Soon-Gu Kwon

Development of a simple scattering model for vegetation canopies and examination of its validity with scatterometer measurements of green-onion fields

A simple scattering model for radar backscattering from vegetated surfaces is developed based on the radiative transfer technique. The number of input parameters of the model is minimized to simplify the scattering model. The validity of the model is examined by comparison between the computation outputs and scatterometer measurements for the backscattering coefficients of green-onion (scallion) fields.

Study on the Calibration of a Full-Polarimetric Scatterometer System at X-band

A study on the calibration of an X-band HPS(Hongik Polarimetric Scatterometer) system for ground-based operation is presented in this paper. In order to calibrate the scatterometer system, the degree of its distortions are analyzed by comparison between theoretical- and measured-values using the theoretically well-known calibration targets such as a metal sphere, a trihedral corner reflector(CR) and a metal cylinder. The calibration works in the field conditions depend on the precise and stable measurements of those calibration target. we present a measurement technique, so-called, an automatic 2-D target-scanning technique, using the incidence-angle( and directions) control of HPS system. Then, we used STCT(Single-Target Calibration Technique) and GCT(General Calibration Technique) to calibrate a distortion of the scatterometer system, and measured the polarimetric RCS(Radar Cross Section) and phase-difference of a trihedral-CR as a test-target to verify the accuracy of the calibration technique. Then, three different types(i.e., 10, 20, 30 cm) of trihedral-CR were used. we obtained the error ranges about dB, dB in a polarimetric RCS and about and in the co-polarized phase-difference by using the GCT and STCT, respectively.

Soil moisture inversion from X-band SAR and scatterometer data of vegetation fields

The polarimetric radar backscatters of a soybean field were measured using the ground-based X-band polarimetric scatterometer (HPS) in an angular range from 20° to 60° in 2009 and 2010. The backscattering coefficients were also obtained using the COSMO-SkyMed (Spotlight mode, HH-mode) from July to October 2010. The backscattering coefficients of the soybean field were computed using the first-order RTM with field-measured input parameters. It was found that the estimated backscattering coefficients agree quite well with the field-measured radar backscattering coefficients. The surface scattering from the underlying soil surface was retrieved. Then, the soil moisture of the soybean field could be retrieved from the surface scattering using the empirical surface scattering model. The extracted soil moisture contents were compared with the in-situ measured soil moisture contents.

Evaluation of calibration accuracy with HPS (Hongik Polarimetric Scatterometer) system for multi-bands and multi-polarizations

A measurement technique for calibrating the HPS(Hongik Polarimetric Sactterometer) for L-, C-, X-band frequencies is presented in this paper. Especially, to overcome the integration problem with a large L-band sensor part, we propose a new compact OMT (Orthogonal Mode Transducer) for L-band. The performances of the integrated HPS system for multi-bands on single platform was evaluated and calibrated by the STCT (Single Target Calibration Technique) and the 2DTST (2D Target Scanning Technique). The measurement results show that the correlation among the measurements is larger than 0.95 and the error-rate is less than 5% at multi-frequency ranges, 1.02∼1.52 GHz, 5∼5.5 GHz, and 9.4∼9.9 GHz. The evaluation of the calibration accuracy using the STCT shows that 2DTST in the field experiment can secure the precise calibration accuracy within 0.5 dB for magnitude correction and less than 4° for phase correction in each bands.

Soil moisture detection using KOMPSAT-5 SAR data

The applicability of the KOMPSAT-5 (Korea Multi-Purpose Satellite-5) SAR on soil moisture detection is addressed in this paper. At first, the penetration into and reflection from bare soil surfaces at X-band are compared with those at L-band for homogeneous and inhomogeneous moisture profiles. The sensitivities of the X-band radar backscatter on soil moisture are examined for rough bare soil surfaces. Then the sensitivities of the X-band radar backscatter on the soil moisture of vegetated surfaces are also examined for various vegetation densities and incidence angles. The applicability of an X-band radar for soil moisture detection may be as good as L-band radar for bare soil surfaces. For vegetated surfaces, the soil moisture can be detected using an X-band radar at lower incidence angles, where the upper limit of the incidence angles depends on vegetation density.

A rotational high-resolution SAR on a tower at multi-frequencies

A rotational SAR system with the HPS (Hongik Polarimetric Scatterometer) tower platform for multi-frequency bands is presented in this paper. It was designed to obtain high-resolution SAR images using the existing tower platform of HPS and, in additional, 1.6m sub-arm for rotational synthetic aperture. Rotational SAR system was compared to the conventional SAR system using FDTD (finite-difference time-domain) simulations. Performances were measured at L-, C-, and X-bands by analysis of the reconstructed SAR images. Measured resolutions which are extracted from IRF (impulse response function) of targets in the rotational SAR images are (14.8cm, 9.2°) (7.2cm, 4°) and (5.1cm, 2.1°) in range- and cross-range directions at L-, C-, and X-bands, respectively.

Improved Method of Moments Using Hybrid Technique of Galerkin`s and Interpolation Methods for Numerical Analysis of Electromagnetic Waves

An improved method of moments using a hybrid Galerkin-interpolation technique for numerical analysis of electromagnetic wave scattering in the 3-dimensional space is presented in this paper. Basically, the EFIE(electric field integral equation) and RWG(Rao-Wilton-Glisson) basis function are used to compute a property of electromagnetic wave scattering. We propose a hybrid technique combining the existing Galerkin`s method with the interpolation method to improve the efficiency of the numerical computation. Then, an index of relative distance of each cells was defined to distinguish the relatively far elements, which interpolation method can be applied. To verify the performance of the proposed technique, the analytical Mie-series solution was used to compute the theoretical RCS of a conducting sphere for the purpose of comparison. We also applied this hybrid technique to various scatterers such as trihedral/omni-directional corner-reflectors to analyze the radar backscattering properties.

Ground-Based Rotational SAR System for Field-Experiments

A C-band ground-based rotational SAR system is presented in this paper. The rotaional SAR system is a test-bed for future rotational SAR systems which can be deployed in space and on a tower. The test-bed system is designed for imaging the electromagnetic scattering from earth surfaces and buried targets. This paper also presents the examination results of the generated SAR images. This rotational SAR system is basically consisted of the network-analyzer based HPS(Hongik Polarimetric Scatterometer) and a horizontally rotating arm. Several SAR images were obtained using the rotational SAR system for various target areas. To verify this system, we simulated the SAR images for the rotational SAR using the FDTD algorithm and compared between the measured and simulated SAR images. The rotational SAR system is operated at the center frequency of 5 GHz and various frequency bandwidth within 0.5~2 GHz to change the resolution of SAR images.

RCS Extraction of Trihedral Corner Reflector for SAR Image Calibration

This paper presents an algorithm for retrieving precise radar cross sections(RCS) of various trihedral corner reflectors (TCR) which are external calibrators of synthetic aperture radar(SAR) systems. The theoretical RCSs of the TCRs are computed based on the physical optics(PO), geometrical optics(GO), and physical theory of diffraction(PTD) techniques; that is, the RCS computation includes the single reflections(PO), double reflections(GO-PO), triple reflections(GO-GO-PO), and edge diffractions(PTD) from the TCR. At first, we acquire an SAR image of the area that five TCRs installed in, and then extract the RCS of the TCRs. The RCSs of the TCRs are extracted accurately from the SAR image by adding up the power spill, which is generated due to the radar IRF(Impulse Response Function), using a square window. We compare the extracted RCSs with the theoretical RCSs and analyze the difference between the theoretical and experimental RCSs of the TCR for various window sizes and various backscattering coefficient levels of the adjacent area. Finally, we propose the minimum size of the integration area and the maximum level of the backscattering coefficients for the adjacent area.

Verification of a Calibration Technique for a Full-Polarimetric Scatterometer System at C-band

This paper presents a study on the calibration of a C-band HPS(Hongik Polarimetric Scatterometer) system using the DMMCT(Differential Mueller Matrix Calibration Technique). For calibration of the polarimetric scatterometer system, a fully-polarimetric antenna pattern(magnitudes and phase-differences) of the antenna main-beam is measured using a conducting sphere at anechoic chamber. The polarimetric scatterometer system could be accurately calibrated after retrieving its distortions using the DMMCT. Unlike a single-polarimetric system, in a fully-polarimetric system, not only backscattering coefficients but also phase differences are important parameters. This calibrated HPS system can be used to measure accurate Mueller matrices of bare soil surfaces, rice paddies, and vegetation fields. The phase-difference parameters as well as the backscattering coefficients for co- and cross-polarizations can then be obtained. The accuracy of calibration was verified by comparing the measured backscattering coefficients with a scattering model. The measured polarization response of a plowed bare field was also compared with the polarization response which was synthesized using a polarimetric scattering model for verifying the calibration technique.