Bistatic Full-wave Radar Tomography Detects Deep Interior Voids, Cracks, and Boulders in a Rubble-pile Asteroid Model

Abstract

In this paper, we investigate full-wave computed radar tomography (CRT) using a rubble-pile asteroid model in which a realistic shape (Itokawa) is coupled with a synthetic material composition and structure model. The aim is to show that sparse bistatic radar measurements can distinguish details inside a complex-structured rubble-pile asteroid. The results obtained suggest that distinct local permittivity distribution changes such as surface layers, voids, low-permittivity anomalies, highpermittivity boulders, and cracks can be detected with bistatic CRT, when the total noise level in the data is around -10 dB with respect to the signal amplitude. Moreover, the bistatic measurement set-up improves the robustness of the inversion compared to the monostatic case. Reconstructing the smooth Gaussian background distribution was found to be difficult with the present approach, suggesting that complementary techniques, such as gravimetry, might be needed to improve the reliability of the inference in practice.

A Time-Domain Multigrid Solver With Higher-Order Born Approximation for Full-Wave Radar Tomography of a Complex-Shaped Target

“…It seems that, whereas the Bistatic I reconstruction is superior to the Bistatic II on the surface part, there is not such a difference in the deep interior, which we interpret to follow from the longer and more "multi-way" signal propagation in comparison to the background prediction in the latter case. The findings of this study suggest that, in addition to the noise-robustness [5], [33], the reconstruction quality can be improved via a bistatic measurement with regard to both structural similarity and value accuracy, especially if a higherorder BA is used. A bistatic configuration had also been chosen for CONSERT to provide tomographic travel-time data [6], [8].…”

“…Regarding the goal of reconstructing the interior structure of an asteroid, it will be important to implement and evaluate the present higher-order BA for a 3D computation geometry. This could be done by extending the linearized multigrid solver introduced and used in our recent studies [32], [33]. Applying the multigrid approach is crucial in discretizing the unknown permittivity distribution, as the sparsity of the inversion grid enables the fast performance of the wave-field updating routine.…”

“…It is necessary to tackle the potential modeling errors arising from the complexity, for example, the effects related to anisotropic structures or scattering effects with high uncertainty. In particular, to optimize the reconstruction quality in a backscattering measurement, it is necessary to filter out the signal peak arising from the wave scattered by the rear wall of D [5], [33]. Namely, the amplitude of such a peak can be larger compared to the earlier arriving peaks which are essential for detecting the scatterers in the interior structure.…”

“…We have previously introduced a multigrid-FETD inversion approach [32] in which the first-order BA of the wave scattering is used. It has been successfully applied in linearized and iterative TV regularization to reconstruct a synthetic permittivity distribution within a real 3D asteroid shape in [5], [33]. Here, building on our previous work, we introduce an iterative and fully non-linear inverse solver in which the unknown permittivity is updated via subsequent linearized approximations akin to the DBI approach and the numerical wave-field can be updated through a higher-order BA.…”

Tomographic inversion of gravity gradient field for a synthetic Itokawa model

“…Hence, a radar observation will be also less dependent on the observation distance in comparison to gravity measurements. Our recent findings suggest that radar inversion emphasizes high-contrast details with the cost of smooth variations such as large areas of denser or more porous regolith which might be detected by a gravimeter [19]. Therefore, it will also be vital to analyze the interconnection and correlation between the electric permittivity and mass density distributions observed by the radar and gravimeter, respectively [49].…”