Eric S. Li

Estimation of forest biophysical characteristics in Northern Michigan with SIR-C/X-SAR

A three-step process is presented for estimation of forest biophysical properties from orbital polarimetric SAR data. Simple direct retrieval of total aboveground biomass is shown to be ill-posed unless the effects of forest structure are explicitly taken into account. The process first involves classification by (1) using SAR data to classify terrain on the basis of structural categories or (2) a priori classification of vegetation type on some other basis. Next, polarimetric SAR data at L- and C-bands are used to estimate basal area, height and dry crown biomass for forested areas. The estimation algorithms are empirically determined and are specific to each structural class. The last step uses a simple biophysical model to combine the estimates of basal area and height with ancillary information on trunk taper factor and wood density to estimate trunk biomass. Total biomass is estimated as the sum of crown and trunk biomass. The methodology is tested using SIR-C data obtained from the Raco Supersite in Northern Michigan on Apr. 15, 1994. This site is located at the ecotone between the boreal forest and northern temperate forests, and includes forest communities common to both. The results show that for the forest communities examined, biophysical attributes can be estimated with relatively small rms errors: (1) height (0-23 m) with rms error of 2.4 m, (2) basal area (0-72 m/sup 2//ha) with rms error of 3.5 m/sup 2//ha, (3) dry trunk biomass (0-19 kg/m/sup 2/) with rms error of 1.1 kg/m/sup 2/, (4) dry crown biomass (0-6 kg/m/sup 2/) with rms error of 0.5 kg/m/sup 2/, and (5) total aboveground biomass (0-25 kg/m/sup 2/) with rms error of 1.4 kg/m/sup 2/. The addition of X-SAR data to SIR-C was found to yield substantial further improvement in estimates of crown biomass in particular. However, due to a small sample size resulting from antenna misalignment between SIR-C and X-SAR, the statistical significance of this improvement cannot be reliably established until further data are analyzed. Finally, the results reported are for a small subset of the data acquired by SIR-C/X-SAR. >

Microstrip ring resonator for soil moisture measurements

Development of a successful remote sensing instrument for soil moisture relies on the accurate knowledge of the soil dielectric constant (/spl epsiv//sub soil/) to its moisture content. Two existing methods for measurement of dielectric constant of soil at low and high frequencies are, respectively, the time domain reflectometry and the reflection coefficient measurement using an open-ended coaxial probe. The major shortcoming of these methods is the lack of accurate determination of the imaginary part of E/sub soil/. In this paper, a microstrip ring resonator is proposed for the accurate measurement of soil dielectric constant. In this technique the microstrip ring resonator is placed in contact with soil medium and the real and imaginary parts of E/sub soil/ are determined from the changes in the resonant frequency and the quality factor of the resonator, respectively. The solution of the electromagnetic problem is obtained using a hybrid approach based on the method of moments solution of the quasistatic formulation in conjunction with experimental data obtained from reference dielectric samples. Also, a simple inversion algorithm for E/sub soil/=E/sup //sub r/-j/spl epsiv/E/sup //sub r/ based on regression analysis is obtained. It is shown that the wide dynamic range of the measured quantities provides excellent accuracy in the dielectric constant measurement. A prototype microstrip ring resonator at L-band is designed and measurements of soil with different moisture contents are presented and compared with other approaches.

Modeling and measurements of scattering from road surfaces at millimeter-wave frequencies

Millimeter-wave radar-based sensors are being considered for a number of automotive applications including obstacle detection and collision warning, true-speed, and road-surface recognition. The interaction of electromagnetic waves with asphalt road surfaces, possibly covered with ice or water, at millimeter-wave frequencies is studied. First, an experimental procedure for determining the effective dielectric constant of bituminous mixtures used in road-surface constructions is developed. In this procedure, the effective dielectric constant is derived using a simple inverse-scattering algorithm to the measured radar cross sections of cylindrical specimen of a standard asphalt mixture. Then the vector radiative transfer equation is used to formulate the scattering from a multilayer medium representing an ice- or water-covered asphalt surface. The University of Michigan polarimetric 94-GHz radar system was deployed for characterizing the polarimetric backscatter responses of asphalt surfaces under many physical conditions near grazing incidence angles (70/spl deg/-88/spl deg/). The measured backscatter coefficients and parameters of copolarized phase difference statistics of a dry asphalt surface with smooth interface at one incidence angle were used to derive the phase and extinction matrices of the asphalt medium. The experimentally determined phase and extinction matrices are substituted in the radiative transfer formulation to predict the scattering from asphalt surfaces under all conditions. Excellent agreement between theoretical predictions and measured quantities is obtained.

Low grazing incidence millimeter-wave scattering models and measurements 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/spl deg/-88/spl deg/). Comparison of the measured and theoretically predicted backscattering coefficients and polarimetric phase difference statistics shows excellent agreement.

Characterization of optimum polarization for multiple target discrimination 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.

Polarimetric characterization of debris and faults in the highway environment 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/spl deg/-86/spl deg/). 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.

Characterization of soil moisture using a microstrip resonator

The development of a successful remote sensing instrument for soil moisture relies on the accurate knowledge of the soil dielectric constant relative to its moisture content. Two existing methods for measurement of dielectric constant of soil at low and high frequencies are, respectively, the time domain reflectometry (TDR) and the reflection coefficient measurement using an open ended coaxial probe. The accuracy of the measurements provided by both methods is limited. A microstrip resonator dielectric probe (MRDP) is proposed for the dielectric constant measurement of soil medium. In this technique the microstrip resonator is placed in contact with soil medium and by measuring the shift in the resonant frequency and change in the quality factor of the resonator, the real and imaginary parts of the dielectric constant are calculated. A prototype microstrip resonator at L-band is designed and measurements of soil with different moisture content are presented.<<ETX>>

Effect of radar polarizations on detectability of debris on roads at millimeter-wave frequencies

Numerous automotive accidents are reported each year. Many of these are attributed to road surface conditions and objects or obstacles on road surfaces. A desirable feature of automotive radar sensors is the ability to detect debris on the highway environment and the provision of an advance warning to drivers. Debris may be detected by comparing the radar cross section (RCS) values of the targets on the road surface with the RCS threshold level defined in terms of the backscatter from the road surface alone, the radar antenna parameters, and a prescribed false alarm rate. Since the RCS of a point target is influenced by many physical parameters such as material, size, shape, and target orientation, a complete polarimetric characterization of radar backscatter of debris on road surfaces may be done analytically. A physical optics model is developed for predicting scattering from solid objects with perpendicular facets such as bricks and right-angled iron. The validity of the theoretical models was verified by comparing the simulation results with the backscatter measurements of a brick and a piece of right-angled iron on an asphalt surface at 94 GHz.

Polarimetric backscatter characterization of road surface faults at millimeter-wave frequencies

To design a reliable obstacle detection radar sensor for automated highway system applications, thorough knowledge of the backscatter response of various road surface conditions is required. In this paper, the polarimetric backscatter behavior of typical road surface faults, such as surface cracks and potholes, is considered. Although cracks do not pose any safety threat, the study of their backscatter response is important as far as the false alarm rate caused by cracks is concerned. Two theoretical models are developed to predict the backscatter response of cracks perpendicular to the antenna boresight. The first model simulates a crack by two impedance wedges next to each other. The method developed by Maliuzhinets is used to calculate the total field in the presence of an impedance wedge at normal incidence condition. A non-uniform expression for impedance wedge diffraction is chosen to represent the diffraction coefficient. The diffracted fields from two adjacent wedges are then added coherently. At MM wave frequencies the radii of curvature of asphalt or concrete crack edges become comparable with the wavelength and the wedge model may become invalid. For this situation a second model for curved edges based on scattering from impedance cylinders is developed. The validity of the theoretical models is verified by the experimental results. The simulation results of the second model exhibit better agreement with the measured data at 94 GHz than do those of the first model. Also, the experimental backscatter behavior of potholes with different depths over a wide range of incidence angles is presented.

Polarimetric radar measurements for vehicle control and warning applications

The idea to use a radar for automotive braking and even steering is not new. Some 35 years ago, in the mid-sixties, the Radiation Laboratory made RCS measurements on tree trunks, stop signs, cement walls, and humans to determine if the targets could be identified from the (limited) RCS data. Unfortunately the results were not encouraging. For example, a tree truck along the side of a road could have similar return magnitude as a human. At that time only single-frequency and co-polarized data, are available. The introduction of radar polarimetry technique provides a new approach to achieve the goal of the early study. Since 1995, the Sarabandi group from the Radiation Laboratory has successfully developed models to predict the backscatter response from road surfaces and to detect debris and faults on road surfaces. In this article, the theory behind the analytical modeling is explained. Then, the experimental results are presented to justify the validity of the developed models.