A bistatic SAR(synthetic aperture radar) system based on GPS(global positioning system) time synchronization module is proposed in this study. To configure a bistatic SAR system, two GPS time-synchronized SAR transceivers are operated as a moving transmitter and a stationary receiver, respectively. The GPS time synchronization module generates a reference clock of 10MHz and a trigger signal to control the time synchronization of both SAR transceivers for data acquisition. To implement the proposed method, the experimental device consisting of a ready-made GPS-DO(GPS disciplined oscillator) and a self-manufactured GTSM(GPS time synchronization module) was added to the Echo-SAR system, which is a chirp pulse-based radar transceiver. These synchronization modules make it possible to temporally synchronize two same SAR transceivers separated physically. To reconstruct a bistatic SAR image, optimum synchronization parameters were extracted from raw data collected through a field campaign, and an optimized procedure of bistatic SAR signal processing based on the back-projection algorithm was established.

Along with vessel detection, vessel recognition in high-resolution SAR images was necessary in order to monitor marine vessels effectively. However, lack of target data and phase defocusing of target from its velocity limited the recognition performance, especially when using detectors based on artificial intelligence. This study accordingly proposed effective vessel recognition in high-resolution ICEYE spotlight SAR images consecutively utilizing (i) vessel detector robust to defocused moving vessels and (ii) mitigation of moving target phase distortion. In order to apply quantitative and qualitative training data enhancement, a target velocity SAR phase refocusing function was developed. The proposed target velocity SAR phase refocusing function generated defocused SLC image with respect to different target azimuth velocity, which can be utilized for both training data augmentation and refocusing of velocity-induced phase distortion. Achievement of stable vessel recognition performance was enabled from (i) robust vessel detection on defocused moving vessels and (ii) well-focused detected vessel targets, both of which were consecutively applied using the proposed target velocity SAR phase refocusing function. Vessel detection results demonstrated robust performance regardless of vessel motion and vessel recognition results significantly improved after phase refocusing, both of which were subject to quantitative and qualitative training data enhancement. Performance of the proposed algorithm was analyzed both in terms of phase focusing and velocity estimation. Refocusing performance outperformed that of conventional state-of-the-art autofocusing algorithm, modified Phase Gradient Autofocusing, while azimuth velocity estimation derived the average offset of 0.68 m/s, which was regarded more accurate than previous azimuth velocity estimators based on single-channel SAR image.

Mitigation of geometric calibration offset in ocean without utilizing ground control points was investigated in this study. Real-time AIS information on vessels was exploited after preprocessing and accordingly tested against the detected vessels in the SAR image. Repetitive procedure of measuring the offset between the AIS sensor and the vessel detection was conducted and derived the SAR image of which the positioning offset was ameliorated. The proposed geo-location enhancement algorithm demonstrated the possibility of application in real-time vessel monitoring from remote sensing.

Monitoring of tidal creeks is important for safety in tidal flats and for understanding coastal changes due to sedimentation and erosion. The tidal creek is a small stream developed on tidal flats and requires very precise Digital Elevation Model (DEM) to detect it. Interferometric SAR (InSAR) technique was usually applied for generating DEM over inland regions. In tidal flat regions, low backscattering due to flatness and remnant water of the tidal flat prevents from generating high precision DEM. In this study, optimum InSAR baseline condition was investigated to generate such a precise DEM in the tidal flat. The long-baseline TanDEM-X and airborne InSAR system were found to be useful, and the tidal creek information (width and depth) was successfully extracted from the DEM that was generated from optimal baseline InSAR systems.

In this study, we investigated signal processing algorithm to reconstruct SAR image obtained from Frequency Modulated Continuous Wave - Synthetic Aperture Radar (FMCW-SAR). The airborne SAR raw data is usually obtained with continuous motion and non-linear (fluctuating) flight path. In order to compensate these effects, Back-Projection Algorithm (BPA) which can compensate the continuous motion effect was developed in this study. The algorithm was evaluated by applying it to the simulated and real-airborne SAR data.

Intertidal flats which are located between land and ocean are productive and rapidly changing places. But these places are now facing many environmental challenges related to climate change and human-induced impacts. Topographic change related to sedimentation or erosion in intertidal flats is the most evident sign of the environmental changes. The intertidal flats usually have small topographic variations (less than 5m) and experience ebb and flood tides every day. Thus, the conventional SAR interferometric techniques (repeat-pass InSAR) cannot be applied for generating DEMs in these intertidal flats. In this study, we developed a long-baseline single-pass airborne interferometric SAR (InSAR) system that has small ambiguity height and collected airborne InSAR data in several intertidal flats, the west coast of Korean peninsula. The constructed topographies using the long-baseline airborne InSAR system were compared with TanDEM-X DEM obtained during long-baseline mission phase.

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 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.

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.

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.

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 Cand X-band data. These scattering signatures were also verified from in-situ measurements 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 Scatterometer) 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.

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 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.

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.