Imaging the rotor blades of hovering helicopters with SAR

An elementary analysis of X-band SAR images of the flashes from the rotating blades of a hovering helicopter is presented. The analysis is supplemented by simulations based on a physical optics approximation. It is shown that the SAR image consists of a set of ghost images. The ghosts have the form of a dasiacombpsila of periodically repeated point spread functions. Each one corresponds to the point spread function of the unmodulated SAR for the leading and trailing blade edges weighted by an envelope consisting of a much broader dasiasincpsila function. Due to interference effects the blade scattering is effectively confined to a small length of the blade. Some effects of varying the rotation rate are presented.