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There has been an increasing interest in the use of spaceborne very high frequency ultra high frequency (VHF-UHF) synthetic aperture radar (SAR) for measuring forest biomass and for detecting underground facilities. The propagation characteristics of the low-frequency electromagnetic wave are severely affected by the ionosphere. Recently, Faraday rotation effects and SAR image degradation have been studied using an analytical model and a homogeneous ionosphere. In this paper, a numerical model is developed to investigate the SAR image degradation caused by an inhomogeneous ionosphere. Both horizontal and vertical structures of the ionosphere are considered in this model. Three different cases are studied. The first is a vertically homogenous ionosphere, where the simulation condition is the same as in the analytical study by Ishimaru and others. The second is a vertical profile, which is introduced using the Chapman formula. The ray-bending effect is added for the ionosphere with a layered structure. Finally, both the vertical profile in electron density and the horizontal gradient in total electron content are considered in the simulation. Simulation results show good agreement with the theoretical analysis under the same conditions of the ionosphere. When both horizontal and vertical structures and the inhomogeneity of the ionosphere are considered in the model, the simulation result shows further image degradation and shift caused by the ray-bending effect. The simulation results also show the strong frequency dependence of the SAR image resolution.