ON PFISTER THEOREM AND ITS GENERALIZATION

The relation between angle of arrival and Doppler shift that has been called Pfister's theorem is shown to be a special case of a result tha t applies to ionospherically transmitted signals between fixed ground sites, whenever the ionosphere moves without change of shape. This gen eral result does not require the existence of an effective mirror refl ector or any special symmetry of the ionosphere, but depends only on t he validity of ray theory. The formula obtained by PFISTER [(1971) J. atmos. terr. Phys. 33, 999] is shown to apply, provided the ray return ing from the ionosphere follows the same path as the incident ray.