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The second-order high frequency (HF) radar cross section (RCS) of the ocean surface, normalized to the area of the scattering patch, is derived for the case in which the radar transmitting and receiving antennas are mounted on a swaying platform or barge. The second-order result includes both electromagnetic and hydrodynamic contributions. The derivation for the hydrodynamic patch scatter component, for time pulsed radars, is based on the first-order RCS found in the counterpart of this paper by replacing the first-order ocean wave spectrum with the second-order ocean wave spectrum. The electromagnetic patch scatter development begins with a general expression for the bistatically received second-order electric field in which platform sway is introduced. Based on an assumption that the ocean surface can be described as a Fourier series whose coefficients are random variables, the second-order monostatic RCS is developed. The resulting second-order cross section is found to consist of Bessel functions and no singularity exists in the newly derived electromagnetic coupling coefficient. Simulation results for the new RCS are also provided to indicate the effects of barge motion under a variety of sea states. [ABSTRACT FROM PUBLISHER]

A comparative analysis of the rain attenuation evaluated from beacon measurements and from a single polarization S-band Radar is performed. The beacon measurements are obtained for two slant satellite paths with different elevation angles in the Ku- and Ka-band. The single polarization S-band Radar reflectivity data is used to predict the attenuation along the satellite propagation paths. This is done by first converting the reflectivity into rain rate using the Z-R relations suitable for the tropical region and, afterwards, by evaluating the slant path attenuation through the integration of the specific rain attenuation derived from the point rainfall rate. An empirical calibration factor for the Radar reflectivity is provided. A comparison of the rain fade suffered from the two satellite paths is presented both on event basis and in terms of cumulative distribution functions. The empirical calibration factor and a single Z-R relation suitable for the tropical region are used for rain attenuation evaluation from single polarization Radar data. [ABSTRACT FROM PUBLISHER]

Radar clutter in a non-standard atmosphere usually is modeled based on a single grazing angle at each range. Instead, the angular distribution of incident power can be used to obtain a more accurate model of the clutter. Angular spectral estimation provides the grazing angle distribution of propagating power. However, a large gradient in the refractivity profile, e.g., an evaporation duct, distorts plane wave propagation which in turn violates assumptions of plane wave spectral estimation. Ray tracing is used in these situations, but has its own limitations (e.g., shadow zones). We suggest using curved wave spectral estimation (CWS) that yields reliable results for any refractivity profile, in contrast to plane wave spectral estimation. CWS is used to derive multiple grazing angle clutter, a model for ocean surface clutter in the microwave region that depends on all incident angles at each range and their corresponding powers. [ABSTRACT FROM PUBLISHER]

The detection and tracking of targets in highly cluttered environments poses a challenge due to the strong clutter backscatter present in the scene. A design concept based on a planar retro-reflective array to aide in the standoff detection and tracking of large metallic objects in a warehouse setting is presented. While traditional methods to achieve enhanced detection involve an increase in gain of the target, this retro-reflector design seeks to minimize the footprint of the tag by utilizing contrasting polarization properties of the target compared to the surrounding clutter. This allows enhanced detection from a reduced size by maximizing the ratio of target signal to that of the clutter. To accomplish this, a unique series-fed dual polarized antenna is designed and implemented with minimal cross-coupling among elements. The isolation allows for the integration of amplifiers with switchable gains into the design architecture to further reduce size, enhance detection and provide the array with unique polarization properties further contrasting those of the clutter. These retro-reflector tags are provided a unique identification by modulating their radar cross section in a prescribed manner. This compact array is implemented at 26 GHz and design details including measured performance for this cross-polarized retro-reflective tag are presented. [ABSTRACT FROM PUBLISHER]

Radar absorbing honeycomb composites with different coating thicknesses are prepared by impregnation of aramid paper frame with solutions containing conductive carbon blacks (non-magnetic) and PI (polyimide). Expressions for the effective permittivity of the composites are studied and validated both in theory and experiment. It is found that a theoretical equivalent panel with given permittivity can be obtained to represent the honeycomb structure in the quasistatic approximation, which provides a feasible way to optimize the design of radar absorbing honeycomb structure by connecting the effective electromagnetic parameters with the unit cell dimensions. The effective permittivity is measured by a network analyzer system in the frequency range of 8–12 GHz and compared with the theoretical result. [ABSTRACT FROM PUBLISHER]

In order to correctly identify a remote target, a robust target signatures identification technique is required. Radar target identification based on complex natural resonances (CNRs) has drawn the interest of many researchers following the development of the singularity expansion method (SEM). CNRs are popular due to the fact that they are theoretically independent of the aspect angle between the radar and the target, and they form a minimal set of parameters by which the target can be identified thus assisting the classification problem. As evident from the literature, statistical techniques such as the generalized likelihood ratio test (GLRT) have produced a better identification result, in the presence of noise, compared to some other SEM based identification methods such as the extinction pulse (E-pulse) technique. In this communication, we develop yet another novel statistical method based on canonical correlation analysis (CCA) to perform target classification. Simulation results using various targets show that our method is comparable to the GLRT method in the presence of white Gaussian noise. [ABSTRACT FROM PUBLISHER]

A phase-only method of generating notches in the beam pattern of a phased array antenna is described. It is shown how the problem can be formulated as a semidefinite programming problem, and solved with readily available solvers. Numerical results simulating the performance of a 32 element uniform linear array are presented. [ABSTRACT FROM PUBLISHER]

Ultra-wideband (UWB) radar with its high range resolution and applicability to optically harsh environments, offer great promise for near field sensing systems. It is particularly suitable for robotic or security sensors that must identify a target in low visibility. Some recently developed radar imaging algorithms proactively employ multiple scattered components, which can enhance an imaging range compared to synthesizing a single scattered component. We have already proposed the synthetic aperture radar (SAR) method considering a double scattered, which successfully expanded a reconstructible range of radar imagery with no a priori knowledge of target or surroundings. However, it requires a multiple integration of the received signals, requiring the fifth times integration in the 3-D case. Thus, this method requires an intensive computation and its spatial resolution is insufficient for clear boundary extraction such as edges or specular surfaces. As a substantial solution, this paper proposes a novel shadow region imaging algorithm based on a range derivative of double scattered signals. This new method accomplishes high-speed imaging, including a shadow region without any integration process, and enhances the accuracy with respect to clear boundary extraction. Results from numerical simulations verify that the proposed method remarkably decreases the computation amount compared to that for the conventional method, especially for the 3-D problem, enhancing the visible range of radar imagery. [ABSTRACT FROM PUBLISHER]

This communication presents results for the low sidelobe synthesis using amplitude-only tapering applied on the turned ON elements of large circular thinned arrays. The low sidelobe synthesis was carried out with the same iterative Fourier transform method that was developed earlier to restore the original sidelobe performance in case of defective array elements. The presented low sidelobe results refer to highly thinned circular arrays with diameters ranging from 25 to 133.3 wavelengths and involved both sum and difference patterns. In addition, sector nulling in combination with low sidelobes was numerical investigated. [ABSTRACT FROM PUBLISHER]

Knowledge of the basic characteristics of the wall is especially important for the proper function of through-wall radars (TWR). In this paper, we introduce an estimation method that can provide the unknown wall parameters in real time. Our algorithm is based on a modified version of common midpoint processing, a technique adopted from geophysics. Using this method, it is possible to estimate the thickness and the permittivity of the wall from time-delay-only measurements performed at several different transceiver-receiver separations. Time-delay estimation (TDE) of echoes backscattered from the wall at each antenna separation is performed entirely in the frequency domain by a subspace super-resolution method. Our TDE approach is capable of resolving overlapping echoes reflected from the wall even in the case of only one snapshot of the measured data, suboptimal calibration possibilities, imperfect deconvolution of the measurement system, and limited radar bandwidth. The performance of our estimation algorithm is verified by simulations, and we also present the results for two real brick walls measured by using the time-domain technique. The estimated wall parameters can be utilized for improving the accuracy of real-time locating and tracking of moving humans by TWR. [ABSTRACT FROM AUTHOR]

MultiEXCELL, a new rainfall model oriented to the analysis of radio propagation impairments which was developed on the basis of a comprehensive rain field database collected by the weather radar sited in Spino d'Adda (Italy), is presented. Single rain cells are modeled by an analytical exponential profile which best represents real single-peaked rain structures. The rain cells' probability of occurrence is analytically derived from the local rainfall statistics. The spatial features of the rain field at mid- and large-scale are investigated through their natural aggregative process. The clusters (aggregates) of cells are studied in terms of distance between individual cells, number of cells per aggregate, and distance between aggregates. Finally, the fractional area covered by rain, which the rainfall spatial correlation strongly depends on, is derived from radar data through the comparison with the same quantity provided by global long-term numerical weather products. The MultiEXCELL procedure for the generation of spatially correlated synthetic rain fields is duly presented and the model's accurateness is preliminary assessed against the available radar dataset. Although MultiEXCELL is mainly oriented to propagation-related applications, its cellular approach may reveal useful also in hydrology, for the prediction/management of water resources, and in meteorology, for the nowcasting of the temporal evolution of rain structures. [ABSTRACT FROM AUTHOR]

Site diversity is an effective rain attenuation mitigation technique, especially in the tropical region with high rainfall rate. The impact of different factors such as site separation distance, frequency, elevation angle, polarization angle, baseline orientation and wind direction is assessed. Results are compared to those reported in existing literature and also compared to the commonly used ITU-R site diversity prediction models. The effect of the wind direction on site diversity is also presented. It can be observed that diversity gain is highly dependent on the site separation distance, elevation angle and wind direction but independent of the frequency, baseline angle and polarization angle of the signal. This study is useful for the implementation of site diversity as a rain attenuation mitigation technique. [ABSTRACT FROM AUTHOR]

A method has been developed to adjust radar cross section (RCS) scattering models for two-body effects, particularly shadowing. The method described here involves using the near field scattering solution of one object to apply two-body corrections to another. By assuming that the second object can be represented by a known set of point scatterers, the fields at these scattering centers are used to calculate the adjusted RCS in the presence of the shadowing object using the principle of reciprocity. Since this approximation does not require a full-wave electromagnetic solution, it is considerably more computationally efficient than moment method solvers or other similar techniques, and thus could be adapted to real-time applications. The results of the method are compared against measurement and Moment Method simulation for the case of two cylinders, a cylinder and a cone, and two spheres. Good agreement is generally obtained between the proposed approximate model and measurements or Moment Method simulations. [ABSTRACT FROM AUTHOR]

The shooting and bouncing ray (SBR) method may give rise to a divergence problem when ray tubes intersect discontinuous parts of the target, such as the boundary, and this affects the accuracy to some extent. This paper proposes an adaptive aperture partition algorithm to solve this problem. The proposed algorithm adaptively splits the virtual aperture into continuous irregular beams instead of discrete uniform ray tubes according to the geometry of the target during the recursive beam tracing. These beams form a beam tree, the level of which represents the number of reflections. Geometric optics is applied to the representative propagation path of each leaf beam to obtain the exit field, and then physical optics is used to evaluate each leaf beam's scattered field. The proposed algorithm could generate the convergent solution of the SBR method when the ray-tube size tends toward infinitesimal. Additionally, this paper describes the beam-triangle intersection in detail and utilizes the kd-tree to accelerate the beam-target intersection. Numerical experiments demonstrate that adaptive aperture partition can greatly improve the accuracy of the SBR method, and the computational efficiency can be also significantly enhanced in several applications, such as the RCS prediction in the THz band. [ABSTRACT FROM AUTHOR]

We propose an approach to the design of array-based, 2D echo generators. The radiating elements are located on non-uniform grids and the quiet zone (QZ) design specifications are enforced at non-uniformly spaced sampling locations. The approach, based on a singular values optimization process, allows dimensioning the size of the echo generator to meet the QZ specifications, defining the number and locations of the QZ sampling points and of the radiators to control the ill-conditioning when searching for the excitation coefficients, dramatically reducing the number of radiating elements, and finding the excitations by a Singular Value Decomposition approach. The performance of the method in terms of QZ field behavior and robustness against realization errors is numerically assessed. An experimental validation of the technique is also presented. [ABSTRACT FROM AUTHOR]

Previous work on rain spatial classification for terrestrial PMP radio systems has been extended in this communication to stratospheric systems—high altitude platform systems. It was found that the rain spatial parameter concept can also be effectively applied to HAP systems, although specific adjustments have had to be made to the method. The differences between terrestrial and stratospheric systems are discussed with regard to the new derived dependences. [ABSTRACT FROM PUBLISHER]

Antenna effects on a monostatic multiple-input-multiple-output (MIMO) Radar for direction estimation are studied by analyzing the Cramèr-Rao lower bound (CRLB). The CRLB is derived for a multi-band MIMO Radar system, and is in a form that incorporates the characteristics of a practical antenna array. Two different uniform linear antenna arrays, one narrowband and the another wideband, are investigated by exploring the CRLB. The estimation performance for the real antenna array is compared to that for an ideal array composed of isotropic antenna elements. It is found out that the mutual coupling between antenna elements alters the radiation patterns a lot, and therefore tremendously affects the estimation performance. Furthermore, the study demonstrates that by distributing the transmitted power over several frequency bands, more accurate estimation can be achieved due to frequency diversity, and the negative effects of radiation pattern deviation can be alleviated. [ABSTRACT FROM PUBLISHER]

Attenuation measured on terrestrial links can be used to estimate surface rainfall rates along a link path. There is current interest in the use of inverse methods to estimate rainfall over areas inferred from terrestrial links. A power-law relationship can be used to estimate rainfall rate from attenuation measured by a terrestrial link. However, the inverse method is simplified with a linear relationship between attenuation and rainfall rate when determining path averaged rainfall rates. This paper investigates power-law and linear relationships between rainfall rate and attenuation. It examines their goodness of fit for a range of frequencies and rainfall rates. Six years of disdrometer data from Chilbolton in the U.K. has been used to study both linear and power-law fits to attenuation and quantify the associated errors. [ABSTRACT FROM AUTHOR]

A high-efficiency self-shielded microstrip-fed Yagi-Uda antenna has been developed for 60 GHz communications. The antenna is built on a Teflon substrate (\varepsilonr = 2.2) with a thickness of 10 mils (0.254 mm). A 7-element design results in a measured S 11 of < -10~dB at 56.0 – 66.4 GHz with a gain> 9.5 ~dBi at 58 – 63 GHz. The antenna shows excellent performance in free space and in the presence of metal-planes used for shielding purposes. A parametric study is done with metal plane heights from 2 mm to 11 mm, and the Yagi-Uda antenna results in a gain> 12 ~dBi at 58 – 63 GHz for h = 5 -8~mm. A 60 GHz four-element switched-beam Yagi-Uda array is also presented with top and bottom shielding planes, and allows for 180^\circ angular coverage with < 3~dB amplitude variations. This antenna is ideal for inclusion in complex platforms, such as laptops, for point-to-point communication systems, either as a single element or a switched-beam system. [ABSTRACT FROM AUTHOR]

A novel procedure for high resolution radar scattering signature prediction of near field targets is proposed and applied to radar-target end-game electromagnetic (EM) modeling as well as hard-ware-in-the-loop (HIL) simulation. By introducing the concept of distinct wave propagation vector (DWPV), this paper develops formulations of near field version of physical optics (PO), Michaeli's equivalent edge currents (EEC) and iterative physical optics (IPO) to predict the high frequency wideband EM scattering from targets with perfect electric conducting (PEC) surfaces in the near field region. In order to characterize the variation of the near field scattering from a complex target with respect to time variant radar-target interaction by using solely one succinct radar image, an approach for two-dimensional (2-D) high resolution inverse synthetic aperture radar (ISAR) image sequence generation and synthesis is developed. Numerical results demonstrate the applicability of the proposed procedure to generate high resolution radar scattering signatures of near field targets with high fidelity. [ABSTRACT FROM AUTHOR]

The need to reduce the number of active elements in radar arrays claims for developing new synthesis approaches able to reduce the control points, i.e., the number of transmit/receive modules (TRM) needed to fulfill the design constraints, is presented. As the large part of the TRM costs are related to the active parts (HPA—high power amplifier), a hybrid method for reducing the number of amplitude controls is proposed. This allow the design of new array architectural solutions where the scanning capabilities only rely on phase controls. [ABSTRACT FROM PUBLISHER]

The first-order high frequency surface wave radar (HFSWR) cross section of the ocean surface is derived for the case of the transmitting and receiving antenna being mounted on a floating, but otherwise fixed, ocean platform. It is assumed that the sway component of the platform or barge motion is responsible for observed differences in the cross section compared to that for the fixed antenna case. Based on earlier work, a general expression for the bistatically received first-order electric field, which consists of a two-dimensional spatial convolution, is presented and reduced to integral form. Then, it is assumed that the surface can be described by a Fourier series whose coefficients are zero-mean Gaussian random variables, and from there the analysis proceeds for the backscatter case. The integrals are taken to the time domain, with the source field being that of a barge-mounted omnidirectional vertically polarized pulsed dipole antenna. Subsequently, the first-order monostatic radar cross section is developed and found to consist of Bessel functions. Simulation results for the new cross section are also provided to show the effects of barge motion under different sea states and operating frequencies. It is seen that the results have important implications in the application of HFSWR technology to ocean remote sensing. [ABSTRACT FROM AUTHOR]

A novel transmit array beamforming approach is introduced that offers low probability of intercept (LPI) for surveillance radar systems employing phased array antennas. Radar systems are often highly visible to intercept receivers due to the inherent two-way versus one-way propagation loss. In this paper, the traditional high-gain antenna beam scanned across a search region is replaced with a series of low-gain, spoiled beams. Keeping the transmit antenna gain low reduces the radar visibility, but the radar's antenna performance remains unchanged as the original high-gain beam can be formed by processing the set of spoiled beams. Large transient power density radiated in a traditional scan is replaced with low power density persistently radiated at the target throughout the scan time. The detection performance of the radar is not affected since the total energy on the target is the same. Derivation of the complex weights to synthesize the high-gain patterns from the low-gain basis patterns is presented for both one-way and two-way beam patterns. [ABSTRACT FROM AUTHOR]

The analysis, design, and realization of a modified bow-tie antenna optimized for impulse ground penetrating radar (GPR) applications is described. The proposed antenna shows improved properties important for GPR, which include its compact size (in comparison with a conventional bow-tie antenna) and ability to radiate UWB pulses with increased amplitude and very small late-time ringing. A substantial increase in the amplitude of the transmitted pulse is achieved by utilizing radiation from discontinuities introduced by the resistive loading employed in the antenna to suppress late-time ringing. By choosing an optimal distance between the antenna's feed point and the location of the resistive loading, radiations that occur from the antenna's feed point and the mentioned discontinuities at the resistive loading will combine constructively in the boreside direction of the antenna. As a result, one will observe a substantial increase of the amplitude of the transmitted pulse in the boreside direction. Furthermore, an analytical expression describing approximate time-harmonic current distribution is derived to indicate an optimal resistive loading profile for the proposed antenna. Additionally, the traveling-wave current distribution of the antenna is theoretically analyzed to examine the applicability of the obtained time-harmonic expression for pulse excitation. It has been found that when the antenna is resistively loaded both the time-harmonic and traveling-wave currents decay to approach nearly the same value at the end section of the antenna. As the amount of current at the antenna ends corresponds to the level of reflection which occurs there, the derived expression is found to be useful to indicate an optimal loading profile for the proposed antenna. A theoretical model of the proposed antenna has been developed to perform numerical analysis using a modified NEC-2 code. In addition, an experimental verification has been carried out and both the simulation and experiment confirmed the improved properties of the proposed antenna. [ABSTRACT FROM AUTHOR]

A balanced antipodal Vivaldi antenna is designed to be used as a sensor for a microwave breast cancer detection system. The antenna has the ability to send short electromagnetic pulses into the near-field, with low distortion, low loss and in a directional manner. The antenna directivity is further improved by the inclusion of a novel feature in the antenna aperture called a "director" which consists of a profiled piece of higher dielectric constant material. Several simulated results are successfully confirmed with measurements. Reflections of a tumor placed in a breast model are simulated for two cases, namely a balanced antipodal Vivaldi antenna with and without a director. Greater tumor responses are recorded with the director present, demonstrating the potential of this feature for microwave breast imaging. [ABSTRACT FROM AUTHOR]

This paper presents an improved antenna array for radar-based breast cancer imaging. The improvement was achieved by increasing the number of antennas in the array to 31 elements, as well as by improving the antenna design itself. Using an experimental setup, with homogeneous curved breast phantoms, we have demonstrated substantial imaging improvement with the new antenna array. The new system is also able to detect 7 mm-diameter tumor phantoms in any location within the breast, even as close as 4 mm from the skin layer. Additionally, we have shown good imaging results in low-contrast scenarios, where the dielectric contrast between tumor and normal tissue was reduced to 2:1. Presented results clearly demonstrate the large impact of antenna's characteristics on imaging performance. [ABSTRACT FROM AUTHOR]

In the resonance region, the radar scattering response of any object can be modelled by natural poles with the formalism of the singularity expansion method. These natural poles are resonance parameters which provide useful information for the discrimination of radar targets as their general shape, characteristic dimensions and constitution. In the case of an open radar target, high-Q internal resonances and low-Q external resonances occur respectively inside the target and on its surface. Because internal resonances have a higher Q, they may have a higher total energy and can thus be used for target identification. In this paper, we choose to study the resonance behavior of a perfectly conducting rectangular cavity with a rectangular aperture. With this simple example, we intend to show how to distinguish between the two origins of these resonances: external resonances corresponding to traveling waves on the surface of the target and internal resonances corresponding to cavity waves. Indeed, this can be applied to characterize aircrafts, whose apertures (such as inlets, open ducts, air-intakes, cavities etc.) contribute significantly to the overall radar cross section. [ABSTRACT FROM AUTHOR]

The extinction pulse (E-Pulse) technique has been widely applied to problems involving radar target identification. In this paper a fast approximate target detection and recognition scheme based on the E-Pulse technique is proposed and applied to a subsurface target detection and recognition scenario. Previous studies have demonstrated that the target resonances for subsurface targets are closely related to the target resonances for a target within a homogenous environment. In the proposed method, the target resonance for the target in the homogenous medium will be used to construct the E-Pulse for target detection and recognition purposes. The details of the proposed method will be described in this paper. The obvious example of a target below a dielectric halfspace is the use of ground penetrating radar (GPR) for detecting and recognizing unexploded ordnance (UXO). However, instead of a GPR related scenario, a numerical example of a biomedically related problem, of a hip prosthesis model sited within a halfspace of homogenous human tissue model with realistic dielectric properties will be used to demonstrate the feasibilities of the proposed technique for target detection and recognition. The reasons for the choice of this particular example will also be explained in the paper. [ABSTRACT FROM AUTHOR]

We exploit the joint fractal properties of sea clutter extracted from detrended fluctuation analysis (DFA) for targets detection. We find that two specific fractal statistics, i.e., the intercept at the crucial scale and the Hurst exponent of optimal scales provide valuable information for targets detection. The first statistic measures the discrepancy between sea clutter and low observable targets at the crucial fractal scale, and the second one evaluates the average fractal difference within the optimal multi-scales. A target detection method integrating these two statistics is proposed, which is validated by real-life IPIX radar datasets. We find that this joint fractal detection approach achieves more accurate results for low observable targets detection. [ABSTRACT FROM AUTHOR]

A deterministic analysis of spatial diversity is presented in connection with radar systems. A numerical technique based on physical optics is used for our analysis. Contrary to statistical models, the proposed technique takes into account accurate near-field radar cross section of the target, and radiation characteristics of transmitting and receiving antennas. The power scattered by the target and received by multiple antennas as a function of the target aspect angle and distance is analyzed. Two combining methods of received powers are tested and statistical analysis is performed showing that, using spatial diversity, the angular range can be increased significantly and the standard deviation of the target response can be reduced. In order to validate our analysis and proposed scheme, experimental measurements were carried out using a metallic plate and a car as targets. This work has potential applications in automotive collision warning/avoidance radar systems. [ABSTRACT FROM AUTHOR]

Spatial mapping of electric field distribution is an important objective in many applications including antenna pattern, radar cross-section, and specific absorption rate measurements. Modulated scatterer technique (MST) based on small loaded dipoles has been successfully used for these purposes for many years. However, MST suffers from several inherent limitations. These limitations become more severe as the frequency increases. This paper introduces a new and efficient method based on a multiple loaded scatterer (MLS) approach. The proposed method allows for accurate recovery of an unknown incident electric field with measurements conducted at a single observation point. The formulation of the MLS method along with several key simulation results illustrating its efficacy for electric field distribution measurement is presented. In addition, the results of several comparisons with conventional MST are also provided. [ABSTRACT FROM AUTHOR]

Signal processing techniques to improve building interior images are introduced. Specifically, an adaptive CLEAN algorithm is introduced in conjunction with target refocusing techniques to de-emphasize undesired signal effects from external walls, and thus more accurately image the lesser contrast objects. Instead of using the point spread function as in the conventional CLEAN algorithm, adaptive CLEAN incorporates a delay spread model found more appropriate for subtracting sidelobes that could obscure interior objects. Refocusing also allows for correcting target distortion and displacement due to through-wall propagation. It is shown that by combining adaptive CLEAN with target refocusing techniques, we are able to reconstruct interior targets (with minimal exterior wall distortion). The introduced processing techniques are demonstrated via simulations generated with a well validated high frequency ray tracing code, and with measured data. [ABSTRACT FROM AUTHOR]

In this paper, an alternating-direction implicit (AD!) scheme is applied to the finite-difference time-domain (FDTD) method for solving electromagnetic scattering problems in a generalized coordinate system. A formulation for two dimensional problems is presented and its numerical dispersion and stability property are discussed. In our generalized approach, the nonorthogonal grid is used to model the complex region of a scatterer only, whereas the standard FDTD lattice is used for the remaining regions. As a result, accurate griddings with a simple algorithm can be obtained using the new scheme, and the complexity of the algorithm is minimal. The perfectly matched layer (PML) is used to truncate the boundary. To illustrate the theory, a sinusoidal plane wave and a Gaussian pulse that propagates through,a space modeled by locally nonorthogonal grids are used, with the stability of the code examined. The radar cross section of a perfectly conducting cylinder with a thin coating, a large curvature, and/or a sharp edge is calculated using the proposed method, and the result is compared with those using other conventional FDTD methods. It is found that the proposed algorithm is much more efficient than its FDTD counterpart when a complex object is analyzed. [ABSTRACT FROM AUTHOR]

Through-the-wall imaging techniques with ultrawideband(UWB) radars are promising candidates for non-destructive testing and reliable human detection, especially in disaster areas, where victims are buried under collapsed walls. These applications require high-resolution target imaging to identify the object shape, such as a human body. We have already proposed a highquality 3-dimensional (3-D) imaging algorithm in the form of Envelope, that is aimed at near field sensing for non-contact measurement or target identification for robots. Envelope achieves realtime accurate 3-D imaging with group mapping from multiple observed ranges to target points, and offers a reliable image even in noisy situations. However, this method does not maintain its quality for through-the-wall imaging because an observed range shift due to wall penetration causes a serious distortion in the image. This paper presents a high-resolution 3-D imaging algorithm by modifying the original Envelope, and which gives a more accurate object shape behind a wall. Furthermore, to enhance the resolution of the estimated images, this method is combined with a direct waveform compensation method, known as spectrum offset correction. Numerical simulations and an experiment verify that our proposed method achieves high-resolution 3-D imaging for through-the-wall radar applications. [ABSTRACT FROM AUTHOR]

We consider the estimation of moving targets located behind concrete walls reinforced with metallic bars, using radar measurements. The periodic structure of the rebar severely attenuates and distorts transmitted waveforms, which produces defocused images with ghost target estimates. We apply beamforming to estimate permittivity and thickness of the wall and number and position of the targets. The proposed solution is based on accurate physical models calculated using the method of moments.We show that the estimation is significantly improved by modeling the waveform distortion due to the bars. The resulting images are focused and clearly represent the contours of the targets. The algorithm is robust to the ambiguities in bar parameter values. In addition, the minimal necessary S N R is lower compared with the case in which the influence of the bars on the signal shape is ignored. [ABSTRACT FROM AUTHOR]

An extended and rigorous analysis of retrodirective cross-eye jamming in a radar system scenario is presented. This analysis removes the approximations that limit the validity of other analyses of cross-eye jamming. These results imply that under certain conditions, a monopulse radar system can be more easily deceived than suggested by conventional cross-eye analyses. Furthermore, the cross-eye jammer antenna patterns do not affect the induced monopulse error. [ABSTRACT FROM AUTHOR]

A triangular grid single polarized tapered-slot array antenna for radar applications is studied. Compared with a rectangular grid an equilateral triangular grid allows a larger unit cell without any onset of grating lobes. Since single polarized tapered-slots in triangular grids support guided modes, which cause scan blindness, the increase in unit cell size is smaller than the optimal 15%. The design presented in the paper is capable of scan angles out to 600 from broadside in the E and H planes. To improve the match over the radar band a local minimum in the active reflection coefficient is positioned at the most critical scan direction, resulting in a reflection coefficient that is less than -12 dB in the X-band. To reduce the radar cross section for the cross-polarization an absorbing layer is positioned above the ground plane, which affects some of the guided modes that lead to scan blindnesses. An experimental antenna with 16 × 16 elements was built, and it was found that the H-plane performance for large scan angles for the finite antenna deviates more than expected from the infinite array approximation. Otherwise both mutual coupling measurements and embedded element patterns agrees well with the numerical results. [ABSTRACT FROM AUTHOR]

Estimating building layouts using exterior radar measurements is a challenging task involving electromagnetic modeling, many unknown parameters, and a limited number of sensors. We propose using the jump-diffusion algorithm as a powerful stochastic tool that can be used to estimate the number of walls, their unknown positions and other parameters. We develop an efficient iterative procedure that first uses low-frequency transmissions to obtain rough estimates of the building layout. These estimates are then used to initialize the estimation at higher frequencies, thus obtaining more accurate results; We show that with proper frequency selection, the building layout can be estimated using radar measurements at only a few frequencies. Numerical examples are used to illustrate the performance of the proposed algorithm in practical situations. [ABSTRACT FROM AUTHOR]

A new method to measure radar cross section (RCS) parameters of antennas is presented in this paper. This method relies on an equation derived for the received signal power which is represented as the superposition of structural-mode scattered, antenna-mode scattered, and leakage signals. The method also measures relative phase and provides the effect of the load connected to the antenna. The received signal power equation is solved using minimum mean square error estimation, and the solution is used to obtain structural-mode RCS, antenna-mode RCS, and relative phase of an antenna by applying short, open, and match load cases. A measurement example of RCS parameters of a microstrip patch antenna using a simple experimental setup is presented. The structural-mode RCS, antenna-mode RCS, and relative phase factor cos (øo) of the antenna, obtained using minimum mean square error estimation solution, are -18.307 dBsm, -20.386 dBsm, and 0.970, respectively. The present method requires less lengthy experimental measurements, while providing results that are more informative and accurate as obtained from previous methods. [ABSTRACT FROM AUTHOR]

Two years of high temporal- and spatial-resolution radar data is used to quantify the rain-attenuation diversity gain available in 26 GHz fixed wireless access systems having sight of the second transmitter. The service area defined by attenuation contours on the least-faded link is determined; the results show that an expansion of the service area is possible even over short paths. Service area contours with and without diversity are derived from radar observations of rainfall rate. Good agreement is obtained with similar results for a statistical model based on a gamma probability distribution and a negative-exponential spatial correlation function for rain rate (which are shown to fit observed data). For the service area defined by a bit error rate of 10-4 and availability of 4 × 10-5, site diversity expands the service area by up to 20% with a 10 km access point spacing. [ABSTRACT FROM AUTHOR]

A method for scanning of the main beam of phased array antennas in a wide angular range is presented. The method utilizes several smaller flat antenna panels, arranged in an array of panels, each of the panels pointed in the direction of the main beam of overall antenna. The output signal of the overall antenna is summation of the signals from all antenna panels. Investigation of the array performance of such structure with respect to the distance between the panels and sidelobe level of the antenna structure is presented. [ABSTRACT FROM AUTHOR]

Large-scale full-wave simulations are performed to investigate radar wave propagation inside buildings. In principle, a radar system combined with sophisticated numerical methods for inverse problems can be used to determine the internal structure of a building. The composition of the walls (cinder block, rebar) may effect the propagation of the radar waves in a complicated manner. In order to provide a benchmark solution of radar propagation in buildings, including the effects of cinder block and rebar, we performed large-scale full-wave simulations using a finite-element time domain (FETD) method. This particular FETD implementation is tuned for the special case of an orthogonal Cartesian mesh and hence resembles finite-difference time domain (FDTD) in accuracy and efficiency. The method was implemented on a general-purpose massively parallel computer. In this paper we briefly describe the radar propagation problem, the FETD implementation, and we present results of simulations that used over 10-billion elements. [ABSTRACT FROM AUTHOR]

Higher order nodal basis functions for representing equivalent surface currents on antennas and scatterers are introduced. The performance of the nodal basis is evaluated by comparing two existing higher order edge bases, using the magnetic field integral equation (MFIE) formulation for scattering by a perfect electric conductor (PEC) sphere and icosahedron as test problems. Both nodal and edge bases are implemented on rational Bézier patches, giving an exact representation of the surfaces, free from geometrical error. The accuracy of the numerical solutions obtained with the three different bases for both the surface current and the radar cross section (RCS) are compared, and it is shown that in general the nodal bases give better accuracy than the edge bases for equal computational cost. [ABSTRACT FROM AUTHOR]

An original method is proposed which associates rigorous 3D finite element methods (FEM) with Floquet modes through FACTOPO multidomain formalism for computing the radar cross section (RCS) of large array antennas. The most original aspect of the method is the fact that the source unit cell is characterized by a generalized scattering matrix (GSM) which does not depend on the plane wave incidence impinging the structure. Consequently, the factorization step of the FEM matrix, which is the main computational effort, is performed only once. The assembling procedure consists then in expressing the FEM GSM matrix in the Floquet mode basis for each incidence and to solve a linear system giving the complex amplitude of the Floquet modes on the array/cell interface. Associated with this formalism, lateral and back structure effects can be treated using physical optics (PO) combined with the equivalent current method (ECM) to improve the accuracy for far broadside incidence. [ABSTRACT FROM AUTHOR]

Analysis of radar scattering from targets with curved boundaries, such as objects comprising cylindrical and conical shapes, is important to many aerospace applications. The radar return is composed of a well-characterized physical optics response in the illuminated region where the transmitter and receiver are not shadowed by the object, and a combination of modal responses (e.g., creeping waves and edge-diffracted fields) in the shadow region. The modal responses have longer down-range than scattering centers located on the object, and therefore, produce extended (or off-body) returns in ISAR images, which are not well-understood. However, these returns are strongly dependent on local features of the object, and thus contain valuable information with regard to the target's geometrical and physical composition. Multiple reflections from illuminated facets, as well as multiply diffracted waves, can also add coherently in the direction of the receiver and produce such returns. This paper applies a robust, coherent-processing system identification technique, originally developed for radar sensor fusion, to estimate amplitude and phase of the scatterers that characterize extended returns in the target signature. Examples are presented that highlight the extraction of creeping waves using measured data on a cone-sphere. [ABSTRACT FROM AUTHOR]

The MUSIC algorithm is a high-resolution direction finding technique which has been successfully applied to enhance radar imaging in inverse synthetic aperture radar (ISAR). Al- though this signal subspace-based method has proven effective when dealing with point targets and high signal-to-noise ratio (SNR), it may fail to work when directly applied to extended targets or target returns of low SNR. The Beamspace MUSIC (BS-MUSIC), in which the MUSIC algorithm is applied to multiple beams, is capable of locating spatially extended targets in low SNR environments. In this paper, we consider BS-MUSIC as an image formation method for indoor radar imaging problems and sensing through-the-wall applications. We compare BS-MUSIC performance to conventional beamforming and element-space MUSIC. Imaging results, using both synthesized and real data, demonstrate the advantages of the proposed algorithm in depicting targets behind walls. [ABSTRACT FROM AUTHOR]

A novel miniaturized cavity-backed composite slot loop antenna (CBCSLA) with low profile and omnidirectional radiation similar to a short dipole is presented. The antenna displays vertical polarization while having less than λ/100 dimension in height. The geometry of the antenna is inspired from a small magnetic loop which is realized using a slot configuration on metallic ground. Since a small slot loop antenna cannot be impedance matched, a composite geometry is devised that acts like as a small slot loop and can easily be impedance matched using this configuration. The diameter of the CBCSLA can be made as small as λ/10 or smaller. The feeding mechanism is designed in a manner to create a circumferentially-constant current distribution around the composite loop while effectively matching to a 50 Ωcoaxial feed from the antenna center. Additional methods are discussed to further enhance the bandwidth and achieve a dual-band response. The antennas are designed, fabricated, and measured. It is shown that despite their small lateral dimension and extremely small height, the miniaturized cavity-backed slot loop features extremely well input impedance match, uniform radiation pattern, low cross-polarization levels and expected gain. [ABSTRACT FROM AUTHOR]

A double rhombus antenna fed through a microstrip 1800 phase shifter is presented. The 1800 phase shifter is a novel design of microstrip-to-coplanar waveguide (CPW)-to-microstrip transition. The proposed phase shifter is obtained without changing the layer of the second microstrip line for an easy embedding into the feedline. Via holes are used to transfer the current from the top to the bottom substrate layer and vice versa. The antenna with the phase shifter is operating in a wide bandwidth from 5.7 and 18.6 GHz (106%), and provides stable endfire radiation patterns. This design is tested in a modified two-element array configuration and proved to have a wide usable bandwidth. [ABSTRACT FROM AUTHOR]

We show that by introducing a gap at the center of the helical sections (where the current is minimum) of a λ/2 quadrifilar helix antenna (QHA) and varying the axial length and radial gap between the overlapping volutes, the antenna gives a 28% impedance bandwidth which is nine times the bandwidth of a conventional QHA. A 16% bandwidth with a front to back ratio of ≥ 14 dB is achievable with 5-14% reduction in the size of the QHA. The structure can yield a monopole radiation pattern suitable for terrestrial applications or a hemispherical pattern suitable for satellite use. The simulation results are validated by measurements at L-band. [ABSTRACT FROM AUTHOR]

A two-layer reflectarray is proposed as a central station antenna for a local multipoint distribution system (LMDS) in the 24.5-26.5 GHz band. The antenna produces three independent beams in an alternate linear polarization that are shaped both in azimuth (sectored) and in elevation (squared cosecant). The design process is divided into several stages. First, the positions of the three feeds are established as well as the antenna geometry to produce the three beams in the required directions. Second, the phase distribution on the reflectarray surface, which produces the required beam shaping, is synthesized. Third, the sizes of the printed stacked patches are adjusted so that the phase-shift introduced by them matches the synthesized phase distribution. Finally, the radiation patterns are computed for the central and lateral beams, showing a shaping close to the requirements. A breadboard has been manufactured and measured in an anechoic chamber, showing a good behavior, which validates the designing methodology. [ABSTRACT FROM AUTHOR]