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The coverage provided by the GPS constellation has grown more robust over time. These color contour plots show the cumulative amount of time per day that portions of the globe experience degraded accuracy (i.e., positioning error exceeding 9 meters or so). The first plot (top left) shows the amount of degraded accuracy for the full 18-satellite nonuniform constellation. The second plot (top right) shows that very few areas of degraded accuracy remain for the full 21-satellite nonuniform constellation. The third plot (middle left) shows that the 21-satellite constellation is sensitive to a single satellite failure. The fourth plot (middle right) shows that the 24-satellite constellation is less sensitive than the 21-satellite constellation to a single satellite failure. The fifth plot (bottom left) shows that the 24-satellite constellation is sensitive to a dual satellite failure. The sixth plot (bottom right) shows that the 27-satellite constellation is less sensitive than the 24-satellite constellation to a dual satellite failure. Hence, the ability of the constellation to withstand sudden satellite failures improves as the size of the nonuniform constellations increases. Moreover, the transition from one constellation to another generally involves only minor amounts of satellite repositioning.