In some cases, natural or man made, the ionospheric electron density is highly disturbed and becomes a random medium. Satellite signals propagating through such a medium suffer rapid phase and amplitude variations called scintillations. The coherence time and bandwidth of the channel are therefore reduced, and the fading can reach Rayleigh type in the most severe cases. The design of communication systems able to operate in such conditions requires a precise knowledge of these parameters. The aim of this paper is to present a new numerical technique, based on the multiple phase screens (MPS) approach, which allows the evaluation of the channel characteristics. The method, called MPC, avoids the plane wave hypothesis and is especially designed for upward links, where the electromagnetic wave source may be close to the turbulent medium and the receiver is far away (geostationary satellite case). The source antenna gain pattern is included in the modeling, along with the medium drift speed, which can vary with height. Interesting results show that the scintillation index and scattering loss at the receiver are closely related to the transmitter antenna directivity and tracking precision. ¿ 1998 American Geophysical Union