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A sea surface imaging technique for an emergency response using a ready-made frequency modulated continuous wave&ndash, synthetic aperture radar (FMCW SAR) system and its experimental results are described in this paper. The optimal range of radiowave incidence angle for sea surface imaging was analyzed by a theoretical scattering model and measurement data, and it was properly applied to the FMCW SAR system by readjusting the delayed-dechirp process. Raw data acquired through flight experiments were reconstructed to SAR image by the range-doppler algorithm. To verify the performance of the reconstructed sea surface image, dual-channel images collected by the configuration of the along-track interferometry were used, and then performance indicators such as signal attenuation, coherence, and phase difference were analyzed. Through this experimental study, it was confirmed that the ready-made FMCW SAR system without a function of the incident angle control can also conduct limited missions for maritime observation. It is possible to be an alternative resource for emergency response, in which the cases are requiring urgent maritime disaster detection and analysis.

Topographic change due to sedimentation and erosion of tidal flat is important for recognizing environmental changes and possible threatens. The tidal flat generally has small topographic variations and high moisture soil surfaces. The conventional SAR interferometric techniques cannot be used for generating DEMs in these tidal flats due to relatively short baseline. In this study, we developed a long-baseline airborne interferometric SAR (InSAR) system that has small ambiguity height and collected airborne InSAR data in the western coast of Korean peninsula. The constructed topographies using the long-baseline airborne InSAR were compared with GPS-RTK measurements.

Intertidal flats are productive and rapidly changing places. However, these places are facing many environmental challenges related to climate change and human-induced impacts. Topographic change due to sedimentation or erosion in intertidal flats can be the key indicator for recognizing these environmental changes. The intertidal flats usually have small topographic variations (less than 5m) and high-moistured soil surfaces (greater than 50%). The conventional SAR interferometric techniques cannot be used for generating DEMs in these intertidal flats. In this study, we developed a long-baseline airborne interferometric SAR (InSAR) system that has small ambiguity height and collected airborne InSAR data in Jebu intertidal flat, west coast of Korean peninsula. The constructed topographies using the long-baseline airborne InSAR were compared with TanDEM-X DEM (acquired during long-baseline mission phase) and GPS-RTK measurements.

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.

Measurement and analysis results of the extracted radiation-patterns from the field-experiments which were conducted to acquire the generic technology for calibration and validation of the satellite SAR system(Synthetic Aperture Radar) are presented in this study. Prototype of active transponder is adjustable within maximum 63.1 dBsm of RCS (Radar Cross Section) and includes the receiving-function with external receiver. To increase an accuracy of these field experiments, we repetitively measured satellite SAR systems of the same operating mode(i.e., COSMO-SkyMed No. 2 & 3, hh-pol., strip-map himage mode, 3 m resolution). Then, the reliability of experimental results was cross-checked through analysis of the RCS of active transponder on SAR image. The property of azimuth radiation patterns of satellite SAR system extracted from them has of HPBW(half-power beamwidth), of FNBW(first-null beamwidth), and 11.17 dB of PSLR(peak to side lobe ratio), respectively.

This paper presents a scattering model and measurements of backscattering coefficients for soybean fields. The polarimetric radar backscatters of a soybean field were measured using the ground-based X-band polarimetric scatterometer in an angular range from 20° to 60°. The backscattering coefficients were also obtained using the COSMOSkyMed (Spotlight mode, HH-polarization) from July to October 2010. The backscattering coefficients of the soybean field were computed using the 1st-order radiative transfer model (RTM) with field-measured input parameters. The soybean layer is composed of the stems, branches, leaves, and soybean pods. The stems, branches, and pods are modeled with lossy dielectric cylinders, the leaves are modeled with lossy dielectric disks. The estimated backscattering coefficients agree quite well with the field-measured radar backscattering coefficients.

Exposed oyster reefs in tidal flats have complex and rough surfaces because of their unique surface texture, which are quite distinct from the surrounding mud or sand flats. Here we investigate the microwave signatures, backscattered from naturally distributed oyster reefs in tidal flats, utilizing the polarimetric analysis techniques to fully polarimetric RADARSAT-2 (C-band) and ALOS PALSAR (L-band) data. The study areas include the tidal flats around Jebu Island and Hampyung Bay on the west coast of the Korean peninsula. We analyzed the microwave scattering mechanisms associated with oyster reefs and surrounding areas using the polarimetric target decomposition theorem and quantitatively measu ρ HHVV red target depolarization effects (the cross-polarized ratio (HV/VV), the co-polarized correlation coefficient ( ρ HHVV ), and the co-polarized phase difference between HH and VV). On the basis of a large increase in the cross-polarized backscattering (HV) in the C-band SAR data, one can observe strong volume (or multiple) scattering and depolarization effects over oyster reefs areas, whereas only surface scattering was dominant in most parts of the background tidal areas. In oyster reefs, the proportion of volume scattering and the cross-polarized ratio were greater than 0.7 and −8 dB, respectively. These scattering characteristics were also verified from in-situ measurements in the field using a ground-based polarimetric scatterometer system. However, almost no difference was observed between the scattering signatures of oyster reefs and background mudflat areas from L-band data, which have a considerably longer wavelength than C-band. The study clearly suggests that multi-frequency (C- and L-band) polarimetric SAR systems can be used to detect the naturally distributed oyster reefs in tidal flats.

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.

A study on the RCS of a self-manufactured active transponder, which has a manually adjustable RCS, for SAR(Synthetic Aperture Radar) external calibration and image analysis at X-band is presented in this paper. The RCS of the active transponder was comparably analyzed using the lab-test and the COSMO-SkyMed SAR system, and also precisely analyzed the difference between the adjusted and estimated RCSs. A maximum RCS of the designed and manufactured active transponder is 60 dBsm. The active transponders with 60 dBsm and the adjusted RCS of 40 dBsm were measured using the single target calibration technique(STCT) and 2D target scanning technique(2DTST). And the extracted RCS using power-spill integration technique in a SAR image was compared with the measured RCS of active transponder. The comparison results show that the measured and extracted RCSs are 59.7 dBsm, 40.2 dBsm and 57.3 dBsm, 39.2 dBsm, respectively.

The radar backscatter from various earth surfaces is sensitive to the frequency of the incident wave. This study examined the radar sensitivities for surface parameters such as soil moisture content and surface roughness of both bare and vegetated surfaces at X-band. Because L-band frequencies are often used for sensing the surface parameters, the sensitivities of X-band are also compared with those of the L-band. The sensitivities of the X-band radar backscatter were examined with respect to soil moisture content and surface roughness of rough bare soil surfaces. These sensitivities were also examined using the same parameters for vegetated surfaces for various vegetation densities and incidence angles. Use of the X-band radar for soil moisture detection was as effective as L-band radar for bare soil surfaces. For vegetated surfaces, the soil moisture could be detected using an X-band radar at lower incidence angles, where the upper limit of the incidence angles was dependent on vegetation density.

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.

A full-polarimetric scatterometer(HPS: Hongik Polarimetric Scatterometer) for X-band is designed, fabricated, and verified using the theoretically well-known point-targets in this paper. The X-band full-polarimetric scatterometer consists of an OMT(Orthogonal-Mode Transducer)+horn antenna, the angle control part for the OMT+horn antenna, a transmitter/receiver with a network analyzer and a frequency-conversion circuitry, and a movable support of these parts. We use an inclinometer sensor to control the vertical and horizontal incidence angles. The full polarimetric data can be obtained because of the polarization switches and the OMT. The accuracy of the scatterometer system is verified by measuring the polarimetric RCS(Radar Cross Section) of one of the theoretically well-known point-targets, i.e., a corner reflector.

This paper presents the development of an accurate and simple scattering model for radar backscatters of agricultural fields using the water-cloud model (WCM) and the first-order radiative transfer (RT) model. This new model is accurate and simple enough to be used for soil moisture retrieval from the measured backscattering coefficients of vegetation fields. To improve the accuracy of the scattering model, we modified the WCM with an additional term for accurately estimating the angular effect of scattering particles in a vegetation canopy, using the relatively accurate first-order RT model. The accuracy of the scattering model was verified with the COSMO SkyMed radar data and ground-truth in-situ measurements of a bean field and a wheat field. The simple model was applied to retrieve soil moisture from the radar measurements. The estimated soil moisture contents agree well with the in-situ measured soil moisture contents for the bean and wheat fields with the correlation coefficients of 0.91 and 0.96, respectively.

In this study, we developed a soil moisture retrieval technique for vegetation fields using a modified water-cloud model. The water-cloud model has been a typical algorithm used to analyze scattering from vegetation fields for a long time, because it is simple to be used for retrieving a variety of information such as soil moisture and vegetation water mass. However, its accuracy has been questionable because the water-cloud model contains lots of approximations in the process of simplification. To improve the accuracy of the algorithm, we modified the water-cloud model with the estimation of parameters using a radiative transfer model. Soil moisture is retrieved from SAR images by the modified water-cloud model for vegetation fields and compared with in-situ measured ground truth data.

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.

Polarimetric signatures of oyster reefs in tidal flats were investigated using multi-frequency polarimetric SAR data. The scattering mechanism in tidal flats was analyzed by the Freeman-Durden target decomposition and the target depolarization effect was quantitatively measured using the cross-polarized ratio. Strong multiple scattering and depolarization effect were observed in oyster reefs areas from C- and X-band data. These scattering signatures were also verified from in-situ measurenments using a ground-based polarimetric scatterometer system. However, no difference was observed between the scattering signatures of oyster reefs and mud flat areas from L-band data, which have a relatively longer wavelength. These results suggest that multi-frequency polarimetric SAR observations can be used to quantitatively detect the spatial distribution of oyster reefs in tidal flats.

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.

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.

An improved field-measurement technique for enhancing the calibration accuracy of a ground-based scatterometer is proposed in this paper. The automatic two-dimensional (2-D) measurement of a calibration target, so-called ‘2-D target scanning technique (2DTST)’, gives a precise alignment between a point calibration target and an antenna bore-sight. Averaging of the calibration-target measurements increases further the accuracy and reliability of the automatic alignment. The measurement results show that the correlation among the measurements is larger than 0.95 and the error-rate is below 5% at a frequency range from 9.4 GHz to 9.9 GHz. The evaluation of the calibration accuracy using the STCT (Single Target Calibration Technique) shows that 2DTST in the field experiment can secure the precise calibration accuracy within 0.5 dB for magnitude correction and 4° for phase correction.

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.

In this paper, we discussed the functionalities of KOMPSAT-5 from the emergency response perspective to oil spill detection, describing its technical abilities in terms of maneuverability, system response time, image quality, and damping ratio. The damping ratio at the X-band was higher than that at other available frequencies, which increased the probability of oil spill detection from SAR data. However, the detection of oil spills depends significantly on the wind condition and the presence of look-alikes. Oil spill detection algorithm for KOMPSAT-5 estimates wind information directly from SAR data, and is expected to enhance the detection capability.

The method of moments/ Monte Carlo technique is used to numerically compute the Mueller matrix for radar backscattering from stem-cut cluster sections of a harvested farm field, and a semi-empirical surface scattering model is employed for estimating the radar backscatter of a rough soil surface. The full-wave numerical technique and the surface scattering model are combined to compute the Mueller matrix and consequently the backscattering coefficients of a harvest farm field. This hybrid technique is verified comparing the numerical computations with the experimental measurements for the backscattering coefficients of harvested rice fields.