Ki-mook Kang

Development of a Cost-Effective Airborne Remote Sensing System for Coastal Monitoring

Coastal lands and nearshore marine areas are productive and rapidly changing places. However, these areas face many environmental challenges related to climate change and human-induced impacts. Space-borne remote sensing systems may be restricted in monitoring these areas because of their spatial and temporal resolutions. In situ measurements are also constrained from accessing the area and obtaining wide-coverage data. In these respects, airborne remote sensing sensors could be the most appropriate tools for monitoring these coastal areas. In this study, a cost-effective airborne remote sensing system with synthetic aperture radar and thermal infrared sensors was implemented to survey coastal areas. Calibration techniques and geophysical model algorithms were developed for the airborne system to observe the topography of intertidal flats, coastal sea surface current, sea surface temperature, and submarine groundwater discharge.

Doppler Velocity Characteristics During Tropical Cyclones Observed Using ScanSAR Raw Data

Doppler velocity can be derived by calculating Doppler shift anomalies between predicted and estimated Doppler centroids. The predicted Doppler centroid is calculated based on a geometric model of satellite assuming that the target is not moving. The estimated Doppler centroid can be directly extracted from the raw SAR signal data by applying the average cross-correlation coefficient method. It is known that wind-generated ocean waves can significantly contribute to Doppler velocity due to the correlation between orbital motions of the waves and (tilt and hydrodynamic) modulated radar cross sections, in addition to what sea surface current contributes. In this study, the characteristics of Doppler velocities under hurricane conditions were investigated using RADARSAT-1 ScanSAR raw data. Five different hurricanes (Hurricane Dean, Hurricane Ivan, Hurricane Kyle, Hurricane Lili, and Typhoon Xangsane) and sequential acquisitions of two hurricanes (Hurricane Kyle and Hurricane Lili) were selected to study the contribution of wind-induced waves to Doppler velocities and compared with in situ measurements of drifting buoys. The results show that hurricane-generated seas and associated winds and waves appear to be different from ordinary sea state. This leads to lower estimates of Doppler velocities than expected and much closer to sea surface current velocities.

Comparison of coastal sea surface temperature derived from ship-, air-, and space-borne thermal infrared systems

The coastal sea surface temperature (SST) is one of the important oceanic environmental factors in determining the change of marine environments and ecological activities. In order to extract coastal SST, we developed ship- and airborne thermal infrared remote sensing systems. The acquired thermal infrared images are radiometrically calibrated using an atmospheric radiative transfer model as well as a thermometer/hygrometer sensor and geometrically calibrated using a GPS/IMU sensor. In this study, the ship- and airborne SST data were compared and verified with spaceborne SST data and in-situ data. The comparison and validation results showed that the ship and air-borne thermal infrared remote sensing could be used in generating SST map, especially in mapping fine-scale coastal features.

Retrieval of sea surface velocity during tropical cyclones from RAD ARS AT-1 ScanSAR Doppler centroid measurements

This paper presents here aims at the retrieval of sea surface velocity during tropical clones using RADARSAT-1 ScanSAR Doppler centroid measurements. The sea surface velocity can be estimated by comparing Doppler shift anomalies between predicted and estimated Doppler centroids. The predicted Doppler centroid, defined as the Doppler centroid assuming that the target is not moving, is calculated based on the geometry model. While the estimated Doppler centroid, representing the moving target, can be extracted directly from SAR raw signal data by applying the Average Cross Correlation Coefficient (ACCC) method. In this study, we present characteristics of the sea surface response to tropical cyclones such as the size of hurricane eye or intensity and the spatial patterns of surface velocity from the center of hurricane. And we present the first attempt to derive sea surface velocity using RADARSAT-1 ScanSAR Doppler centroid measurements during two different hurricane (Typhoon Xangsane and Hurricane Lili) and three sequential cases of Hurricane Lili, especially when using inaccurate satellites orbit and attitude information with wide coverage, such as RADARSAT-1 ScanSAR wide mode. Although the result was compared with one drifting buoy data in a single location during Typhoon Xangsane, it clearly shows that the sea surface velocity during tropical cyclones can represent the sea surface current.

Nonlinear internal wave spirals in the northern East China Sea

Oceanic internal waves are known to be important to the understanding of underwater acoustics, marine biogeochemistry, submarine navigation and engineering, and the Earth’s climate. In spite of the importance and increased knowledge of their ubiquity, the wave generation is still poorly understood in most parts of the world’s oceans. Here, we use satellite synthetic aperture radar images, in-situ observations, and numerical models to (1) show that wave energy (having relatively high amplitude) radiates from a shallow sill in the East China Sea in all directions, but with a significant time lag dependent on background conditions, (2) reveal that wave fronts are locally formed with often favorable conditions for re-initiation, and (3) demonstrate the resulting variety of wave patterns. These findings would be the case for any broad shelf having shallow sills with time-varying conditions, and therefore have significant implications on the redistribution of energy and materials in the global as well as regional ocean.

Intertidal flat topographies measured by long-baseline airborne sar and tandem-x

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.

A long baseline airborne SAR interferometry for tidal flat mapping

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.

Measurements of intertidal flat topography using a long-baseline airborne interferometric SAR

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.

Design and Manufacture of FMCW Radar with Multi-Frequency Bandwidths

Design of X-band frequency FMCW based imaging radar with multi-resolutions and performances of the self-manufactured radar system are presented in this study. In order to implement the multi-bandwidths, a ramp sequence of the FMCW signal is consisting of two kinds of ‘saw-tooth’ waveform with different bandwidth, and a receiver circuit consisting of L-band source and frequency converter circuit is used to effectively extract spectra of beat-frequency from the received signal of X-band frequency. The system setups for performance measurement of self-manufactured radar system are maximum output power of 35 dBm, sampling frequency of 1.2 MHz and sweep time of 1 ms. Then, the measured resolutions of the modulated signal having bandwidth of 500 MHz and 300 MHz in range & azimuth-direction are (0.28 m, 0.26 m) and (0.44 m, 0.27 m), respectively.

Feasibility study on estimating sea surface currents from single (Envisat ASAR) and dual (TanDEM-X) SAR system

Sea surface current plays an important role in understanding physical oceanography and can provide critical information for preventing natural disasters such as oil spill and ship distress. In this study, two techniques using space-borne Synthetic Aperture Radar (SAR) systems were processed to evaluate their feasibility to extract sea surface current velocities. The first technique employs the Doppler shift algorithm from a single SAR system which is generated when moving targets are imaged in East Sea of Korean Peninsula. The second technique utilizes Along-Track Interferometry (ATI) from dual SAR system to estimate sea surface current velocity in Strait of Korea. Envisat ASAR and TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) data were used in each technique, respectively.