Guanxin Liu

Understanding the penetrability of synthetic aperture radar (SAR) signals into near-surface materials is a prerequisite for using SAR observations to infer subsurface physical properties. However, direct measurements of penetration depth under controlled material conditions remain limited, especially for comparisons across radar bands, soil water content, sand, and hard ground surfaces. This study provides direct laboratory measurements of L-band and S-band SAR signal penetration using a ground-based SAR system in a microwave anechoic chamber. Unlike penetration depth inversion studies, dihedral corner reflectors were buried at known depths. We identified the depth at which each reflector response became indistinguishable from the background. The reported values represent effective signal penetration intervals under the laboratory geometry. The chamber effectively reduced thermal noise and electromagnetic interference. At an incidence angle of 40°, the L-band signal penetrated 45–50 cm and 30–35 cm in clay samples prepared at 4% and 18% volumetric soil water content (SWC), respectively. The L-band penetration depth was 85–90 cm in dry sand with 3% SWC, about 9 cm in asphalt, 5 cm in gravel, and 2 cm gravel plus 7 cm asphalt in a composite hard-surface layer. The S-band penetration depth was 20–25 cm in loose clay, 15–20 cm in compacted clay, and 15–20 cm in sandy loam under the tested conditions. These results show that SAR penetration depends strongly on wavelength, water content, material type, and compaction/surface condition. We also demonstrate that thin hard ground surfaces can allow measurable L-band penetration. Our findings provide experimental benchmarks for interpreting SAR signals in subsurface parameter retrieval and sand-layer characterization.

The penetration depth of Synthetic Aperture Radar (SAR) signals is a critical parameter for assessing subsurface characteristics and detecting concealed structures. Although numerous empirical and scattering models have been proposed to estimate signal penetration depth (SPD), there remains a lack of understanding of SPD in various settings. In this study, we deployed a ground-based SAR system equipped with L-band (1.5 GHz) and S-band (3.0 GHz) SAR sensors and a microwave anechoic chamber to carry out experimental studies of SPD in soil with different volumetric water content (VWCs) and in hard surfaces of varying thicknesses of gravel and asphalt. At an incidence angle of 40°, the laboratory measurements indicate that the L-band SPDs in homogeneous clay with VWC levels of 4% and 18% are approximately 45–50 cm and 30–35 cm. The L-band signal can penetrate asphalt up to 9 cm, gravel up to 5 cm, and a composited layer of 2 cm gravel and 7 cm asphalt. The S-band can penetrate loose and compacted clay of 20‒25 cm and 15‒20 cm, respectively. The SPD in silt loam was about 15‒20 cm.

Multi temporal InSAR (MTInSAR) has been widely used in ground deformation monitoring. The accuracy of MTInSAR is often limited by the atmospheric phase screen (APS), even though many studies focus on atmospheric correction. One efficient approach utilizes wavelet correlation analysis (WCA) to separate APS based on the polarimetric independence of InSAR signals. However, the traditional WCA-based method fails to separate the APS when the deformation signal existed. To address this issue, we propose a novel MTInSAR approach that utilize dual-polarization data and an improved WCA to effectively separate the deformation and APS. In this method, we generate a series of zero temporal baseline interferograms (ZTBIs) with original interferograms to reduce the effect of deformation on the WCA. A hybrid indicator is then designed to determine the optimal wavelet decomposition scale in WCA, which enables more accurately separation of APS phase component of the ZTBI. The APS for the original interferogram is retrieved using the least squares (LS) from ZTBI. The APS-compensated interferograms can be then used to accurately estimate the ground deformation. The effectiveness and improvement of the proposed method when compared with the traditional methods is demonstrated with experiments using a Sentinel-1 polarized SAR dataset over the Santa Ana Basin, Los Angeles. The results demonstrate that the proposed method can effectively mitigate the effects of APS on the InSAR-derived deformation, with an RMSE less than 4.4 mm compared with GPS measurement. This represents an enhancement of 22.8% and 45.7% over the traditional SBAS and GACOS-based results, respectively.