  We consider a mathematical model of synthetic aperture radar (SAR) with a
known, possibly non-flat, topography. In this context we consider the problem
of recovering the wavefront set of the ground reflectivity, given radar data
measured along a curved flight path. We show that if singularities are located
at "mirror points," then the resulting data may be smooth; in effect, the
singularities "cancel." With a flat topography, these mirror points are always
discrete, but we show that in a non-flat topography there may be infinite
families of mirror points.
