Global Positioning Systems

Glen T. Huettl

Agricultural Education

Garrison High School

Global Positioning System (GPS)

• Navigation and Positioning System

• Determines Exact Location

• Establishes Longitude, Latitude, Altitude

GPS Components

• Satellites in Space Send Signals

• Receivers on the earth Receive the Signals

• Software Converts Satellite Signals

• Position Monitoring Equipment Validate Satellite Location

How GPS Works

• Receivers calculate position by reading 3 or more satellite signals

• Called “Triangulation”

• Theory used before GPS to locate earthquakes

GPS Developed By:

• U.S. Department of Defense

• Available for both Civilian and Military Uses

• Available 365 days a year

Commerical Applications

• Emergency Vehicles

• Electronic Maps in Cars

• Aircraft Landing Systems

• Delivery Vehicles

• Sea Navigation

History of GPS

• Developed by Department of Defense for Security Purposes

• Previous navigational and positional methods were not exact

• GPS is answer to age-old problem

Satellites

• NAVSTAR (Navigation Satellite Timing and Ranging)

• 24 Satellites

• Circle the earth every 12 hours

• 4 - 10 Satellites visible from any location in the earth at any time

• Send radio signals

Satellites (cont’d)

• Act as Reference Points

• Position known and precisely measured

• Control Segment continually monitor accuracy of signal

• Atomic Clocks provide timing accuracy

The Satellite Signal

• Information sent is 1) Time stamp of when signal sent and 2) Satellite position

• Complicated digital codes referred to as “Pseudo-Random” Code

• Radio signals transmit two L-Band frequencies: L1 and L2

Receiver

• Small hand-held computers

• Attach to an Antenna that receives satellite signals

• Uses integrated circuit technology

• Costs vary from $200 to several $1000’s

• Affordable to a wide audience

The Receiver Calculates Position

• Travel time of signals plus high powered math calculates position

• Speed of Light X Time = Distance

• Converts signals sent from 3 to 4 satellites

• Intersecting point of three measurements gives positional reading (“Triangulation”)

Control Segment

• Monitor Satellite Signal for Accuracy

• Located around the world

• Satellites pass over monitors every 12 hours

• Measure signals and correct positional errors

Error Sources

• Selective Availability

• Particles in the Ionosphere

• Atomic Clocks

• Speed of Light Calculations

• Multipath Errors

• Receiver Errors

Error Correction Methods

• Control Segment Error Correction

• Differential Global Positioning System (DGPS)

DGPS

• More advanced form of GPS

• Offers more Accuracy

• DGPS adds correctional signal to GPS signals

• Uses a reference station to broadcast corrections to receivers

DGPS (cont’d)

• Cancels out natural and man-made errors

• Makes GPS positioning very precise

• Broadens GPS applications because of accuracy of Differential signal.

DGPS Uses

• Coast Guard

• Natural Resource Management

• Forest Service

• Offshore Exploration

• Agricultural Uses

Commercial USES

• Yield Monitors

• Variable Rate Technology

• Field Mapping and Acreage Management

• Noting Underground Locations

WAAS Wide Area Augmentation System

• Government supplied correction

• Approx 25 land based stations