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Apis

Member Since: July 31, 2011

Country: United States

  • "Why four? You would think you only need three for triangulation, right?"
    GPS determines position by trilateration, triangulation involves angles (see wikipedia). You need 4 satellites for 3D trilateration and 3 for 2D trilateration, no need to consider time. GPS uses time to measure distance to satellites though.
    The 2D case is easiest: if you know the distance to a satellite (of known position) you know your position is on a circle with the satellite in the center and the radius equal to the distance. Two satellites gives two circles, your position will be on the intersection which is usually 2 points. A third satellite will give you a single point. In the 3D case one satellite gives a sphere, two a circle, three two points and four a single point.

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