A previously developed simulation platform for localization purposes has been extended and customized to accurately analyze and simulate ultra wideband (UWB) through-wall imaging (TWI) synthetic aperture radar (SAR) systems as well. The newly added features/modules include electromagnetic simulator to account for wall presence and target scattering, a wideband backprojection imaging algorithm, and different transceiver architectures and antenna array configurations. The developed platform is capable of time gating to suppress early wall reflections and simulates various discrete components and functions of the UWB SAR system using both linear and nonlinear analysis. The simulator has been experimentally validated for both dielectric and metallic targets. The developed simulator can also be used to study various effects related to operation frequency, pulse width, pulse shape, wall dispersion, and carrier leakage. It predicts UWB system performance such as detection range, image resolution, and receiver dynamic range. The system's components can be re-optimized for high performance 3-D imaging based on our developed models. The simulation platform is also suitable for designing the optimal signal waveform that could improve the recovered image quality for TWI applications.