Mitigating ground clutter effects with lightweight artificial dielectrics

Ground surface roughness is problematic when using a radar impulse to detect and locate land mines. Waves scatter from a random rough ground surface in unpredictable ways, contributing to clutter that is particularly hard to suppress. This clutter has proven experimentally and computationally to distort and obscure the desired scattered field from a buried target. To overcome this effect we have developed a lightweight, artificial dielectric that can be placed over a chosen area that will mimic flat ground and mitigate clutter effects. An artificial dielectric of close-packed array of small insulated metal-coated plastic spheres and lossless uniform plastic spheres can be formulated to match the dielectric properties soil. The ratio of these two spheres in the collection is adjusted to match a particular soil type and the moisture content. Placing them in a conformable bag and ensuring a flat upper interface with the air, ground reflections from an impulse radar can effectively be removed to reveal a target scattering signature. Furthermore, a matched filter can be used to distinguish between a landmine and a false alarm (such as a rock) The artificial dielectric was matched by running experiments in the frequency and time domains. A 1 GHz center frequency impulse ground penetrating radar was used to collect time signals and compare different cases: flat ground, rough ground and rough ground with artificial dielectric. Results indicate excellent rough surface reflection removal and target signal enhancement.

We propose a radiation concept to realize phased arrays with wideband and wide-scanning performance. The array is based on connected-slot elements that radiate in the presence of artificial dielectric superstrates. The array can be implemented with a single multi-layer printed circuit board. Artificial dielectrics are used in place of real dielectrics to minimize the losses due to surface waves and avoid the occurrence of scan blindness over a wide scan range. The achievable bandwidth depends on the number of layers composing the artificial dielectric. A design example is shown that achieves a bandwidth (VSWR<;2.5) of about 5:1 when scanning up to 50° in all azimuth planes. Both single- and dual-polarization designs can be implemented.

Summary form only given. Artificial dielectrics (ADs) are composed of periodic electrically small metallic structures embedded in a host material in order to increase its equivalent relative permittivity. ADs have been extensively studied and used for decades for radar development. Recently, there is an renewed interest in planar ADs, driven by the ever increasing demand for high efficiency silicon integrated antennas (K. Takahagi and E. Sano, IEEE Trans. Antennas Propag., 59-10, 2011). In integrated technology, artificial dielectric layers (ADLs) can be realized by multiple layers of patches with small electrical dimensions. The sub-wavelength sizes of the patches ensure keeping the ohmic losses very low, since no resonant currents are supported. These structures have been shown to enhance the front-to-back ratio and the gain of planar antennas, without supporting surface waves (W. H. Syed and A. Neto, IEEE Trans. Antennas Propag., 61-11, 2013).In this contribution, we present the design and experimental results of a 300 GHz ADL-loaded antenna prototype. The radiator consists of a double slot, fed by a co-planar waveguide and loaded with an AD slab to increase the forward radiation and the gain. The antenna and ADL are shown in Fig. 1(a). The ADL is constituted by 7 layers of periodic patches with 5 m inter-layer distance. An in house silicon based integrated circuit process has been used to realize the prototype. The simulated and measured reflection coefficients are shown in Fig. 1(b). The measurement of the antenna radiation pattern are under progress and will be presented at the time of the conference.

This thesis is a contribution towards developing cost-effective ways for reducing outdoor traffic noise in outdoor environments by exploiting the interaction between sound travelling directly to a listener from the source and sound reflected by the intervening ground. Sound propagation over different kinds of porous, rough and mixed impedance ground surfaces have been studied experimentally and numerically. Measurements of short-range acoustic level difference spectra over outdoor ground surfaces and artificially-created surfaces outdoors and in the laboratory have been compared with predictions to establish suitable impedance models. Sound propagation over mixed impedance ground having single or multiple impedance discontinuities has also been studied. Acoustic transmission loss through vegetation, crops and hedges has been investigated. The phenomenon of sound diffraction and periodicity due to rough periodic ground surfaces has been explored through artificially created rough surfaces in the laboratory and outdoors. The phenomenon of surface wave propagation over rough hard surfaces and porous surfaces has been explored through laboratory experiments. Measured data indoors and outdoors have been used to validate numerical (BEM and FEM), empirical and analytical (MST) prediction techniques. The validated numerical methods have been used to make predictions at scales suitable for attenuating traffic noise by means of carefully designed ground treatments. The work has also been extended to railway and tramway noise. It has been found that replacing hard ground with porous ground, introducing single or multiple impedance discontinuities, growing vegetation and introducing low height roughness can all contribute between 3 and 15 dB additional attenuation of traffic noise. In respect of replacing hard ground by porous ground, it is concluded that the ground with lowest flow resistivity i.e. grassland left untouched and allowed to grow wild gives the best attenuation performance. However, dividing a single width of soft ground into alternating strips of hard and soft surfaces does not improve the insertion loss. The overall width of the soft surface is the main factor. Cultivating crops over porous ground can enhance the attenuation but the effect is not very significant for A-weighted levels as most of additional attenuation occurs at higher frequencies above 3 kHz. A 0.3 m high and at least 3 m wide lattice structure design is found to be very useful for traffic noise attenuation since it offers greater insertion loss than the same width and height of parallel low walls and the resulting attenuation is azimuthal angle independent. It has been shown also that the potentially negative effect on insertion loss due to propagation of roughness-induced surface waves over rough surfaces can be reduced by introducing sound absorbing material in between the walls.

Study on the ground surface quality of Cr 2O 3 coatings

In this paper, the ground surface is equivalent to a multi-layer structure of two lossy media (such as dry soil and wet soil). Four different periodic dielectric layers are investigated, and their radio wave shielding effectiveness is analyzed and evaluated by the propagation matrix method. In view of the main structure factors such as the thickness of different dielectric layers and the arrangement of different dielectric layers, the shielding effectiveness of different dielectric layer structures in the frequency band of 300MHz is explored. The results show that the layered structure represented by structure c exhibits better shielding effectiveness in different situations, indicating that the more layers and thickness of the strong lossy medium, the better the shielding effectiveness. At the same time, this arrangement of the strong lossy dielectric layer and the weak lossy dielectric layer has the best increase effect on the shielding effectiveness compared to structures a, b, and d.

ABSTRACTThe polarization reversal was studied in single crystals of lithium tantalate with intermediate deviation from stoichiometric composition (([Li]/([Li]+[Ta]) = 48.9%) with and without surface photoresist layer. The switching current was analyzed according to the stages of domain structure evolution revealed during in situ visualization of domain kinetics. The characteristic times of the switching process were extracted by fitting of the experimental data using modified Kolmogorov-Avrami model. The formation of the self-assembled domain chains was observed during polarization reversal with photoresist dielectric layer on one polar surface.

Ground Penetrating Radar (GPR) is a promissing sensor for landmine detection, however there are two major problems to overcome. One is the non-planer (e.g. rough and/or undulating) ground surface. It remains unremovable clutters on a sub-surface image output from GPR. Geography adaptive scanning is useful to image objects beneath non-planer ground surface. The other problem is the distance between the antennas of GPR. When imaging the small objects such as an anti-personnel landmine buried in shallow depth, it increases the nonlinearity of the relationship between the time for propagation and the depth of a buried object. In this paper, we modify Kirchoff migration so as to account for not only the variation of height and pose of the sensor head, but also the antennas alignment of the vector radar. The validity of this method is discussed through application to the signals acquired in experiments.

In this paper, we demonstrate, at 300 GHz and with integrated technology, the effectiveness of artificial dielectric layers to enhance the front-to-back ratio of printed antennas. This concept was previously proposed at microwave frequencies and using printed circuit board technology. The artificial material is now realized by introducing non-resonant metallic inclusions in a silicon dioxide host material. This allows to enhance the permittivity of the host medium and renders it anisotropic. By loading an electrically thin dielectric with these metallic inclusions, an engineered slab with effectively quarter wavelength thickness has been realized. Despite the large effective height and density of the artificial dielectric, the surface wave efficiency of the antenna is 99%. This is entirely due to the anisotropic properties of the material. A prototype antenna was built using an in-house CMOS back-end compatible integrated circuits (IC) process. Measured results from the antenna are presented and show a good agreement with the expected results.

In this letter, the reflection elements based on artificial dielectric layers (ADLs) are proposed for broadband wide-angle bistatic radar cross section (RCS) reduction. ADLs are introduced to improve the interference cancellation bandwidth of the reflection elements, and avoid the dilemma of mixing multiple dielectric mediums. The reflection elements achieve an approximate 180° phase difference over the frequency range of 4.8–17.5 GHz (113.9%) under normal incidence, and the bandwidth under TE 40° incidence and TM 55° is 113.9% and 100%, respectively. To validate this design, a coding scatterer surface is simulated and fabricated. Measured results show that the proposed coding scatterer surface achieves a 10 dB bistatic RCS reduction from 5 to 17.5 GHz under normal incidence for both polarizations. Bistatic RCS reduction under oblique incidence is analyzed and presented as well.

We present an analytical formulation to describe the plane-wave propagation through artificial dielectric layers (ADLs) of finite thickness. The formulation is based on a spectral domain approach derived for connected arrays. An analytical circuit model based on this approach is reported, which makes use of lumped loads to represent the reactance associated with each layer and the coupling between adjacent layers. Furthermore, a zero order mode transmission lines is used to characterize the transmission and the reflection of an incident plane wave. The circuital representation is first derived for an infinite number of layers and then extended to treat the case of finite height ADL slabs.

This paper discusses the potential of Large-eddy simulation for predicting the turbulent characteristics in a spatially developed turbulent boundary layer over a rough ground surface. Lund et al. proposed the method using quasi periodic boundary condition for smooth-wall turbulent boundary layer. Here we modify this method so that it could be applied to the flow over a rough ground surface. Roughness elements are arrayed on the ground to match the mean velocity profile with power law with exponent a = 0.25. The roughness length, the roughness displacement and the turbulence intensities are in good agreement with experimental data. The ratios of integral scales of turbulence almost conform to previous investigations. However, we necessarily can not simulate all the turbulent quantities, so the complete periodic boundary condition is required for elimination of unfavorable peak in the spectra of a velocity fluctuation.

This paper discusses the detection of small three-dimensional (3-D) dielectric objects with low contrast (relative to their surrounding media) buried near the 3-D rough ground surface by means of stand-off ground penetrating radar. This case scenario was derived from the requirements on detecting small non-metallic anti-personnel (AP) mines in realistic minefields, where AP mines are buried a few centimeters beneath rough ground surfaces. If the dielectric contrast between the objects and their surrounding media is low, and if the objects are small and buried just beneath rough ground surfaces, then scattering from the rough ground surfaces will dominate and hence mask the weaker scattering from these buried objects. Various analyses and numerical simulations have shown the considerable complexity of scattering from rough surfaces and/or the targets buried beneath rough surfaces. In this paper, we present the methodology using measured GPR phase data to deal with this complex problem without using a-priori knowledge about the target and/or ground characteristics.

A rigorous three-dimensional (3-D) electromagnetic model is developed to analyze the scattering from anti-personnel (AP) nonmetallic mine-like target when it is buried near a clutter object under two-dimensional (2-D) random rough surfaces. The steepest descent fast multipole method (SDFMM) is implemented to solve for the unknown electric and magnetic surface currents on the ground surface, on the target and on the clutter object. A comprehensive numerical investigation of two clutter sources; the ground roughness and the nearby benign object, is presented based on using more than 800 random rough surface realizations which could not be achieved without using fast algorithms such as the SDFMM. The statistics of the scattered near-electric fields are computed using the Monte Carlo simulations for both polarizations. For the parameters used here, the results show that the average and the standard deviation of the target signature represent 5-7% and 3-3.5% of the total scattered signal, respectively, while they represent 16-20% and 7-12% of the signal for the clutter object, respectively. This study indicates the high possibility of a false alarm during the detection process when the target is located nearby a realistic object such as a piece of a tree root.

In this work we present a novel adaptive ground clutter removal (AGCR) algorithm for ground penetrating radar (GPR) applications. The reduction or even removal of the disturbing ground clutter, which mostly obscures the wanted reflections of buried targets, is very important especially for the detection of shallowly buried anti-personnel mines (APMs). Most of the ground clutter removal algorithms used nowadays, show weak performances in the case of a rough ground surface or suffer from high computation complexity. This algorithm is not designed to detect flush buried APMs. Further work is needed to ensure that the algorithm can detect all APM mines from flush buried to the maximum depth of cover of 13 cm as required by the UN SOPs. The algorithm presented in this contribution is capable to remove the ground clutter resulting from undulated surfaces by estimating not only the position but also the varying signal shape. This technique enables a near optimal ground clutter reduction with a moderate computation complexity. Furthermore, the proposed AGCR algorithm preserves the characteristic impulse responses of the buried objects, which affords a following target classification based on these significant reflections. The performance improvements of the proposed AGCR algorithm compared to standard algorithms are shown on measured data. The GPR data were collected with a laboratory test set-up which is also described briefly in this work.