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 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