Through-wall/through-barrier motion sensing systems are becoming increasingly important tools to find people hidden behind barriers and under rubble. The sensing performance of these systems is best determined with dynamic calibration targets that mimic human breathing and respiration. Potential advantages of these targets are in their reproducibility and accuracy in support of calibration methods for human detection testing in through-wall and through-rubbles situations. Essential ingredients of an ideal calibration target are: (1) its RCS should be as close to that of a human, (2) it must be easy to fabricate and deploy, (3) it must present essentially a constant RCS value at all aspect angles, and (4) it must be durable with respect to laboratory and field test conditions. An ideal calibration target is a sphere. However, we have determined that it is possible to mimic a sphere using a polyhedron with a limited number of faces. We are in the process of constructing an expandable metallic polyhedron target of appropriate size and number of faces for experimental measurements under various through-wall and through-rubble operational scenarios. This paper presents simulation results on the dynamic RCS variations of the calibration target as it expands and contracts to simulate human breathing.