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Remote sensing of ocean surface winds, currents, waves, and internal waves around the Korean peninsula using Space-borne Synthetic Aperture Radar (SAR)

Duk-jin Kim(1), Junsu Kim(2), Kyung-Ae Park(3) and Wooil M. Moon(3)

(1) Korea Aerospace Research Institute, -, Daejeon, 305-333, Korea
(2) Seoul National University, -, Seoul, Korea
(3) Seoul National University, -, Seoul, 151-742, Korea

Abstract

Synthetic Aperture Radar (SAR) remote sensing is one of the most cost effective and powerful tools for application research of the ocean. The SAR system can observe the Earth’s surface with the very high resolution and large spatial coverage, which makes it a valuable tool for measuring geophysical parameters such as ocean surface winds, waves, currents and internal waves. We have developed and improved the algorithms for extracting quantitative information of those parameters using various types of SAR data available both space-borne SAR (ERS-1/2, RADARSAT, and ENVISAT ASAR) and airborne SAR (AIRSAR) acquired around the Korean Peninsula.

SAR is the only system that can provide a synoptic view of find wind fields near the coastal area. Many SAR images including RADARSAT and ENVISAT ASAR’s alternating polarization mode and wide swath mode were obtained off the eastern coast of the Korean Peninsula to investigate the ability of retrieving sea surface wind field. We installed corner reflectors on the nearby beach to calibrate the SAR data, and we obtained in-situ measurements from the several coast-based automatic weather systems and ocean buoys. Using the simultaneously acquired polarization ENVISAT ASAR data (HH and VV), the most appropriate polarization ratio was evaluated and applied for improving the wind retrieval model. In addition, the best combinations depending on given sea states and incidence angle ranges were investigated.

The characteristics of short-period and long-period (near-inertial) internal waves observed in the East Sea (Sea of Japan) were analyzed. The possibility of inferring characteristics of the interior ocean dynamics from the SAR image associated with internal solitary waves was tested using a hydrodynamic interaction model (action balance equation) and a radar backscattering model (two-scale tilted Bragg model). These models were used iteratively to fit the observed SAR data to the simulated SAR. The estimated results were compared with in-situ measurements. The typical scales and the spatial and temporal characteristics of internal waves observed in the western part of the East Sea were also estimated from more than 140 SAR images. On the other hand, very long wave patterns (13-10 km) were observed in two successively acquired ENVISAT ASAR images and interpreted as near-inertial internal waves based on the hydrographic data.

ATI-SAR, utilizing two SAR antennas separated along the platform flight direction and combined interferometrically, was used to derive ocean surface current and wave information. The phase of ATI-SAR is related to the line-of-sight velocity of the water scatterers. The surface current extraction from the ATI-SAR velocity is still an open question, because the Doppler shift is not simply proportional to the component of the mean surface current. It also includes other types of contributions associated with the phase velocity of the Bragg waves and orbital motions of all ocean waves that are longer than Bragg waves. For accurate current estimation, a new and practically useful method was developed using simultaneously measured L- and C-band ATI-SAR data. The influence of Bragg resonant waves and long ocean wave motions on the ATI-SAR velocity according to the radar-frequency was analyzed and effectively eliminated. The method was applied to L- and C-band ATI-SAR measurements acquired at the Ulsan coast in the southeastern part of the Korean Peninsula. The resulting ocean surface current vectors were compared with in situ measurements collected by an RCM (Recording Current Meter). Furthermore, ocean surface wave information was extracted from the ATI-SAR data using a quasi-linear transform. The limitations of the transform were also discussed.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry