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Synthetic aperture radar (SAR) imaging of near Earth surface natural material volumes (soil, snow, and vegetation) and man-made objects corresponds to an equivalent real-aperture scenario with temporally short and spatially focused pulses. This equivalence holds as long as the imaged scene can be considered static within both fast (chirp duration) and slow (synthetic aperture) times. The electromagnetic backscatter recorded in a properly focused SAR image resolution cell that meets this condition has an amplitude and phase, which represents the coherent sum of the returns from individual scatterers inside the small volume represented by the cell. For some types of scatterers, the backscattering properties can be accurately characterized by analytical expressions. However, there are many interesting scattering phenomena where a purely analytical treatment leads to unrealistic simplifying assumptions. One approach to investigating these phenomena is with numerical methods, such as the finite-difference time-domain (FDTD) method. The FDTD method is computationally expensive, but it can model the complete physical interaction of electromagnetic waves according to Maxwell’s equations with arbitrary materials and shapes. This article describes the development of a specialized 3-D FDTD software package capable of simulating the real-aperture equivalent of individual SAR image resolution cells given the spatial distribution of permittivity and conductivity within the cells. We demonstrate the usefulness of our new software tool by first recreating published 2-D simulation results examining the link between soil moisture and InSAR phase and then expanding these results to more realistic 3-D soil volumes.