The average power received at a space craft from a reciprocity-tracking transmitter is shown to be the free-space diffraction-limited result times a gain-reduction factor that is due to the point-ahead requirement. For a constant-power transmitter, the gain-reduction factor is approximately equal to the appropriate spherical-wave mutual-coherence function. For a constant-average-power transmitter, an exact expression is obtained for the gain-reduction factor.
Jeffrey H. Shapiro, "Point-ahead limitation on reciprocity tracking," J. Opt. Soc. Am. 65, 65-68 (1975)