How GPS Works


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      -There are two devices necessary to utilize GPS navigation:
  • GPS satellite in orbit
  • GPS receiver on the ground
      -24 GPS satellites continuously orbit the earth.  At any one time, there are more than 3 satellites above the horizon at the receiver's location, the minimum number needed to determine its position on the earth's surface.

      -The GPS receiver utilizes 3-D trilateration in order to find it's position:
  • The receiver picks up radio waves broadcast from each satellite. Based on the calculated time it takes for the radio waves to travel from the satellite to the reciever,  the receiver can calculate its distance from each satellite.
    • First Satellite:  With a signal from one satellite, the receiver can calculate that it is located at some point on an imaginary sphere of the calculated distance from the satellite.
    • Second Satellite:  With a signal from two satellites, the receiver can calculate that it is located somewhere on the imaginary circle where the two spheres of calculated distance intersect.
    • Third Satellite:  With a signal from three satellites, the receiver can calculate that it is located at one of two points.  When the third sphere of distance is combined with the other two, the location of the receiver can be narrowed down to 2 points.  The receiver can automatically eliminate one of the points because it is typically located somewhere off in space.  The receiver is then left with the single point of its location.
GPS Trilateration



Works Cited:

Brain, Marshall, and Tom Harris.  "How GPS Receivers Work."  25 September 2006.  HowStuffWorks.com. <http://electronics.howstuffworks.com/gadgets/travel/gps.htm>  18 April 2010.

"Trilateration Method." Circuits Today. Web. 18 Apr 2010. <http://www.circuitstoday.com/wp-content/uploads/2009/12/Trilateration-GPS.jpg>.


April 2010
By Wooten Gough, Graham Billings, and Jason McMahon