Triangulation
The basic concept of GPS is triangulation. Suppose a person is standing in a valley surrounded by several towering mountain peaks. By using a compass to measure the direction to each peak, this person could locate his or her exact location on a map by using triangulation. After writing down the three measurements (remembering that there are 360 degrees in a circle), a line should be drawn from each peak in the opposite direction just measured.
Then 180 degrees is added or subtracted so that the direction the lines are drawn from each peak will fall between 0 and 360 degrees. For example, if one of the measurements is 270 degrees to peak A, the line from peak A back to the person's position would be 90 degrees. The point at which the three lines intersect is the point at which the person is standing.
The GPS satellites are like mountain peaks; they are known points in space from which lines can be drawn in order to specify a location. Each satellite transmits a radio signal that can be received on Earth and recognized by a GPS receiver.
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