Measurements of the propagation parameters of tree canopies at MMWfrequencies

The presence of trees in a given scene can hamper detection of nearby targets by millimeter-wave (MMW) radars especially at near grazing incidence. Proper characterization of scattering and attenuation in tree canopies is important for optimal detection algorithms. In this paper, a new technique for determining the extinction and volume backscattering coefficients in tree canopies using the measured radar backscatter response is proposed and verified experimentally. The technique, which can be applied to already available wideband radar backscatter data, is used to compute the extinction and volume backscattering coefficients of different tree canopies under various physical conditions. The dynamic range of these coefficients are presented and results at 35 GHz are compared with results at 95 GHz     

On the use of permanent symmetric scatterers for ship characterization

The symmetric scattering characterization method (SSCM) has been recently introduced for high-resolution characterization of certain targets under coherent conditions. SSCM is based on the Poincare/spl acute/ sphere representation, which supports a high-resolution decomposition of symmetric target scattering, as well as assessment and validation of the backscatter coherence. In this paper, the SSCM is investigated for ship characterization using Convair-580 polarimetric synthetic aperture radar (SAR) data. It is shown that the target Poincare/spl acute/ parameters permit identification of dominant scatterers with a significant symmetric scattering component. The polarization orientation angle of these quasi-symmetric scatterers is used to derive an estimate of the ship's pitch angle, under certain conditions. The effect of SAR system focus setting errors and Doppler centroid mistracking on the SSCM performance is investigated. It is shown that the SSCM is sensitive to the system focus setting and Doppler centroid shift. The first-order effects of these errors can be removed prior to the application of the SSCM method.     

Millimeter-wave radars for remotely sensing clouds andprecipitation

Millimeter-wave radars have been used since the early 1950s to study clouds and precipitation, but until recently these early instruments were limited to simple backscatter power measurements and were plagued by hardware problems. However, development of solid-state millimeter-wave componentry and high-power klystron amplifiers has spurred the evolution of reliable, coherent radars operating up to 95 GHz. In addition, advances in digital signal processing technology have resulted in single-card processors that can simultaneously execute algorithms to compute reflectivity, Doppler, and polarimetric quantities in real time. A review of the current state of the art in millimeter-wave cloud radars is presented, including a discussion of transmitters, antennas, and receiver components. Two radar systems built by the University of Massachusetts are described, including a mobile, dual-frequency (33- and 95-GHz) polarimetric radar, and an airborne 95-GHz polarimetric radar that was recently flown in a cooperative experiment with the University of Wyoming. Spaceborne applications are also discussed, especially the use of satellite-based 95-GHz radars for measuring the vertical distribution of clouds     

Low grazing incidence millimeter-wave scattering models andmeasurements for various road surfaces

Systematic characterization of the scattering behavior of traffic targets, clutter, and their associated interactions are required in order to design and assess the performance of millimeter-wave-based sensors for automated highway system (AHS) applications. In this paper, the polarimetric radar backscatter response of various road surfaces is investigated both theoretically and experimentally. In general, it is found that the overall scattering response of road surfaces is composed of volume and surface scattering components. Previously a hybrid volume scattering model was developed for predicting the backscatter response of smooth asphalt surfaces at millimeter-wave frequencies. There, only the volume scattering was accounted for, however, experimental results show that the surface scattering cannot be ignored when the surface roughness parameters become comparable to the radar wavelength. In this paper, the previous study is extended to include the radar backscatter response of concrete surfaces, snow-covered smooth surfaces, and rough asphalt or concrete surfaces. Radiative transfer (RT) theory is used to model the volume scattering and the integral equation model is used to describe the surface scattering. Asphalt and concrete mixtures are dense random media whose extinction and phase matrices are characterized experimentally. Ice and water over asphalt and concrete surfaces are modeled by homogeneous layers. Fresh snow is modeled by a sparse random medium whose extinction and phase matrices are obtained analytically. The University of Michigan 94-GHz polarimetric radar system was used to perform polarimetric backscatter measurements of the aforementioned road surfaces at near grazing incidence angles (70°-88°). Comparison of the measured and theoretically predicted backscattering coefficients and polarimetric phase difference statistics shows excellent agreement     

A novel bistatic scattering matrix measurement technique using amonostatic radar

Presents a new technique for measuring the bistatic scattering matrix of point targets using a monostatic radar. In this technique, the complexity of the traditional bistatic measurement setup and difficulties in retaining the phase coherence between the transmitter and the receiver are circumvented completely. The bistatic measurement is performed using a wideband, polarimetric, monostatic radar in conjunction with a rotatable ground plane positioned behind the target. Assuming that the distance between the target and the ground plane is larger than the radar resolution, the desired bistatic response (image contribution) can be isolated from the unwanted backscatter. Noting that the radar operates in the backscatter mode and using the reciprocity theorem, it is shown that the measured cross-polarized responses (σ_vhand σ_hv) cannot be determined uniquely. To rectify this problem, additional independent measurements are required. Additional equations for characterizing the cross-polarized components are obtained by placing an anisotropic lossless slab over the perfectly conducting flat surface. The validity and accuracy of the new bistatic measurement technique is demonstrated by measuring a number of point targets with known theoretical bistatic responses. Also, a new approach for determining the effective dielectric constant of dense random media based on the new bistatic measurement technique is developed     

Calibration of linearly polarized polarimetric SAR data subject to Faraday rotation

A model for linearly polarized fully polarimetric backscatter measurements is used, incorporating the effects of system noise, channel amplitude, phase imbalance, crosstalk, and Faraday rotation. A step-by-step procedure is outlined for correction (or calibration) of fully polarimetric data subject to Faraday rotation, to recover the true scattering matrix. The procedure identifies steps for crosstalk removal and correction of channel imbalances that are robust in the presence of Faraday rotation. The final steps in the procedure involve a novel strategy for estimation and correction of Faraday rotation. Three approaches to estimate the (one-way) Faraday rotation angle /spl Omega/ directly from linear (quad-) polarized synthetic aperture radar (SAR) backscatter data obtained by a spaceborne SAR system are described. Each approach can initially be applied to the signature of any scatterer within the scene. Sensitivity analyses are presented that show that at least one of the measures can be used to estimate /spl Omega/ to within /spl plusmn/3/spl deg/ to 5/spl deg/, with reasonable levels of residual crosstalk, noise floor, channel amplitude, and phase imbalance. Ambiguities may be present in the estimates of /spl Omega/ of /spl plusmn/n/spl pi//2 - the impact of this is discussed, and several approaches are suggested to deal with this possibility. The approach described in this paper is relevant for future L-band spaceborne SARs and removes one key obstacle to the deployment of even longer wavelength SARs (e.g., an ultrahigh frequency or P-band SAR) in Earth orbit.     

A calibration technique for polarimetric coherent-on-receive radarsystems

A new technique for calibrating a coherent-on-receive polarimetric radar system is proposed. A coherent-on-receive polarimetric radar is capable of measuring the Mueller matrix of point or distributed targets directly by transmitting at least four independent polarizations and measuring the vertical and horizontal components of the backscatter signal simultaneously. The technique requires the use of two calibration targets, a target with known scattering matrix (such as a metallic sphere or a trihedral corner reflector) and any depolarizing target (for which knowledge of its scattering matrix is not required) to determine the system distortion parameters. The system distortion parameters, which include the channel imbalances, the cross-talk factors of both the transmit and the receive antennas, and the phase shifts and amplitude variations of the transmitter polarizers, are determined by measuring the calibration targets for four different transmit polarizations. The validity of the new calibration technique is examined by measuring the scattering matrices of spheres and cylinders as test targets using a coherent-on-receive radar operating at 34.5 GHz. Excellent agreement between the theoretical and the measured scattering matrices for the test targets are obtained     

Characterization of optimum polarization for multiple targetdiscrimination using genetic algorithms

A stochastic optimization algorithm is used to characterize the polarization states of a nonpolarimetric radar transmitter and receiver antennas for optimal target classification. Specifically, the optimized solution is sought when a multitude of targets are to be categorized. It is shown that the objective function of the optimization problem is highly nonlinear and discontinuous, hence, classical optimization algorithms fail to provide satisfactory results. The stochastic optimization algorithm used is based on a genetic algorithm (GA) which operates on a discretized form of the parameter space and searches globally for the optimum point. In this process, it is assumed that the polarimetric responses of the targets are known a priori. The optimization algorithm is applied to two sets of data: (1) a synthetic backscatter data for four point targets with similar radar cross sections (RCSs) and (2) a set of polarimetric backscatter measurements of asphalt surfaces under different physical conditions at 94 GHz. The purpose of the latter study is to come up with the optimal design for polarization states of an affordable millimeter-wave radar sensor that can assess traction of road surfaces     

Analysis and applications of backscattered frequency correlationfunction

The application of the radar backscatter frequency correlation for classification and inversion of physical parameters of terrestrial targets is investigated. Traditionally, in radar remote sensing, the backscattering coefficients and the backscatter phase difference statistics of a distributed target are considered for estimating the biophysical parameters of interest. Because of the complex nature of random media scattering problems, however, target classification and parameter inversion algorithms are very convoluted. One obvious way of enhancing the success and accuracy of an inversion algorithm is to expand the dimension of the input vector space. Depending on the radar parameters, such as footprint (pixel) size, incidence angle, and the target attributes (physical parameters), the backscatter signal decorrelates as function of frequency. In this paper, analytical and experimental procedures are developed to establish a relationship between the complex frequency correlation function (FCF) of the backscatter and the radar and target attributes. Specifically, two classes of distributed targets are considered: 1) rough surfaces and 2) random media. Analytical expressions for the frequency correlation function are derived and it is shown that the effect of radar parameters can be expressed explicitly and thus removed from the measured correlation functions. The University of Michigan wideband polarimetric scatterometer systems are used to verify the theoretical models and inversion algorithms developed in this study     

On the detection of Faraday rotation in linearly polarized L-band SAR backscatter signatures

The potentially measurable effects of Faraday rotation on linearly polarized backscatter measurements from space are addressed. Single-polarized, dual-polarized, and quad-polarized backscatter measurements subject to Faraday rotation are first modeled. Then, the impacts are assessed using L-band polarimetric synthetic aperture radar (SAR) data. Due to Faraday rotation, the received signal will include other polarization characteristics of the surface, which may be detectable under certain conditions. Model results are used to suggest data characteristics that will reveal the presence of Faraday rotation in a given single-polarized, dual-polarized, or quad-polarized L-band SAR dataset, provided the user can identify scatterers within the scene whose general behavior is known or can compare the data to another, similar dataset with zero Faraday rotation. The data characteristics found to be most sensitive to a small amount of Faraday rotation (i.e., a one-way rotation <20/spl deg/) are the cross-pol backscatter [/spl sigma//spl deg/(HV)] and the like-to-cross-pol correlation [e.g., /spl rho/(HHHV/sup */)]. For a diverse, but representative, set of natural terrain, the level of distortion across a range of backscatter measures is shown to be acceptable (i.e., minimal) for one-way Faraday rotations of less than 5/spl deg/, and 3/spl deg/ if the radiometric uncertainty in the HV backscatter is specified to be less than 0.5 dB.     

极化有源雷达校准器的幅相失真分析及修正

极化有源雷达校准器（Polarimetric Active Radar Calibrator,PARC）可用于宽带高分辨雷达的定标,以使高分辨成像技术能准确刻画雷达目标的细微特征,但其作为有源器件会引入额外失真从而影响系统失真补偿。本文基于成对回波理论及频域失真补偿原理,通过将频域补偿转换为时域滤波,提出截取滤波器系数的方法以修正PARC失真。实测数据结果表明了理论分析和提出的修正方法的正确性,修正后的PARC可近似理想地补偿系统失真。     

Unsupervised segmentation of polarimetric SAR data using thecovariance matrix

A method for unsupervised segmentation of polarimetric synthetic aperture radar (SAR) data into classes of homogeneous microwave polarimetric backscatter characteristics is presented. Classes of polarimetric backscatter are selected on the basis of a multidimensional fuzzy clustering of the logarithm of the parameters composing the polarimetric covariance matrix. The clustering procedure uses both polarimetric amplitude and phase information, is adapted to the presence of image speckle, and does not require an arbitrary weighting of the different polarimetric channels; it also provides a partitioning of each data sample used for clustering into multiple clusters. Given the classes of polarimetric backscatter, the entire image is classified using a maximum a posteriori polarimetric classifier. Four-look polarimetric SAR complex data of lava flows and of sea ice acquired by the NASA/JPL airborne polarimetric radar (AIRSAR) are segmented using this technique     

Fuzzy-Fusion Classifier for MMV Full-Polarization Radar Target Recognition Using One-Dimensional Scattering Centers

One-dimensional (1-D) scattering centers on a target can be used for radar target recognition purpose. In this paper, a new classifier based on fuzzy techniques is proposed, which is very much suitable for target recognition using 1-D scattering centers. Furthermore, information fusion techniques for full polarimetric receiver data are utilized at the decision level. The performance of the proposed fuzzy-fusion classifier is evaluated using a dataset of five aircraft models measured by millimeter-wave (MMW) full-polarization stepped-frequency radar in the compact range facility.     

Polarimetric characterization of debris and faults in the highwayenvironment at millimeter-wave frequencies

In this paper, measurements and models for the polarimetric backscatter response of various paint targets on roads and road surface faults are presented. Of particular interest are debris and faults that could lead to fatal accidents and damage of property. A desired safety feature for automotive radar sensors is the capability of detecting such debris and faults. The detectability of a point target is evaluated by comparing its RCS value with the RCS threshold value defined by the backscatter response of the road surface. Extensive backscatter measurements at W-band were conducted to obtain the backscatter response of typical debris and faults on asphalt surfaces at near grazing incidence angles (76°-86°). On the other hand, theoretical models, based on diffraction from impedance wedges and scattering from impedance cylinders, respectively, as well as physical optics approximation, were developed to predict the backscatter response of road surface faults and targets with planar facets on road surfaces. Experimental results indicate that detectability in all cases is a function of target size, its azimuthal angle with respect to radar boresight, and the polarization state of the system. The measured backscatter response is used to verify the validity of the theoretical models. Angular polarimetric backscatter measurements of targets defining roadside boundaries such as a concrete curb, a guardrail, and a pebble surface are also presented. The results of these measurements could be used to alert fatigued drivers should their vehicles be heading sideward     

Measurement and calibration of differential Mueller matrix ofdistributed targets

A rigorous method is presented for calibrating polarimetric backscatter measurements of distributed targets. By characterizing the radar distortions over the entire mainlobe of the antenna, the differential Mueller matrix is derived from the measured scattering matrices with a high degree of accuracy. It is shown that the radar distortions can be determined by measuring the polarimetric response of a metallic sphere over the main lobe of the antenna. The radar distortions are categorized as distortions caused by the active devices or distortions caused by the antenna structure (passive). Since passive distortions are immune to changes once they are determined, they can be used repeatedly. The active distortions can be obtained by measuring the sphere response only at boresight, reducing the time required for calibration under field conditions. The calibration algorithm was applied to backscatter data collected from a rough surface. The results indicate that removal of the radar distortions from the cross products of the scattering matrix elements cannot be accomplished with traditional calibration methods     

Traffic Surveillance System Based on a High-Resolution Radar

Traffic surveillance is an important civilian application of radars. The current high-resolution radars give new opportunities so that the traffic application may be redefined. In this paper, a traffic scenario with a high-resolution radar is presented. A range-bin alignment method, the Global Range Alignment, which comes from the focusing techniques in inverse synthetic aperture radar, is applied to obtain further capabilities than the usual velocity measurement: distinction between vehicle types via length estimation and adequate management in situations with simultaneous targets. Preliminary results from a real scenario using a high-resolution linear frequency-modulated continuous-wave millimeter-wave radar are shown.     

Joint parameter estimation of DOD/DOA/polarization for bistatic MIMO radar

In the paper,polarization-sensitive array is exploited at the receiver of multiple input multiple output (MIMO) radar system,a novel method is proposed for joint estimation of direction of departure (DOD),direction of arrival (DOA) and polarization parameters for bistatic MIMO radars. A signal model of polarimetric MIMO radar is developed,and the multi-parameter estimation algorithm for target localization is described by exploiting polarization array processing and the invariance property in both transmitter array and receiver array. By making use of polarization diversity techniques,the proposed method has advantages over traditional localization algorithms for bistatic MIMO radar. Simulations show that the performance of DOD and DOA estimation is greatly enhanced when different states of polarization of echoes is fully utilized. Especially,when two targets are closely spaced and cannot be well separated in spatial domain,the estimation resolution of traditional algorithms will be greatly degraded. While the proposed algorithm can work well and achieve high-resolution identification and accurate localization of multiple targets.     

From Glacier Facies to SAR Backscatter Zones via GPR

We present a comparison between data acquired with frequency-modulated ground-penetrating radar (GPR) and satellite synthetic aperture radar (SAR). Both radars are polarimetric and operate at a center frequency of 5.3 GHz. The field site is the polythermal glacier Kongsvegen, Svalbard. Along glacier GPR profiles cover the ablation area and the accumulation area, where the latter consists of superimposed ice (SI) and firn. The glacier facies are clearly identifiable on the GPR profiles, although we show that the copolarized response is better for distinguishing different ice zones, whereas the SI–firn boundary is most obvious in the cross-polarized response. A calibrated backscatter coefficient is calculated for the GPR data and compared with the SAR backscatter coefficient. The SAR zones are in very good agreement with the GPR-derived glacier facies. We show that, in the ablation area, the SAR response is dominated by backscatter from the previous summer surface. In the SI and firn areas, it is dominated by sources below the previous summer surface.      

一种极化-多普勒气象雷达的射频干扰滤波方法

为了滤除极化-多普勒气象雷达中的射频干扰,该文提出利用谱极化滤波器,适用于同时发射同时接收(STSR)和分时发射同时接收(ATSR)体制的极化气象雷达。首先利用C波段STSR气象雷达的实测数据研究射频干扰的时域、频域和极化域特性,建立射频干扰信号模型。然后,在X波段ATSR雷达的数据中仿真加入射频干扰,验证谱极化滤波器的有效性。总体看来,在ATSR雷达中利用谱极化滤波器可以有效保留降雨目标并且滤除射频干扰。最后,针对STSR雷达提出利用数据分集的方法,STSR雷达的实测数据可以模拟ATSR雷达数据,再利用谱极化滤波器实现射频干扰滤除,同样可以取得较好的滤波效果。