1,509 research outputs found
Open-Stopband Suppression via Double Asymmetric Discontinuities in 1-D Periodic 2-D Leaky-Wave Structures
No full textA technique for the open-stopband (OSB) suppression in one-dimensional (1-D) periodic leaky-wave antennas based on a double asymmetric series/shunt discontinuity inside the unit cell is discussed and applied to a canonical 2-D structure. A longitudinal equivalent network formalism is developed here to analyze the unit-cell matching, which confirms that the OSB suppression cannot be achieved using a double symmetric discontinuity. Conversely, an infinite number of matched configurations can be obtained using two unequal passive discontinuities, whose reciprocal distance can be chosen freely inside a continuous range always including a quarter wavelength. The possibility of suppressing the OSB is then shown for both TM and TE leaky modes supported by a metal strip grating on a grounded slab by using independent full-wave modal solvers and considering typical geometries with either narrow metallic strips or narrow slots inside the unit cell
Open-stopband suppression in a canonical 1-D periodic 2-D structure with asymmetric unit cell
No full textPeriodic leaky-wave antennas are typically affected by the open-stopband (OSB) problem, i.e., the pattern degradation as the beam scans through broadside. Over the years, various techniques have been proposed to mitigate, or even suppress, this longstanding issue. However, most of these techniques cannot be easily applied to 1-D periodic 2-D structure. Recently, it has been theoretically and experimentally shown that the OSB can be suppressed by means of a very simple technique: it consists of designing a unit cell with two unequal discontinuities suitably spaced one another. In this work, we apply this novel technique to a canonical 1-D periodic 2-D structure, namely a grounded dielectric slab with a metal strip grating on top. To validate the concept, full-wave results are presented in the 17.5-20.5 GHz band considering an asymmetric unit-cell characterized by two narrow strips for the TM-polarized case
Decorrelation of the near-specular land scattering in bistatic radar systems
No full textSignal fluctuations at the receiving antenna have been studied from decades by the radar community, especially to understand the decorrelation of the scattering in radar interferometry. This was done assuming uncorrelated point-like scatterers, leading to a simple model for the geometric decorrelation. In this case, the scattering is certainly incoherent. The quasi-specular reflections gathered under the illumination of signals of opportunity can exhibit significant temporal fluctuations. They are related to the statistical features of the surface roughness and can be observed even in almost flat regions, where a predominant coherent reflection could be expected. The presence of gentle undulations, however, i.e., those showed by surfaces having variations of the profiles comparable with the wavelength over the vertical scale, but much longer over the horizontal one, can determine transition regions where the scattering is neither coherent nor completely incoherent. In these conditions, the nature of the fluctuations of the scattering is not well understood and it requires additional studies. A discussion about the dominance of coherent or incoherent reflection in the Global Navigation Satellite System Reflectometry (GNSS-R) community is presently ongoing. To describe the nature of the scattering, and to understand the decorrelation of the near-specular components in GNSS-R, we propose a numerical study of the field collected by a moving airborne receiver based on the Kirchhoff approximation. Our study demonstrates that the near-specular scattering collected over representative natural landscapes by a GNSS-R receiver is partially coherent and essentially incoherent in most cases. Its correlation time is a function of the roughness parameters, namely standard deviation and correlation length, as well as of the system parameters (i.e., spatial resolution and height). The analysis can provide useful information for the interpretation of GNSS data, which present intrinsic variability that can significantly affect the retrieval of the relevant bio-geophysical parameters
Decorrelation of the near-specular scattering in GNSS reflectometry from space
No full textTo understand the temporal decorrelation of the near-specular component of land-scattered signals in Global Navigation Satellite System Reflectometry (GNSS-R), and to describe the nature of the scattering considering spaceborne receivers at arbitrary altitudes, we propose here an analytical solution of the covariance of the field under the Kirchhoff approximation. Both the case of infinite and finite illumination on ground are studied. Surfaces with gentle undulations are considered, i.e., those having small slopes and showing slow variations of the profiles over the horizontal scale. This allows for investigating scattered fields that can be neither coherent nor completely incoherent over land surfaces that are nearly flat. In a recent work from the Authors, an extensive numerical evaluation of the decorrelation of the near-specular land scattering was presented. The phenomenology of the problem was studied and discussed numerically, solving for airborne receivers the relevant scattering integral, both as a function of the geometry of the system and of the statistical parameters of the illuminated surface. Such numerical results are used here to validate the proposed closed-form formulation. It is demonstrated how the near-specular scattering collected over land targets by a GNSS-R receiver from space decorrelates as a function of the receiver movement and of the statistical parameters describing the illuminated surface (namely, height standard deviation and correlation length). The proposed analysis provides information of interest for the design of future GNSS-R missions. The interpretation of GNSS-R data from space, which typically shows strong fluctuations, can also be supported by this approximated analytical study
Temporal decorrelation of scattered gnss signals
No full textThe Global Navigation Satellite System Reflectometry (GNSS-R) is an emerging remote sensing technique based on the exploitation of scattered navigation signals for monitoring bio-geophysical parameters of the Earth surface. The receiver is placed onboard of airborne or spaceborne platforms, whose movement can affect the feature of the gathered signals. These can present strong fluctuations, which depend on the electromagnetic parameters of the illuminated surface and on its statistical features. Fluctuations have been studied from decades by the radar community, especially to understand the decorrelation of the scattering in radar interferometry. It has been done, however, by only considering uncorrelated point-like scatterers, leading to a simple model. To characterize the temporal decorrelation of the near-specular scattering in GNSS-R systems, we describe here some numerical results collected by a moving receiver changing the random surface parameters and accounting for the coherent/incoherent nature of the scattered field
Decorrelation of GNSS-R Reflected Signals: Analytical Modeling
No full textWe propose an analytical solution of the scattering for the evaluation of the decorrelation time of reflected signals of opportunity. We consider spaceborne receivers and an approximated solution of the covariance of the scattered field under the Kirchhoff approximation. The case of an infinite illuminated surface showing gentle undulations is studied. It is discussed how the near-specular scattering, collected over land targets by a receiver from space, decorrelates as a function of the receiver movement and of the statistical parameters describing the illuminated surface. The study gives information of interest for the design of future bistatic missions, especially for GNSS reflectometry. The interpretation of data from space, which typically shows strong fluctuations, can also be supported
Cross-Correlation of Scattered GNSS Signals
No full textNavigation signals reflected by the Earth surface, collected by a receiver in relative movement with respect to the source and the surface, can exhibit temporal fluctuations. Their features are related to the characteristics of the surface roughness and they can be observed even in the presence of almost flat surfaces with gentle undulations, i.e., those whose horizontal scale can be comparable with the impinging wavelength. In this work, a full-wave solution of the scattering based on the Kirchhoff approximation is implemented to characterize the temporal variability of scattered signals of opportunity. A numerical solution is compared with a simple closed-form expression achieved considering omnidirectional sources. The analysis can provide useful information for the interpretation of GNSS data, especially those collected by means of satellite platforms, which presents an intrinsic variability that can significantly affect the retrieval of bio-geophysical parameters
2-D Beam Scanning with Cylindrical-Leaky-Wave-Enhanced Phased Arrays
No full textDirective pencil beams scannable in both elevation and azimuth are obtained through a planar phased array placed inside a Fabry-Perot cavity. The key element of the proposed approach is the exploitation of a conical element pattern (EP) with high directivity in elevation, obtained through the excitation of a dominant, weakly attenuated cylindrical TM leaky wave of azimuthal order n = 0 by means of a simple coaxial probe. Then, a highly reduced number N of such sources are arranged to form a phased array radiating directive pencil beams. Beam-angle reconfigurability with continuous scanning both in azimuth and elevation inside a wide solid angular range is achieved by varying the array phasing and the operating frequency. An investigation on the features of truncated cylindrical leaky waves is first developed to properly characterize the EP. Then, conventional array theory is exploited to calculate the pattern of the entire array. The radiation efficiency is also evaluated accounting for the spurious surface wave related to the undesired excitation of the quasi-TEM mode. The proposed array design provides a simple and inexpensive innovative solution for obtaining a high-gain pencil beam continuously scanning in the 3-D space without suffering gain losses. In the presented implementation, the elevation angular scan, which is generally constrained by the wideband capability of the feeding system, by the requirements on the sidelobe level, and by the cutoff of the relevant leaky mode, ranges from about 21° to about 68°. Possible applications are envisaged for the next generation of wireless power transfer devices, for advanced radar and surveillance systems, earth observation, as well as for ceiling-mounted indoor localization and tracking
Estado de Derecho y Unión Europea, a cura di D. J. LIñán Nogueras, P. J. Martín Rodríguez, edito da Tecnos, Madrid, 2018, 472 pp.
No full textIl presente scritto recensisce l'opera "Estado de Derecho y Unión Europea", a cura dei professori D. J. LIñán Nogueras e P. J. Martín Rodríguez, edita da Tecnos, Madrid, 201
On the modeling of the bistatic coherent scattering from a rough surface
No full textIn this contribution, we investigate on the coherent bistatic scattering generated by a radar system when illuminating a rough surface. Moving from the well-known scattering theory based on the Kirchhoff approximation, we propose an extended formulation of the Fung-Eom model, by introducing a higherorder expansion on the phase term of the impinging spherical wave. We demonstrate and discuss the importance of such correction for the accurate characterization of the coherent scattering generated in bistatic radar system. We give insight, for the first time, on the role played by both the antenna pattern and the wavefront sphericity in bistatic scattering in the whole incidence plane, not limiting the analysis to the backscattering and specular directions. This is particularly important for practical applications, such as the modeling of the scattering generated by sources of opportunity
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