through

Differential Correction

Rover or Remote

(unknown)

Base Station (known)


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Community Base Stations

  • Semi-Permanent base station for differential correction

  • Automated data collection

  • Multiple rovers within 300 mile radius can use data

  • Internet access to base files

  • 12-channel receiver


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Uncorrected File

18 meter accuracy from known position

Any one point could

be inaccurate by 40

Meters.

Precision:

50% of points

within 25 meter

radius circle

average GPS

position

Known position

approx. 180 seconds

1 point per second


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File After Differential Correction

1-5 meter accuracy from known position using GeoExplorer III

Precision:

50% of points

within 5 meter

radius circle

average GPS

position

Known position

approx. 180 seconds

1 point per second


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Limiting Factors

  • Dense canopy

  • Steep topography

  • Large and numerous structures

  • Microwave antenna interference


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Thank You

  • Sky Harrison

  • Information Center for the Environment

  • University of California, Davis

  • http://ice.ucdavis.edu

  • Phone: (530) 752-0532

  • Email: sky@ice.ucdavis.edu


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