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Geographic Information Systems. Global Positioning Systems (GPS). 1. GPS. A method used in surveying. It uses a constellation of satellites orbiting the earth at very high altitudes. GPS.

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Geographic Information Systems

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Geographic Information Systems

Global Positioning Systems (GPS)

1. GPS

  • A method used in surveying. It uses a constellation of satellites orbiting the earth at very high altitudes

GPS ..

  • The GPS technology allows accurate geodetic surveys by using specially designed receivers that, when positioned at a point on the earth, measure the distance from that point to three or more orbiting satellites

  • Through the geometric calculations of triangulation, the coordinates of the point on the surface of the earth are determined

X, Y, Z (position) and time from 4 satellites to calculate position

Pamela E. Jansma, University of Arkansas


  • NAVigation Satellite Timing And Ranging (NAVSTAR) Global Positioning System

  • Developed by U.S. Department of Defense

  • A constellation system of 28-30 satellites orbiting the earth at a high altitude twice a day

  • Transmitting precise time and position information

  • For world-wide and all weather navigations


  • 24 satellites in 6 orbital planes

  • Orbit the earth at approx. 20,200 km

  • 550 inclination

  • Satellites complete an orbit in approximately 12 hours

3. Operation Principles

  • The basis of GPS is triangulation

  • Satellite

  • GPS receiver

  • Determination of Location

  • Coordinate systems

3. Operation - Distance to One Satellite

  • Satellite

    - continuously broadcasts time and its location

  • GPS receiver

    - receives the signal and calculates the distance between the satellite and the receiver:

    Distance = Velocity x Time

    where time = the time a signal takes to travel from the satellite to the receiver, velocity = ?

3. Operation - Determine Location

  • Determination of location

    - 3 or more simultaneous distance measurements are needed to determine the location of the receiver

  • Coordinate systems

    - A GPS provides its position in geographical/UTM/ State Plane coordinates and altitude

One measurement narrows down our position to the surface of a sphere

A second measurement narrows down our position to the intersection of two spheres, which is a circle

A third measurement narrows down our position to just two points, because the intersection of a circle and a sphere is two points

A four measurement narrows our position down to one point

3. Operation- Determine location ..

Pamela E. Jansma, University of Arkansas

4. GPS Receiver

  • GPS receivers can be hand carried out or installed on aircraft, ships, tanks, submarines, cars and trucks

GPS Receiver ..

  • Radio channels

  • Internal clock

  • Computer

GPS Receiver ..

  • Radio channels

    It needs one or more channels to receive signals of

        (1) the time a signal is sent, and        

    (2) a pseudorandom code to identify the signal

  • Internal clock

    - Synchronized with the satellite in order to calculate the signal travel time precisely

GPS Receiver ..

  • Computer

    - Calculates distance and location

    - Stores location readings

    - Calculates the satellite availability

5. Autonomous vs. Differential GPS

  • Autonomous GPS: one receiver unit

  • Selective availability interference

  • Differential GPS

  • WAAS (Wide Area Augmentation System)

Differential GPS ..

  • Selective availability interference

    -The Us government used to insert random errors in the signals in order to maintain optimum military effectiveness of the system. It was turned off in 2000

  • Differential GPS

Differential GPS

  • Selective availability interference

  • Differential GPS

    - Uses two receiver units, with one placed at a known location, such as a surveyed control point, to detect the inserted errors and calculate the needed corrections. The correction information is used by another receiver to overcome its selective availability interference

Differential GPS (DGPS) ..

  • The DGPS corrections can be applied to the GPS data in real-time (radio modems). Or, can be done later on a PC

  • There are a series of radio beacons to transmit the DGPS corrections for accurate navigation

5. Differential Correction

5. DGPS - WAAS Satellite DGPS

  • Wide Area Augmentation System (WAAS)

      - a real-time differential correction service

    - is based on a network of approximately 25 ground reference stations that cover a very large service area

WAAS Satellite DGPS ..

- Signals from GPS satellites are received by wide area ground reference stations (WRSs)

- Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist

- These WRSs are linked to form the U.S. WAAS network. This service is primarily for aviation purposes

WAAS Satellite DGPS ..

  • Improves the accuracy, integrity and availability of the basic GPS signals

  • Safety - critical navigation system for aviation

  • Owned and operated by Federal Aviation Administration (FAA)

  • Type: real-time

  • Cost: free

WAAS Stations


  • Omnistar

  • Type: Real-time

    - Network of reference stations

    70 reference stations

    3 network control centers

    - Worldwide coverage

  • Cost: ~ $800/year

6. Static vs. Kinematic GPS

  • Static: two or more receivers receive data for a lengthy period of time at a fixed location

  • Kinematic: one receiver is placed at a known location, while one or more receivers move from point to point

  • Real Time Kinematic (RTK)

7. Advantages and Limitations

  • Advantages: fast, more accurate, lower cost than manual approaches

  • Disadvantages: obstructed locations

8. Readings

  • Chapter 2

Omnistar – How it Works

5. DGPS - Omnistar CoverageNorth America

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