Geometrical control of the effective outer scale imposed by detrend filtering, in the presence of highly anisotropic irregularities, explains differences between scintillation indices ratio at different locations. On the basis of weak scatter theory results, the behavior of the geometrical factor G, responsible for static effect of diffraction, is investigated in the case of GPS satellites. A simple comparison is made between GPS satellites and polar orbiting satellites used in the past for scintillation experiments to understand such differences in the behavior of scintillation indices measured by means of GPS scintillation monitors.