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arxiv: 2604.10185 · v1 · submitted 2026-04-11 · 📡 eess.SP

Hybrid Physical and Geometrical Optics Method for Modeling Subsurface Imaging Using mmWave FMCW Radar

Kaito Ichijo , Hang Song , Xin Du , Bo Wei , Junichi Takada This is my paper

Pith reviewed 2026-05-10 16:10 UTC · model grok-4.3

classification 📡 eess.SP
keywords hybrid physical geometrical opticssubsurface imagingmmWave FMCW radarwave propagation modelingsynthetic aperture radarradar simulationimaging reconstruction
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The pith

A hybrid physical and geometrical optics method simulates wave propagation for mmWave FMCW subsurface imaging.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper proposes a hybrid simulation approach for modeling wave propagation in subsurface imaging scenarios using mmWave frequency-modulated continuous-wave radar. Full-wave electromagnetic simulations become computationally prohibitive at millimeter-wave frequencies due to high resource demands. Instead, the method applies physical optics to calculate reflections from objects and geometrical optics to handle wave transmissions through them. By merging these calculations, synthetic data is generated that allows successful reconstruction of images via synthetic-aperture radar processing. Experimental comparisons validate that this approach can effectively model the imaging process.

Core claim

The central claim is that hybridizing physical optics for object reflections with geometrical optics for transmissions through the medium enables accurate simulation of wave propagation in mmWave FMCW subsurface imaging, bypassing the computational cost of full-wave methods while still producing data suitable for SAR imaging reconstruction.

What carries the argument

The hybrid physical-geometrical optics combination, where physical optics computes reflections from the object and geometrical optics computes transmissions through the object to simulate overall wave propagation.

If this is right

  • The simulated data supports successful SAR image reconstruction.
  • Comparison with physical experiments shows the method models subsurface imaging effectively.
  • Computation time and resources are reduced compared to full-wave simulations at mmWave frequencies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Such hybrid methods could extend to other high-frequency radar applications where full-wave simulation is impractical.
  • If the approximations hold, this could enable faster iterative design of subsurface imaging systems.
  • Potential exists for integration with machine learning for enhanced image interpretation from the simulated data.

Load-bearing premise

The combination of physical optics for reflections and geometrical optics for transmissions approximates the full wave propagation effects accurately enough in subsurface mmWave scenarios without significant unaccounted errors at the interfaces.

What would settle it

A direct comparison showing large discrepancies between the hybrid simulation results and either full-wave simulations or experimental measurements in a controlled subsurface setup would indicate the method fails to model the imaging correctly.

Figures

Figures reproduced from arXiv: 2604.10185 by Bo Wei, Hang Song, Junichi Takada, Kaito Ichijo, Xin Du.

Figure 1
Figure 1. Figure 1: Schematic diagram of HPGO for simulating wave propagation in subsurface imaging. (1) [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
read the original abstract

A hybrid physical and geometrical optics method is proposed to model the subsurface imaging using mmWave FMCW radar. Modeling of the wave propagation for subsurface imaging can improve the interpretation of acquired data and imaging results. Full-wave simulation is common in simulating wave propagation. However, when the frequency is high such as mmWave frequency, it is difficult to implement since it costs large computation resource and time. In this paper, the physical and geometrical optics are hybridized to simulate the wave propagation in subsurface imaging scenarios. In the proposed method, physical optics method is utilized to calculate the reflection from the object and geometrical optics method is utilized to calculate the transmission of the wave through object. By combining the results from physical and geometrical optics, the wave propagation in the subsurface imaging scenarios is simulated. The synthetic-aperture radar imaging is applied to the simulated data and the image is successfully reconstructed. Further, the experiment setup is developed and the comparison between simulation and experiment is carried out. The results demonstrated that the proposed simulation method can model the subsurface imaging with mmWave FMCW radar.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper proposes a hybrid physical optics (PO) and geometrical optics (GO) method for simulating wave propagation in mmWave FMCW radar subsurface imaging scenarios. PO is applied to compute reflections from the buried object while GO handles transmission through the object; the combined fields are then used to generate synthetic data for SAR image reconstruction. The authors report that the reconstructed images match experimental results favorably and conclude that the hybrid approach successfully models the imaging process as a computationally lighter alternative to full-wave simulation.

Significance. If the hybrid PO+GO approximation proves sufficiently accurate, the method would offer a practical, lower-cost tool for forward modeling in mmWave subsurface radar applications, aiding system design and data interpretation where full-wave solvers become prohibitive at high frequencies. The approach builds on established asymptotic optics principles without introducing new free parameters, which is a positive attribute.

major comments (3)
  1. [Abstract / Results] Abstract and results section: The central validation claim rests on 'successful' SAR image reconstruction and 'favorable' comparison to experiment, yet no quantitative metrics (RMSE, SSIM, peak sidelobe level, or error bars across the FMCW bandwidth) are reported. This absence leaves the accuracy of the hybrid model unquantified and undermines the assertion that the simulation adequately captures subsurface propagation.
  2. [Method] Method section: The interface between PO reflection and GO transmission is not shown to conserve energy or maintain phase continuity across the FMCW frequency sweep. Because GO neglects diffraction and PO is an asymptotic approximation, their combination may introduce unaccounted amplitude and phase errors at dielectric interfaces typical in subsurface scenarios; a concrete check against a full-wave reference (even for a canonical case) is needed to bound these errors.
  3. [Discussion] Discussion / validation: The paper does not address the validity regime of the GO transmission step (ray optics requires object features ≫ wavelength) or the impact of multiple internal reflections that PO+GO single-pass transmission omits. In mmWave subsurface imaging with dielectric contrasts, these omissions can distort the reconstructed SAR image; the manuscript should either quantify the resulting image error or restrict the claimed applicability.
minor comments (2)
  1. [Method] Notation for the combined PO+GO field expression should be defined explicitly (e.g., an equation showing how the transmitted GO field multiplies the PO-reflected contribution) to improve reproducibility.
  2. [Results] Figure captions for the simulated and experimental SAR images should state the exact frequency range, bandwidth, and standoff distance used so readers can assess the operating regime.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We address each major comment below and indicate the revisions we will make to strengthen the paper.

read point-by-point responses

  1. Referee: [Abstract / Results] Abstract and results section: The central validation claim rests on 'successful' SAR image reconstruction and 'favorable' comparison to experiment, yet no quantitative metrics (RMSE, SSIM, peak sidelobe level, or error bars across the FMCW bandwidth) are reported. This absence leaves the accuracy of the hybrid model unquantified and undermines the assertion that the simulation adequately captures subsurface propagation.

    Authors: We agree with the referee that quantitative metrics are important for validating the hybrid model. In the revised version, we will include RMSE and SSIM metrics for the comparison between simulated and experimental SAR images. Additionally, we will report error bars or variations across the FMCW bandwidth to quantify the accuracy more rigorously. revision: yes

  2. Referee: [Method] Method section: The interface between PO reflection and GO transmission is not shown to conserve energy or maintain phase continuity across the FMCW frequency sweep. Because GO neglects diffraction and PO is an asymptotic approximation, their combination may introduce unaccounted amplitude and phase errors at dielectric interfaces typical in subsurface scenarios; a concrete check against a full-wave reference (even for a canonical case) is needed to bound these errors.

    Authors: The referee correctly points out that energy conservation and phase continuity at the PO-GO interface were not explicitly demonstrated. We will add an analysis in the revised manuscript showing that the combined fields satisfy energy conservation for the transmission and reflection coefficients at the interface. Furthermore, we will include a comparison of the hybrid method against a full-wave simulation for a canonical buried object case to bound the approximation errors. revision: yes

  3. Referee: [Discussion] Discussion / validation: The paper does not address the validity regime of the GO transmission step (ray optics requires object features ≫ wavelength) or the impact of multiple internal reflections that PO+GO single-pass transmission omits. In mmWave subsurface imaging with dielectric contrasts, these omissions can distort the reconstructed SAR image; the manuscript should either quantify the resulting image error or restrict the claimed applicability.

    Authors: We acknowledge that the validity regime and the omission of multiple reflections were not discussed. In the revision, we will add a discussion on the applicability conditions for the GO step, noting that it is valid when object dimensions are much larger than the wavelength. We will also address the neglect of multiple internal reflections as a limitation of the current single-pass model and either provide an estimate of the resulting image distortion or restrict the claims to scenarios where such effects are minimal. revision: yes

Circularity Check

0 steps flagged

No significant circularity; forward simulation from established optics principles

full rationale

The paper defines a hybrid modeling procedure that applies standard physical optics to compute object reflection and geometrical optics to compute transmission, then combines the results to generate simulated FMCW radar signals for subsurface scenarios. SAR imaging is subsequently applied to these simulated signals using conventional algorithms, and the output is compared against separate experimental measurements. No equations or claims reduce a derived quantity to a fitted parameter or self-referential definition by construction; the method is presented as an approximation whose validity is checked externally via experiment rather than asserted through internal consistency alone. Self-citations, if present, are not load-bearing for the central claim.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review provides no explicit free parameters, axioms, or invented entities; the hybrid method implicitly relies on standard assumptions of physical and geometrical optics approximations whose validity for the target regime is not audited here.

pith-pipeline@v0.9.0 · 5494 in / 1184 out tokens · 50445 ms · 2026-05-10T16:10:49.225517+00:00 · methodology

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Reference graph

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