An arrangement of points such that the set of equations for the location of an unknown point in terms of the distances and angles between known points and the unknown point does not have a unique solution. For example, the location of an unknown point in a given plane can be determined, in general, by measuring the angles from that point to three points of known location in that plane. A critical configuration is present if all four points lie on the same circle, or if the three known points lie on a straight line. Many critical configurations are possible in the arrangement of stations observing an artificial satellite.