Image interpretation and prediction in microwave diversity imaging

The microwave image of a metallic object is interpreted from a point of view based on the understanding of the interconnection between the scattering mechanisms, the data acquisition system, and the image reconstruction algorithm. From this understanding it is possible to interpret and predict microwave images reconstructed from data collected over specified and angular windows. The connection between a special scattering mechanism, edge diffraction, and its reconstructed image is established. The microwave image of an edge is two bright points whose locations correspond to the end points of the edge if the normal aspect angle is not included in the angular windows; otherwise a line joining the two end points and representing the edge will appear in the image. Experimental images of a trihedral reflector constructed from data collected over different angular windows support this approach to image interpretation and prediction     

Bird-proof feed-horn windows for microwave radio system antennas

Window units constructed from multiple dielectric plates offer means for combating bird damage to antenna feed horns on microwave radio systems. The need for such devices on Australian systems is increasing as routes are extended into the drier regions of the country. Measurements show that the antenna electrical characteristics are not appreciably altered by the new window units.     

Polarization effects in microwave diversity imaging of perfectlyconducting cylinders

A theoretical vector development is derived for the directly and mutually scattered wavefield of two cylinders in a bistatic measuring system. The approach is first to give an expression for the field scattered from a single cylinder illuminated by a right-hand circularly polarized plane wave. This expression is then extended to the case of the directly scattered (or first-order) field of two cylinders. The mutually scattered (or second-order) field of two cylinders is then formulated in terms of a coherent summation of the scattered field from each cylinder due to the incident wave scattered from the other cylinder. In order to simplify computational tasks, only the second-order scattered field is analytically derived; this is called the mutually scattered field. The total wavefield scattered by two cylinders becomes the coherent superposition of the directly scattered component and the mutually scattered component for each polarization. The use of an automated microwave imaging facility employing frequency, polarization and angular diversity to verify the results of theoretical analysis is described. The analytical and experimental results are shown to be in good agreement. The results show that the effects of polarization state transformation or change in the mutually scattered field component are more pronounced than in the directly scattered component     

Optimal kernels for nonstationary spectral estimation

Current theories of a time-varying spectrum of a nonstationary process all involve, either by definition or by difficulties in estimation, an assumption that the signal statistics vary slowly over time. This restrictive quasistationarity assumption limits the use of existing estimation techniques to a small class of nonstationary processes. We overcome this limitation by deriving a statistically optimal kernel, within Cohen's (1989) class of time-frequency representations (TFR's), for estimating the Wigner-Ville spectrum of a nonstationary process. We also solve the related problem of minimum mean-squared error estimation of an arbitrary bilinear TFR of a realization of a process from a correlated observation. Both optimal time-frequency invariant and time-frequency varying kernels are derived. It is shown that in the presence of any additive independent noise, optimal performance requires a nontrivial kernel and that optimal estimation may require smoothing filters that are very different from those based on a quasistationarity assumption. Examples confirm that the optimal estimators often yield tremendous improvements in performance over existing methods. In particular, the ability of the optimal kernel to suppress interference is quite remarkable, thus making the proposed framework potentially useful for interference suppression via time-frequency filtering     

Bias criteria for the selection of spectral windows (Corresp.)

Bias criteria for evaluating spectral window functions and upper bounds of bias are derived using power spectrum models, and known window functions are evaluated according to the criteria. The function commonly called the "Hanning" window is shown to have the best bias characteristics.     

Coherent Doppler tomography for microwave imaging

A tomographic extension of the type of microwave Doppler imaging typified by synthetic aperture radar has recently been developed and shown experimentally to exhibit a high degree of spatial resolution. When CW irradiation is used, the sidelobes in the pointspread function are inherently high and tend to limit the dynamic range of the reconstructed images. The point-spread function of a system using CW irradiation and an aperture that completely surrounds the object has a central lobe of width of λ/5, but the first sidelobe is only 8 dB below the central peak. The limitation due to the high sidelobes can be partially overcome by using wide-band signals or bistatic diversity. One of the steps in reconstructing a coherent Doppler tomogram is to perform a two-dimensional Fourier transform. The ordinary two-dimensional discrete Fourier transform (DFT) produces points in the transform space on a Cartesian raster. In coherent Doppler tomography (CDT), however, the data are sampled on a polar raster. To diminish the computational burden associated with converting to the Cartesian raster and interpolating, we have developed an alternative algorithm which requires no interpolation and is based on interpreting the two-dimensional Fourier transform as a one-dimensional circular convolution integral. The quality of the images computed in this fashion compares favorably with that for the old method and the computational burden is greatly reduced.     

基于Windows平台的微波折射率仪数据采集系统

给出了在Windows98下折射率仪数据采集系统的软件实现方案，介绍了在Windows98操作系统平台下如何通过虚拟设备驱动程序去控制折射率仪信号采集卡，并对本系统的应用进行了简单的描述。     

A MULTIRATE PROCESSING APPROACH FOR MICROWAVE IMAGING

In this paper,a multirate processing approach for microwave imaging is presented.This approach has the advantages of largely compressing the raw spectral data for imaging,greatly reducing the storage requirement and enhancing the processing efficiency.To demonstrateits applicability,the proposed approach is tested on both simulated and experimental data.     

Parallel GA-based approach for microwave imaging applications

Genetic algorithms (GAs) are well-known optimization strategies able to deal with nonlinear functions as those arising in inverse scattering problems. However, they are computationally expensive, thus offering poor performances in terms of general efficiency when compared with inversion techniques based on deterministic optimization methods. In this paper, a parallel implementation of an inverse scattering procedure based on a suitable hybrid genetic algorithm is presented. The proposed strategy is aimed at reducing the overall clock time in order to make the approach competitive with gradient-based methods in terms of runtime, but preserving the capabilities of escaping from local minima. This result is achieved by exploiting the natural parallelism of evolutionary techniques and the searching capabilities of the hybrid approach . The effectiveness of the proposed implementation is demonstrated by considering a selected numerical benchmark related to two-dimensional scattering geometries.     

Coupling medium for microwave medical imaging applications

Sodium meta silicate gel is proposed as a novel coupling medium with a better coupling and a lesser dielectric loss than the conventionally used media. Dielectric constants between 8 and 20 exhibited for various relative density and pH, makes the material in perfect match with that of the various tissues. It is also useful as a phantom model material in microwave medical imaging.     

频率分集MIMO雷达信号优化设计

针对集中式多输入多输出(multiple input multiple output, MIMO)雷达扩展目标检测识别问题,提出将多频阵列(multiple frequency array, MFA)应用到集中式MIMO雷达中来实现频率分集,增加自由度,在频域基于互信息量(mutual information, MI)优化不同频带天线上的功率分配以设计信号,针对目标依据雷达功率分配情况施放干扰以避免检测识别的情况,雷达在杂波及干扰环境下再次优化信号功率分配,实现雷达认知功能。仿真结果证明,优化信号可综合噪声、杂波及干扰统计特性重新调整功率分配,可提高目标频域响应和目标回波间互信息量,为改善目标检测识别性能奠定基础。      

Three-dimensional nonlinear image reconstruction for microwave biomedical imaging

Active microwave imaging has attracted significant interests in biomedical applications, in particular for breast imaging. However, the high electrical contrasts in breast tissue also increases the difficulty of forming an accurate image because of the increased multiple scattering. To model such strong three-dimensional (3-D) multiple scattering effects in biomedical imaging applications, we develop a full 3-D inverse scattering algorithm based on the combination of the contrast source inversion and the fast Fourier transform algorithm. Numerical results show that our algorithm can accurately invert for the high-contrast media in breast tissue.     

对地观测的微波散射成像方法

以微波为探测手段进行对地成像观测,能够全天候、全天时工作,在许多技术领域得到了重要应用,对微波成像的新思想、新体制、新技术的探索,也是一个非常活跃的研究领域。该文提出了一种新的微波成像方法,称为微波散射成像方法,该方法可采用收发分置工作方式,工作时只需要以单色波照射待成像地域。原理性成像模拟结果表明,使用这种方法可以获得被观测地域高几何分辨率和高辐射分辨率的微波图像。     

Convolutional sparse coding framework for compressive spectral imaging

Spectral images (SI) can be represented as 3D-arrays of spatial information across multiple wavelengths. Compressive Spectral Imaging (CSI) reduces sensing costs by sensing compressed versions of the scene, recovering a suitable version of the original SI solving a sparsity-inducing inverse problem. On the other hand, Convolutional Sparse Coding (CSC) has been successfully proved for representing gray-scale images, however it misses any correlation between images. This work considers the spatial-spectral correlation within SIs introducing an extension of the CSC signal model describing the SI as the sum of convolutions of 3D sparse coefficient maps with their respective 3D dictionary filters. Furthermore, we use the proposed CSC framework for recovering SIs from CSI measurements. The simulations results, using two different CSI acquisition architectures, show that the proposed CSC framework yields better representations of the SIs than those obtained under the traditional sparse signal representation approach, improving the quality of the recovered SIs.     

New approach to Doppler tomography for microwave imaging

A new approach to microwave imaging is presented. The requirements for large bandwidth and the computational burden in conventional range-Doppler imaging are reduced. In conventional microwave imaging, one of the steps in reconstructing a Doppler tomogram is to perform a two-dimensional Fourier transform which utilizes and produces data points in a Cartesian raster. The computational burden associated with this reconstruction is high. In contrast, the approach proposed in this paper is, in essence, a phase-based process rather than an amplitude-based process. Backscattered signals from the illuminated target are received and analysed using the wavelet transform to extract the Doppler variation information which contains the spatial distribution information of the scattering centres. Diagrams which are very similar to the 'sinogram’ used in X-ray tomography are obtained and are used to reconstruct the two-dimensional images. Simulated and experimental results are shown to confirm the practicality of the proposed approach. The quality of the images shown in this paper is satisfactory and the computational burden is greatly reduced.     

Frequency optimization for microwave imaging of biological tissues

Theoretical calculations are provided to show that 2 to 8 GHz represents a frequency region most suited for microwave imaging of biological tissue. The attenuation and spatial resolution are both adequate to yield useful diagnostic information.     

A multiview microwave imaging system for two-dimensional penetrableobjects

A microwave imaging system is based on a multiview numerical solution to the integral equation of 2D transverse magnetic (TM) scattering is proposed. This solution is achieved by the moment method, and a pseudoinversion transformation is used to face ill-conditioning problems. All experimental setup is described that uses a scanning subsystem for measuring the values of the scattered electric field inside an observation domain located outside the investigation one (i.e., the area containing the cross sections of cylindrical dielectric scatters). Rotations of the investigation domain with respect to the scanning subsystem and the transmitting antenna allow a multiview imaging process. The imaging system does not require plane-wave illumination and does not use any first-order approximations; hence, it may be used even in the case of strong scatterers. The offline and once-and-for-all computation of the pseudoinverse matrix allows an inexpensive reconstruction in terms of computer resources. Some tests of the system were carried out, and the results are reported     

On the heating of output windows of microwave tubes by electron bombardment

An account is given of an experimental investigation of the behavior of output windows, the particular type chosen for study being the cylindrical type in a cavity resonator. By feeding microwave power into the cavity and measuring electric field and the heat dissipated in the window and in the metal walls, a non-linear window-heating effect was discovered above a certain critical field strength. In this condition, watts dissipated at the window vary as the fifth or higher power of the electric field strength and the resulting thermal stresses may easily destroy it. Further experiments were performed on various modifications of the original structure in order to isolate and study the phenomena responsible for the heating. Some of these tests were made using an axial magnetic field. It was found that there are two distinct mechanisms of heating, one requiring the presence of the magnetic field and the other one not requiring it. Both involve energy exchange between the electric field and the window by free electrons which dance on the window surface synchronously with the alternating field and multiply by a secondary emission or multipactor process above a certain field strength. The critical field strength may be greatly reduced by the presence of contaminating films on the dielectric surface, and may be greatly increased by suitable surface treatment. It is shown that these effects are not restricted to cylindrical windows but will tend to occur at any dielectric surface in vacuum if the tangential RF field is strong enough.