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     

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 Re-Examination of Radar Terrain Backscattering at Nadir

The theoretical behavior of nadir specular and diffuse radar backscattering from rough terrain is re-examined. Terrain is modeled as a random rough surface in the Kirchhoff approximation. The relative significance of specular (coherent) and diffuse (noncoherent) radar backscatter is compared in terms of the antenna system parameters (beamwidth, height, and frequency) and rough surface statistics. It has been found that in case of a high-altitude (space or airborne) radar, the ratio between coherent and noncoherent terrain backscatter is independent of its altitude of the radar platform. A possible explanation is suggested for the anomalous radar backscatter observed in the Skylab S-193 altimeter experiment.     

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     

Electromagnetic scattering interaction between a dielectriccylinder and a slightly rough surface

An electromagnetic scattering solution for the interaction between a dielectric cylinder and a slightly rough surface is presented in this paper. Taking the advantage of a newly developed technique that utilizes the reciprocity theorem, the difficulty in formulating the secondary scattered fields from the composite target reduces to the evaluation of integrals involving the scattered fields from the cylinder and polarization currents of the rough surface induced by a plane wave. Basically, only the current distribution of isolated scatterers are needed to evaluate the interaction in the far-field region. The scattered field from the cylinder is evaluated in the near-field region using a stationary phase approximation along the cylinder axis. Also, the expressions for the polarization current induced within the top rough layer of the rough surface derived from the iterative solution of an integral equation are employed in this paper. A sensitivity analysis is performed for determining the dependency of the scattering interaction on the target parameters such as surface root mean square (RMS) height, dielectric constant, cylinder diameter, and length. It is shown that for nearly vertical cylinders, which is of interest for modeling of vegetation, the cross-polarized backscatter is mainly dominated by the scattering interaction between the cylinder and the rough surface. The accuracy of the theoretical formulation is verified by conducting polarimetric backscatter measurements from a lossy dielectric cylinder above a slightly rough surface. Excellent agreement between the theoretical prediction and experimental results is obtained     

L-Band Radar Estimation of Forest Attenuation for Active/Passive Soil Moisture Inversion

In the radiometric sensing of soil moisture through a forest canopy, knowledge of canopy attenuation is required. Active sensors have the potential of providing this information since the backscatter signals are more sensitive to forest structure. In this paper, a new radar technique is presented for estimating canopy attenuation. The technique employs details found in a transient solution where the canopy (volume-scattering) and the tree–ground (double-interaction) effects appear at different times in the return signal. The influence that these effects have on the expected time-domain response of a forest stand is characterized through numerical simulations. A coherent forest scattering model, based on a Monte Carlo simulation, is developed to calculate the transient response from distributed scatterers over a rough surface. The forest transient-response model for linear copolarized cases is validated with the microwave deciduous tree data acquired by the Combined Radar/Radiometer (ComRAD) system. The attenuation algorithm is applicable when the forest height is sufficient to separate the components of the radar backscatter transient response. The frequency correlation functions of double-interaction and volume-scattering returns are normalized after being separated in the time domain. This ratio simply provides a physically based system of equations with reduced parameterizations for the forest canopy. Finally, the technique is used with ComRAD L-band stepped-frequency data to evaluate its performance under various physical conditions.      

Millimeter wave backscatter from deciduous trees

Measurements of the millimeter-wave backscatter from deciduous (leafed) trees are explained using a simple scattering model. The backscatter from individual leaves was measured in the laboratory and used to justify the use of an average leaf radar cross section when computing the normalized radar cross section (NRCS) of tree canopies. NRCS measurements of canopies show that the direction in which incident radiation impinges on the canopy is an important factor in characterizing radar backscatter. Comparisons of measured NRCS values demonstrate that planophil and erectophil trees can be distinguished based on their backscatter at 215 GHz     

Monitoring of rain water storage in forests with satellite radar

The sensitivity of radar backscatter to the amount of intercepted rain in temperate deciduous forests is analyzed to determine the feasibility of retrieval of this parameter from satellite radar data. A backscatter model is validated with X-band radar measurements of a single tree exposed to rain. A good agreement between simulation and measurements is observed and this demonstrates the ability of radar to measure the amount of intercepted rain. The backscatter model is next applied to simulate different satellite radar configurations. To account for forest variability, the backscatter difference between a wet and dry forest canopy is calculated for four deciduous tree species, above a wet and a dry soil. On average, the simulated backscatter of a wet forest canopy is 1 dB higher than the backscatter of a dry forest canopy at co-polarized L-band and 2 dB at co-polarized C and X-band. The simulated sensitivity is in agreement with observations. It is argued that current satellites can retrieve the amount of intercepted rain at best with a reliability of 50%, due to the variability in soil moisture, species composition and system noise. The authors expect that the reliability will improve with the launch of the next generation radar satellites. The results of this analysis may also be used to assess the influence of rain, fog or dew upon other radar applications for temperate deciduous forests     

Millimeter-wave radar phenomenology of power lines and apolarimetric detection algorithm

The radar phenomenology of high-voltage power lines and cables is studied for examining the feasibility of detecting power lines along the path of a low-flying aircraft using a millimeter-wave radar system. For this purpose, polarimetric backscatter measurements of power line samples of different diameters and strand arrangements were performed over a wide range of incidence angles with very fine increments at 94 GHz. Also, similar polarimetric backscatter measurements were conducted for cylinders of the same radii and lengths as the power line samples for identifying the scattering features caused by the braiding structure of the power lines. In addition, the effects of a thin layer of water and a layer of ice over the power line surface on its polarimetric scattering behavior are studied by repeating the polarimetric backscatter measurements. Based on this phenomenological study, a polarimetric detection algorithm that makes use of the scattering features caused by the braided structure of power lines is proposed. It is shown that the proposed algorithm is capable of detecting power lines in a relatively strong clutter background with a poor signal-to-clutter ratio. The performance of the algorithm is demonstrated experimentally using a rough asphalt surface and a vegetation foliage as sample clutter backgrounds     

Study of Millimeter-Wave Radar for Helicopter Assisted-Landing System

Landing helicopters on rough unprepared terrain has always been considered hazardous, since the massive dust cloud generated by wind drafts from the helicopter's rotors completely obstructs the pilot's view of the landing area. This article presents a performance assessment of a proposed millimeter-wave (MMW) three-dimensional imaging radar system, specifically meant for helicopter assisted landing. The assessment includes simulation of radar backscattering from the underlying rough terrain, in addition to the signal attenuation and scattering from dust clouds generated by the helicopter's rotors. Terrain scattering is simulated in two steps: 1) generation of a two-scale random rough surface according to prescribed statistics of a large-scale undulation and a small-scale surface roughness; and 2) simulation of the returned signal, including the effects of the real-aperture radar parameters and the terrain-backscatter response. Details of the three-dimensional imaging algorithm are presented. Single-scattering theory is used to simulate the effects of a dust cloud - including signal attenuation and backscatter clutter generation - on the radar's performance. It is shown that operating in the upper millimeter-wave regime (70 GHz-220 GHz) is the most practical solution for a compact, high-resolution, three-dimensional imaging system for this problem.     

Minimization of end-fire radar echo of a long thin body by impedance loading

A theory on the minimization of the end-fire radar echo of a long, thin body by an impedance loading technique is presented. When a long, thin body is illuminated by a plane wave in the region of longitudinal incidence, a large backscatter is observed. This backscatter is due to the induced traveling wave which is reflected from the rear end of the body. By an appropriate impedance loading, this backscatter can be minimized. In this paper, the induced current on a long, thin and loaded cylinder is studied. The optimum impedance loading which minimizes the end-fire radar echo is explicitly obtained. Theoretical results are compared with the experimental data and an excellent agreement is obtained.     

Microwave backscatter modeling of erg surfaces in the Sahara desert

The Sahara desert includes large expanses of sand dunes called ergs. These dunes are formed and constantly reshaped by prevailing winds. Previous study shows that Saharan ergs exhibit significant radar backscatter (/spl sigma//spl deg/) modulation with azimuth angle (f). We use /spl sigma//spl deg/ measurements observed at various incidence angles and f from the NASA Scatterometer (NSCAT), the SeaWinds scatterometer, the ERS scatterometer (ESCAT), and the Tropical Rainfall Measuring Mission's Precipitation Radar to model the /spl sigma//spl deg/ response from sand dunes. Observations reveal a characteristic relationship between the backscatter modulation and the dune type, i.e., the number and orientation of the dune slopes. Sand dunes are modeled as a composite of tilted rough facets, which are characterized by a probability distribution of tilt with a mean value, and small ripples on the facet surface. The small ripples are modeled as cosinusoidal surface waves that contribute to the return signal at Bragg angles only. Longitudinal and transverse dunes are modeled with rough facets having Gaussian tilt distributions. The model results in a /spl sigma//spl deg/ response similar to NSCAT and ESCAT observations over areas of known dune types in the Sahara. The response is high at look angles equal to the mean tilts of the rough facets and is lower elsewhere. This analysis provides a unique insight into scattering by large-scale sand bedforms.     

Laboratory measurements of polarization radios of wind wave surfaces

Polarization ratios (sigma_{vv}/sigma_{HH}) of wind-generated rough water surfaces are studied experimentally by means of radar backscatter power measurements. The measurements were made at 9.23 GHz with incidence angles between45degand55degfor wind speeds between 3 m/s to 10 m/s. Scattering surface statistics at all wind speeds were also measured by means of a wave height gauge and a laser slope gauge. The polarization ratio data are presented against parameters pertinent to scattering theory, which are obtained from the measured surface statistics. The results are compared to "composite surface" theory calculations for an assumed wave-height spectral density of the formF(k) propto k^{-m}, wheremis an adjustable parameter.     

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     

Radar sensitivity to tree geometry and woody volume: a modelanalysis

A model, based on radiative transfer theory and the matrix doubling algorithm, is described and used to compute the backscatter coefficients σ°s of forests. The objective is to investigate the radar sensitivity to woody biomass and some geometrical tree properties like branch dimensions and orientation. Model computations show that the backscatter coefficient is sensitive to the woody volume, particularly at HV polarization, P and L band; however, the radar response is appreciably influenced also by branch dimensions and orientation. Comparisons with experimental results available in the literature are shown. The correspondence between predicted and measured σ°s is generally good, although a slight overestimation is noted at L band, while uncertainties about data calibration and soil properties make the comparisons more difficult, in some cases, at P band     

Radar backscatter characteristics of trees at 215 GHz

Millimeter-wave backscatter measurements are presented for various tree types taken during the 1987 growing season at Amherst, Massachusetts. These measurements were taken with a 215-GHz radar system that is capable of measuring backscatter for the VV (vertical transmit, vertical received), HH (horizontal transmit, horizontal received), VH, and HV polarizations. Geometrical optics modeling was used for the normalized radar cross section (RCS) of deciduous trees and the backscatter is shown to be characterized in terms of gravimetric leaf water content, the leaf area, and the foilage crown cover. Data are also provided for coniferous trees, although the geometrical optics model is not applicable in those cases     

Dependence of Speckle Statistics on Backscatter Cross-Section Fluctuations in Synthetic Aperture Radar Images of Rough Surfaces

A theory is described to relate the statistical properties of the fields backscattered from rough surfaces to those of speckle in synthetic hetic aperture rdar (SAR) images. The expressions are derived for the autocorrelation and cross-correlation functions of speckle intensity in both single-look and multilook images of stationary random rough surfaces in terms of the SAR system parameters and the autocorrelation function of backscatter radar cross-section fluctuations. It is shown that if the correlation scale of cross-section fluctuations is comparable with or greater than the SAR resolution, the correlation functions of speckle intensity depend on those of the cross-section fluctuations. This property, therefore, may be applied to image classification. Comparison on of the theory with computer simulation shows good agreement.     

Analysis of algorithms for the retrieval of rain-rate profiles froma spaceborne dual-wavelength radar

The ability to retrieve rain-rate profiles from a dual-wavelength spaceborne radar system operating at 13.6 and 35 GHz is analyzed. The fundamental problem of extracting either the attenuation and/or the reflectivity from the backscatter echo, which contains both contributions, is addressed. Three algorithms, the backscatter, the attenuation coefficient, and the dual-wavelength methods, are examined. These algorithms are tested using four rain-rate profiles derived from radar measurements. In particular, measured (true) values are compared with calculated (retrieved) rain rates applying the algorithms with superimposed uncertainties assuming a suggested spaceborne dual-wavelength radar system. Error values of rain rates are determined where these values reflect failure of the assumptions utilized in the derivation of the algorithms, rain backscatter noise, and instrument noise     

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