Radar sounders (RS) provide information on subsurface targets for planetary investigations. Several simulation techniques have been developed to support the RS design and the data interpretation. Each technique has different properties and modeling capabilities, achieving different trade-offs between accuracy and computational requirements. The state-of-the-art RS simulation techniques include: i) numerical methods, such as the Finite- Difference Time-Domain (FDTD) technique, which allows the modelling of small-scale scattering phenomena at the cost of high computational requirements; ii) facet modeling and ray-tracing based methods, such as the multi-layered coherent RS (MCS) technique, which requires less computational resources than FDTD, allowing the modeling of large-scale scattering phenomena. Recently an integrated simulation methodology has been presented, for simulating small-scale scattering phenomena in large scenarios. However, this methodology was designed for modeling only surface scattering. In this paper, we propose a method that extends the capabilities of the integrated methodology to model both large and small-scale roughness in a multi-layer scenario. The proposed method uses the FDTD technique to evaluate the effects associated with small-scale roughness in terms of i) scattering phenomena associated with the layers and ii) power losses associated with the signal transmitted through a rough layer. To recursively apply scattering and transmission to multiple layers of the subsurface, a coherent ray-tracing method is used. We experimentally assessed the effectiveness of the proposed methodology on three-layer models by integrating the effect of roughness imposed on the layers and in transmission through them.
Radar sounders (RS) play an important role in planetary investigation. However, the complex tasks of predicting performance and interpreting RS data and predicting performance require to perform data simulations. In the literature, there are different methods for RS data simulation, including: i) numerical methods involving the exact solution of Maxwell's equations based on 3D modelling; and ii) methods based on ray-tracing and facet modelling. Among numerical methods, the Finite-Difference Time-Domain (FDTD) technique allows one to precisely model complex nonlinear targets, both geologically and geophysically, at the cost of high computational load. On the contrary, coherent ray-tracing methods reduce the computational cost by triangulating the targets into facets with size comparable to the wavelength. In this work, we combine the advantages of FDTD and ray-tracing methods into a novel integrated simulation technique for modelling scattering phenomena at two scales of wave interaction, i.e. large (facet) scale and small (sub-facet) scale. The coherent facet method is used to simulate facet- scale scattering phenomena, while FDTD is used to evaluate the sub-facet-scale scattering due to roughness on top of the single facets. We investigate the method's effectiveness by generating a one-layer synthetic DEM as a fractional Brownian motion (fBm) process superimposed by different values of RMS slope of the small-scale roughness and by analysing how the received signal changes in terms of signal-to-noise ratio. The results show the effectiveness of the method in representing small-scale roughness realistically.
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