Speckle noise is a major factor impairing synthetic aperture radar (SAR) imagery interpretation and processing. In this paper, we propose a simple approach to deal with the despeckling of multi-frequency SAR data acquired over bare soil surfaces. The basic idea is to perform a proper normalization step in order to compensate the frequency dependence of the SAR measurements and make histograms of the multi-frequency images comparable. Once this pre-processing step is performed, conventional multi-temporal filtering can be applied. We tested this simple approach on both simulated and real-world multi-frequency datasets. Obtained results show that the proposed idea leads to increased speckle suppression capabilities with respect to both single-channel filtering and pure multi-temporal despeckling.
We present an analytical formulation of the correlation coefficient of the electromagnetic fields scattered at near-specular direction by a rough or gently undulating surface and measured at two spatially separated positions occupied by a moving receiver at slightly different times. This allows us obtaining an explicit expression of the correlation time of the received signal in terms of radar and surface parameters. This work provides a contribution to the discussion, currently ongoing in the Global Navigation Satellite System Reflectometry (GNSS-R) scientific community, about the behavior of received signal fluctuations, especially when surface profile variations are such that the scattering is neither coherent nor completely incoherent. The scattering surface is here modeled as randomly rough, and the Kirchhoff Approximation (KA) or the first-order Small Slope Approximation (SSA1) is employed to compute the scattered field. In fact, the expression of the correlation coefficient is the same for both approximations. The obtained closed-form expression shows that, as the surface correlation length increases, the degree of coherence smoothly increases from the value obtained with the expression already available in literature for very rough surfaces to a value close to unity for gently undulating surfaces. The obtained behavior of correlation time as a function of surface parameters, system resolution and observation geometry is in agreement with numerical simulations available in literature. More in general, obtained analytical results are in agreement with the observed behavior of GNSS-R signals over flat land surfaces.
In this paper, a comparative study of spaceborne Global Navigation Satellite System (GNSS)-Reflectometry for ship detection applications is provided. The analysis is conducted by evaluating the impact of 1) the acquisition geometry and 2) the received signal polarization on ship detectability in GNSS-R data. In particular, the backscattering acquisition geometry is demonstrated to be more suitable for ship detection applications, thus allowing for the detection of 20 m-length ships. Even very large ships are hardly detectable in the conventional forward-scattering geometry. Moreover, receiving right-hand circular polarization is demonstrated to provide significant improvements of the signal-to-noise-plus-clutter with respect to the conventional left-hand circular polarization channel, conventionally exploited in GNSS-R remote sensing. The study is based on a numerical tool for the bistatic radar cross section of the ship, which is presented in a companion paper.
The advent of 5G/B5G wireless communications networks has stimulated research efforts in assessing health effects of human tissues exposure to electromagnetic fields (EMF). In this paper, we investigate the role of skin thickness in the exposure of human body to EMF at mm-Waves. Different skin models and dielectric characterizations, including layered skin, are studied using the equivalent multi-layer model framework. Simulation analyses show that as frequency increases the impact of changes in the dielectric and geometric characterization of the skin is less and less relevant due to the decreasing penetration depth. Additionally, the presence of stratum corneum determines an increased power absorption.
We present an analytical formulation of the correlation of the electromagnetic fields scattered by a rough or gently undulating surface and measured at two spatially separated points. The latter may be the positions of two different bistatic radar sensors receiving the scattered field at the same time, as in bistatic synthetic aperture radar interferometry, or two positions occupied by a single moving receiver at two slightly different times, as in Global Navigation Satellite System Reflectometry. The scattering surface is modeled as randomly rough, and the Kirchhoff Approximation is employed to compute the scattered field. The obtained closed-form expression of the field correlation is substantially coincident with the one already available in literature for far-from-specular scattering directions; conversely, for close-to-specular directions, the obtained formulation shows that, as the surface correlation length increases, the degree of coherence smoothly increases from the value obtained with the expression available in literature for rough surfaces to a value close to unity for very gently undulating surfaces.
