G lobal p ositioning s ystem
1 / 35

G lobal P ositioning S ystem - PowerPoint PPT Presentation

  • Updated On :

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.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'G lobal P ositioning S ystem' - Rita

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
G lobal p ositioning s ystem l.jpg

Global Positioning System

GeoXT Training

January 20, 2003

Instructor sky harrison l.jpg

Training Provided by

Information Center for the Environment

University of California, Davis


Sky Harrison

Swap training objectives l.jpg
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

Slide4 l.jpg

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 l.jpg
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 l.jpg


How Does it Work?

The GPS Segments


Colorado Springs


Space segment l.jpg
Space Segment

  • 24 satellites in 6 inclined orbits

  • 4 satellites per orbit - 12 hour revolutions

  • 12,600 miles (20,000 km) altitude

Slide8 l.jpg




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

Slide9 l.jpg

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 l.jpg
What is GPS Used For?


Point Features

Line and Area Features


GIS Data Capture

Point features l.jpg

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 l.jpg



Fire perimeters

Cultural sites



Timber sale units

Stream channels

Vegetation type

Riparian area

Line and Area Features

Navigation l.jpg

  • Relocate points of interest with known coordinates

  • Locate mapped features using projected or estimated coordinates

Spatial data recording l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Trilateration from satellitesto determine position

  • Satellite positions in space are known

Measuring the distance to a satellite l.jpg
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 l.jpg

distance =

Elapsed time x 186,000 miles

per second

Pseudo Range Trilateration

Slide20 l.jpg

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.

Slide21 l.jpg

Slide22 l.jpg

Gps position accuracy l.jpg

  • Satellite Clock Errors location

  • 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:

Slide25 l.jpg

Satellite Geometry location

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).

Slide26 l.jpg

Satellite Geometry location

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).

Slide27 l.jpg

Satellite Geometry location

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 l.jpg
Selective Availability (S/A) location


  • 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 l.jpg
GPS Position Accuracy location

  • 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

Slide30 l.jpg

Increasing GPS Accuracy through location

Differential Correction

Rover or Remote


Base Station (known)

Community base stations l.jpg
Community Base Stations location

  • 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

Slide32 l.jpg

Uncorrected File location

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

Slide33 l.jpg

File After Differential Correction location

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 l.jpg
Limiting Factors location

  • Dense canopy

  • Steep topography

  • Large and numerous structures

  • Microwave antenna interference

Thank you l.jpg
Thank You location

  • Sky Harrison

  • Information Center for the Environment

  • University of California, Davis

  • http://ice.ucdavis.edu

  • Phone: (530) 752-0532

  • Email: [email protected]