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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

Global Positioning System

GeoXT Training

January 20, 2003

instructor sky harrison

Training Provided by

Information Center for the Environment

University of California, Davis


Sky Harrison

swap training objectives
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

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!
what is gps
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).

how does it work


How Does it Work?

The GPS Segments


Colorado Springs


space segment
Space Segment
  • 24 satellites in 6 inclined orbits
  • 4 satellites per orbit - 12 hour revolutions
  • 12,600 miles (20,000 km) altitude




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

User Segment

  • U.S. Military
  • Civilian shipping
  • Scientific high accuracy applications
  • Resource managers (GIS data capture)
  • Survey and mapping control
what is gps used for
What is GPS Used For?


Point Features

Line and Area Features


GIS Data Capture

point features
Nest trees

Survey monuments

Fence corners

Trail markers

Instream structures

Search and rescue

Monitoring devices

Reference points

Cultural sites



Photo points

Hazard sites


Point Features
line and area features


Fire perimeters

Cultural sites



Timber sale units

Stream channels

Vegetation type

Riparian area

Line and Area Features
  • Relocate points of interest with known coordinates
  • Locate mapped features using projected or estimated coordinates
spatial data recording
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
gis data capture
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
data dictionaries
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.



Source Number



32S/13E-17K01 M





trilateration from satellites to determine position
Trilateration from satellitesto determine position
  • Satellite positions in space are known
measuring the distance to a satellite
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.
pseudo range trilateration

distance =

Elapsed time x 186,000 miles

per second

Pseudo Range Trilateration

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

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:


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).


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).


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

selective availability s a
Selective Availability (S/A)
  • 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
gps position accuracy29
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

Increasing GPS Accuracy through

Differential Correction

Rover or Remote


Base Station (known)

community base stations
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

Uncorrected File

18 meter accuracy from known position

Any one point could

be inaccurate by 40



50% of points

within 25 meter

radius circle

average GPS


Known position

approx. 180 seconds

1 point per second


File After Differential Correction

1-5 meter accuracy from known position using GeoExplorer III


50% of points

within 5 meter

radius circle

average GPS


Known position

approx. 180 seconds

1 point per second

limiting factors
Limiting Factors
  • Dense canopy
  • Steep topography
  • Large and numerous structures
  • Microwave antenna interference
thank you
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