Yisok Oh

An empirical model and an inversion technique for radar scattering from bare soil surfaces

Polarimetric radar measurements were conducted for bare soil surfaces under a variety of roughness and moisture conditions at L-, C-, and X-band frequencies at incidence angles ranging from 10 degrees to 70 degrees . Using a laser profiler and dielectric probes, a complete and accurate set of ground truth data was collected for each surface condition, from which accurate measurements were made of the rms height, correlation length, and dielectric constant. Based on knowledge of the scattering behavior in limiting cases and the experimental observations, an empirical model was developed for sigma degrees /sub hh/, sigma degrees /sub vv/, and sigma degrees /sub hv/ in terms of ks (where k=2 pi / lambda is the wave number and s is the rms height) and the relative dielectric constant of the soil surface. The model, which was found to yield very good agreement with the backscattering measurements of the present study as well as with measurements reported in other investigations, was used to develop an inversion technique for predicting the rms height of the surface and its moisture content from multipolarized radar observations. >

Quantitative retrieval of soil moisture content and surface roughness from multipolarized radar observations of bare soil surfaces

A semiempirical polarimetric backscattering model for bare soil surfaces is inverted directly to retrieve both the volumetric soil moisture content M/sub v/ and the rms surface height s from multipolarized radar observations. The rms surface height s and the moisture content M/sub v/ can be read from inversion diagrams using the measurements of the cross-polarized backscattering coefficient /spl sigma//sub vh//sup 0/ and the copolarized ratio p(=/spl sigma//sub hh//sup 0///spl sigma//sub vv//sup 0/). Otherwise, the surface parameters can be estimated simply by solving two equations (/spl sigma//sub vh//sup 0/ and p) in two unknowns (M/sub v/ and s). The inversion technique has been applied to the polarimetric backscattering coefficients measured by ground-based polarimetric scatterometers and the Jet Propulsion Laboratory airborne synthetic aperture radar. A good agreement was observed between the values of surface parameters (the rms height s, roughness parameter ks, and the volumetric soil moisture content M/sub v/) estimated by the inversion technique and those measured in situ.

Semi-empirical model of the ensemble-averaged differential Mueller matrix for microwave backscattering from bare soil surfaces

A semi-empirical model of the ensemble-averaged differential Mueller matrix for microwave backscattering from bare soil surfaces is presented. Based on existing scattering models and data sets measured by polarimetric scatterometers and the JPL AirSAR, the parameters of the co-polarized phase-difference probability density function, namely the degree of correlation /spl alpha/ and the co-polarized phase-difference /spl sigmav/, in addition to the backscattering coefficients /spl sigma//sub /spl nu//spl nu///sup 0/,/spl sigma//sub hh//sup 0/ and /spl sigma//sub /spl nu/h//sup 0/, are modeled empirically in terms of the volumetric soil moisture content m/sub /spl nu// and the surface roughness parameters ks and kl, where k=2/spl pi/f/c, s is the rms height and l is the correlation length. Consequently, the ensemble-averaged differential Mueller matrix (or the differential Stokes scattering operator) is specified completely by /spl sigma//sub /spl nu//spl nu///sup 0/,/spl sigma//sub hh//sup 0/,/spl sigma//sub /spl nu/h//sup 0/,/spl alpha/, and /spl zeta/.

Condition for precise measurement of soil surface roughness

Whereas it is well known that electromagnetic scattering by a randomly rough surface is strongly influenced by the surface-height correlation function, it is not clear as to how long a surface-height profile is needed and at what interval it should be sampled to experimentally quantify the correlation function of a real surface. This paper presents the results of a Monte Carlo simulation conducted to answer these questions. It was determined that, in order to measure the rms height and the correlation length with a precision of /spl plusmn/10%, the surface segment should be at least 40l long and 200l long, respectively, where l is the mean (or true) value of the surface correlation length. Shorter segment lengths can be used if multiple segments are measured and then the estimated values are averaged. The second part of the study focused on the relationship between sampling interval and measurement precision. It was found that, in order to estimate the surface roughness parameters with a precision of /spl plusmn/5%, it is necessary that the surface be sampled at a spacing no longer than 0.2 of the correlation length.

An inversion algorithm for retrieving soil moisture and surface roughness from polarimetric radar observation

A semi-empirical polarimetric scattering model for bare soil surfaces is developed. This scattering model is constructed based on the existing theoretical models in conjunction with an extensive experimental data collected with polarimetric scatterometer systems at microwave frequencies. The backscattering coefficients as well as parameters of phase difference statistics, degree of correlation (/spl alpha/) and polarized phase difference (/spl zeta/), are expressed in terms of both surface parameters (rms height, correlation length, and dielectric constant) and radar parameters (frequency and incidence angle). The semi-empirical model is used as a basis for an inversion algorithm to estimate the surface parameters from the polarimetric backscatter response of a surface when the radar parameters are known. By performing a sensitivity analysis, a set of optimum parameters are chosen for the inversion algorithm. It is shown that the co-polarized ratio (/spl sigma//sup 0/ /sub hv///spl sigma//sup 0/ /sub vv/) the cross-polarized ratio (/spl sigma//sup 0/ /sub hv///spl sigma//sup 0/ /sub vv/), and the degree of correlation for co-polarized phase difference are most sensitive to the surface parameters and least affected by the measurement errors.<<ETX>>

Measurement and calibration of differential Mueller matrix of distributed 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. >

Power lines: radar measurements and detection algorithm for polarimetric SAR images

Polarimetric radar backscattering measurements of a variety of powerline cables are presented. The objective of the first part of the investigation was to study the effect of braiding of the cables on the backscattering at skew incidence. The measurements were performed for four different actual size powerline samples at C-, X-, and Ka-band over a wide range of incidence angles. The data were collected over a 500 MHz bandwidth at C- and X-band with a 1.25 MHz increment and a 1 GHz bandwidth at Ka-band with a 2.5 MHz increment. Also the effect of nonuniform illumination and measurement in the near field of the cables were studied. Experimental data shows a significant radar backscatter for W-polarization (/spl sigma//sub /spl nu//spl nu//) at angles away from normal incidence. This backscatter is proportional to the number and diameter of the strands on the surface of the cables. There is also noticeable backscatter for the HH and VH components of the scattering matrices. Their magnitudes, relative to that of the W component, are proportional to the pitch angle of the helix. Since detection of these cables is an important safety issue for low-flying airplanes a detection algorithm using polarimetric synthetic aperture radar (SAR) images was developed using the knowledge gleaned from the measurements. The detection algorithm was tested on a simulated image and worked well, detecting a power line whose backscatter power was 6 dB below the average background power. >

A numerical simulation of scattering from one-dimensional inhomogeneous dielectric random surfaces

An efficient numerical solution for the scattering problem of inhomogeneous dielectric rough surfaces is presented. The inhomogeneous dielectric random surface represents a bare soil surface and is considered to be comprised of a large number of randomly positioned dielectric humps of different sizes, shapes, and dielectric constants above an impedance surface. Clods with nonuniform moisture content and rocks are modeled by inhomogeneous dielectric humps and the underlying smooth wet soil surface is modeled by an impedance surface. In this technique, an efficient numerical solution for the constituent dielectric humps over an impedance surface is obtained using Greens function derived by the exact image theory in conjunction with the method of moments. The scattered field from a sample of the rough surface is obtained by summing the scattered fields from all the individual humps of the surface coherently ignoring the effect of multiple scattering between the humps. The statistical behavior of the scattering coefficient /spl sigma//spl deg/ is obtained from the calculation of scattered fields of many different realizations of the surface. Numerical results are presented for several different roughnesses and dielectric constants of the random surfaces. The numerical technique is verified by comparing the numerical solution with the solution based on the small perturbation method and the physical optics model for homogeneous rough surfaces. This technique can be used to study the behavior of scattering coefficient and phase difference statistics of rough soil surfaces for which no analytical solution exists.

Polarimetric Backscattering Coefficients of Flooded Rice Fields at L- and C-Bands: Measurements, Modeling, and Data Analysis

The polarimetric backscattering coefficients (vv-, hh-, hv-, and vh-polarizations) of a flooded rice field are measured using L- and C-band ground-based polarimetric scatterometers. These measurements were made during the rice growth cycle, i.e., from the transplanting period to the harvest period (May to October 2006), to understand the feasibility of modeling and estimating rice growth. We also collected ground truth data that include fresh and dry biomasses, plant height, leaf area index, and leaf size. To study the incidence angle effect, the scatterometer data were collected at four different incidence angles, i.e., 30deg , 40deg, 50deg, and 60deg. In this paper, we show that the backscattering coefficients of a rice field can accurately be modeled using the radiative transfer theory. We also demonstrate that a polarimetric scatterometer is an effective tool for estimating rice growth. The hh-polarized backscattering coefficient is more sensitive to rice growth than its vv-polarization counterpart. The polarimetric ratio can be used to estimate rice growth accurately.

Cross-calibration experiment of JPL AIRSAR and truck-mounted polarimetric scatterometer

When point calibration targets are used to calibrate a SAR image, the calibration accuracy is governed by two major factors. The first factor stems from the stringent requirement on the radar cross section (RCS) of the point calibration target. To reduce the effect of radar return from the background, the RCS of a point calibration target must be much larger than that of the background. Calibration targets with large RCS require large physical dimensions for passive targets or high amplifier gain for active targets, which in practice leads to uncertainty in the nominal RCS of the targets. The second factor is related to the fact that point calibration targets are used to develop a calibration algorithm which is applied to distributed targets. To this end, accurate knowledge of the impulse response (ambiguity function) of the SAR system is required. To evaluate the accuracy of such a calibration process, a cross-calibration experiment was conducted at a test site near Pellston, MI, using the JPL aircraft SAR and the University of Michigan truck-mounted polarimetric scatterometer. Five different types of distributed surfaces, all in the same area, were chosen: three of these were bare surfaces with varying roughnesses, and the other two were covered with vegetation. Trihedral corner reflectors were used for calibrating the aircraft SAR, and the UM scatterometer was calibrated using a metallic sphere. The scatterometer data were collected at L and C bands immediately after the aircraft flew over the test site. This paper presents results of the cross calibration between the polarimetric SAR and ground-based polarimetric scatterometer measurements at L and C bands. Comparison of the data measured by the two radar systems shows that SAR calibration with trihedrals may lead to unreliable results. A distributed-target calibration technique is introduced and applied to the data with good results. >