Takero Yoshida

SAR Image Simulation in the Time Domain for Moving Ocean Surfaces

This paper presents a fundamental simulation method to generate synthetic aperture radar (SAR) images for moving ocean surfaces. We have designed the simulation based on motion induced modulations and Bragg scattering, which are important features of ocean SAR images. The time domain simulation is able to obtain time series of microwave backscattering modulated by the orbital motions of ocean waves. Physical optics approximation is applied to calculate microwave backscattering. The computational grids are smaller than transmit microwave to demonstrate accurate interaction between electromagnetic waves and ocean surface waves. In this paper, as foundations for SAR image simulation of moving ocean surfaces, the simulation is carried out for some targets and ocean waves. The SAR images of stationary and moving targets are simulated to confirm SAR signal processing and motion induced modulation. Furthermore, the azimuth signals from the regular wave traveling to the azimuth direction also show the azimuthal shifts due to the orbital motions. In addition, incident angle dependence is simulated for irregular wind waves to compare with Bragg scattering theory. The simulation results are in good agreement with the theory. These results show that the simulation is applicable for generating numerical SAR images of moving ocean surfaces.

Simulation and Evaluation of Sea Surface Observations Using a Microwave Doppler Radar

해면으로부터의 마이크로웨이브 후방산란 시뮬레이션을 이용하여 마이크로웨이브 도플러 레이더에 의한 파고와 해면 흐름 관측법을 평가하였다. 해면으로부터 후방산란하는 마이크로웨이브의 도플러 스펙트럼은 마이크로웨이브 해면 조사폭과 스펙트럼 해석 시간폭의 영향을 받는다. 본 연구에서는 마이크로웨이브 해면 조사폭과 스펙트럼 해석시간폭의 영향을 조사하기 위하여, 다양한 파랑과 해면 흐름 조건의 수치 해면 생성과 마이크로웨이브 후방산란 시뮬레이션을 통하여 도플러 스펙트럼을 구하였다. 결과에 의하면 마이크로웨이브 해면 조사폭을 파장의 1/5이하, 스펙트럼 해석 시간폭을 파주기의 1/5이하로 설정하면, 충분한 정도의 파고계측이 가능하다. 또한, 파주기에 비해서 충분히 긴 스펙트럼 해석 시간폭을 설정하면 해면 흐름의 상대유속 계측이 가능하다. 시뮬레이션을 이용하여 마이크로웨이브 도플러 레이더에 의한 해면관측의 적절한 계측방법을 찾을 수 있다.

Time-Domain Simulation of Along-Track Interferometric SAR for Moving Ocean Surfaces

A time-domain simulation of along-track interferometric synthetic aperture radar (AT-InSAR) has been developed to support ocean observations. The simulation is in the time domain and based on Bragg scattering to be applicable for moving ocean surfaces. The time-domain simulation is suitable for examining velocities of moving objects. The simulation obtains the time series of microwave backscattering as raw signals for movements of ocean surfaces. In terms of realizing Bragg scattering, the computational grid elements for generating the numerical ocean surface are set to be smaller than the wavelength of the Bragg resonant wave. In this paper, the simulation was conducted for a Bragg resonant wave and irregular waves with currents. As a result, the phases of the received signals from two antennas differ due to the movement of the numerical ocean surfaces. The phase differences shifted by currents were in good agreement with the theoretical values. Therefore, the adaptability of the simulation to observe velocities of ocean surfaces with AT-InSAR was confirmed.

Numerical Research on Clear Imaging of Azimuth‐Traveling Ocean Waves in SAR Images

This paper presents the imaging mechanisms of ocean waves traveling in the azimuth direction by applying a numerical simulation of synthetic aperture radar (SAR) image for moving ocean surfaces. The numerical simulation was originally designed in the time domain to take into account motion-induced modulations. It is known that azimuthal waves are affected by azimuth shifts due to orbital motions of ocean waves, i.e., velocity bunching. The effect of bunching generally results in nonlinear imaging, however a clear wave-like pattern of a homogeneous wave field propagating to the azimuth direction occurs in SAR imagery under a certain condition. More specifically, if azimuth shifts due to the orbital motions are concentrated on wave crests or troughs, bunching causes a single peak in SAR image planes. Theoretically, it is derived noticeably when the maximum orbital motion of an azimuth-traveling ocean wave is nearly equal to one quarter of its wavelength. To demonstrate this, SAR signals were simulated numerically for ocean waves with and without employing the condition of the clear imaging. The results showed that a wavelike pattern relating to the azimuthal waves can be imaged apparently under the specific condition. In addition, the simulations were performed in the case of the other wave and wind conditions. From the results, it was revealed that the specific condition leads to clear wave-like imaging of azimuthal waves in SAR images if noise is relatively small.

Use of Numerical Simulation for Water Area Observation by Microwave Radar

Numerical simulation technique has been developed to calculate microwave backscattering from water surface. The simulation plays a role of a substitute for experiments. Validation of the simulation was shown by comparing with experimental results. Water area observations by microwave radar have been simulated to evaluate algorithms and systems. Furthermore, the simulation can be used to understand microwave scattering mechanism on the water surface. The simulation has applied to the various methods for water area observations, and the utilizations of the simulation are introduced in this paper. In the case of fixed radar, we show following examples, 1. Radar image with a pulse Doppler radar, 2. Effect of microwave irradiation width and 3. River observation (Water level observation). In addition, another application (4.Synthetic aperture radar image) is also described. The details of the applications are as follows. 1. Radar image with a pulse Doppler radar: A new system for the sea surface observation is suggested by the simulation. A pulse Doppler radar is assumed to obtain radar images that display amplitude and frequency modulation of backscattered microwaves. The simulation results show that the radar images of the frequency modulation is useful to measure sea surface waves. 2. Effect of microwave irradiation width: It is reported (Rheem[2008]) that microwave irradiation width on the sea surface affects Doppler spectra measured by a CW (Continuous wave) Doppler radar. Therefore the relation between the microwave irradiation width and the Doppler spectra is evaluated numerically. We have shown the suitable condition for wave height estimation by a Doppler radar. 3. River observation (Water level observation): We have also evaluated algorithms to estimate water current and water level of river. The same algorithms to estimate sea surface current and sea surface level are applied to the river observation. The simulation is conducted to confirm the accuracy of the river observation by using a pulse Doppler radar. 4. Synthetic aperture radar (SAR) image: SAR images are helpful to observe the global sea surface. However, imaging mechanisms are complicated and validation of analytical algorithms by SAR images is quite difficult. In order to deal with the problems, SAR images in oceanic scenes are simulated.

Time domain numerical simulation of microwave backscattering for ocean SAR image

This paper describes a simulation of SAR raw signals in time-domain. We had developed a time domain simulation technique to calculate microwave backscattering from the moving sea surface with a fixed radar. It has been improved with considering movements of a radar which is assumed as a SAR. In the simulation, physical optics approximation is applied to calculate backscattered microwaves. SAR raw signals from a target have been simulated as a part of SAR image simulation. Furthermore, in order to simulate SAR images of the moving sea surface, azimuth shifts from a moving target have discussed. These results show that the simulation can generate ocean SAR images in time-domain for sea surface observations.

Time domain numerical simulation of microwave backscattering from sea surface for radar remote sensing

Time domain numerical simulation technique has been developed to understand microwave backscattering mechanisms and evaluate sea surface observation methods. Backscattering coefficients and Doppler spectra can be obtained by this simulation technique. Physical optics approximation is applied to calculate scattering electric fields. The validity of the simulation has been verified by comparing with the results of the experimental basin. The results indicate that the simulation technique is effective to calculate microwave backscattering from the water surface changing in time. As an application of the simulation, we have simulated microwave backscattering with a pulse Doppler radar to evaluate sea surface observation methods. Radar images of backscattering coefficient and Doppler velocity have been generated. The results of the simulation show that radar images of Doppler velocity are more effective than images of backscattering coefficient for sea surface observation.

Modeling Fluid Force Acting on Single Floating Body in Group of Floating Bodies

細を得ることを目的として,相対的に解像度の粗い 8 億格子 の大規模 LES 計算を実施し,得られた流場の解析から,以 下の知見を得た. ・表面の格子解像度が y= 80 程度の格子による LES 計算 で推定される流場は,与えた条件よりも低いレイノルズ 数条件の実験結果と一致する傾向となるものの,プロペ ラ周りの流場を概ね再現できている. ・実験的な計測,あるいは従来の RANS による CFD で解 像できていなかったプロペラ翼面上の微細な渦構造を, 大規模 LES では解像することが可能である. ・翼面上の渦構造の変化と限界流線が対応しており,横渦 が縦渦に変化する付近で限界流線は大きく半径方向を 向く. ・翼面上での境界層の発達過程では横渦から縦渦に変化す る際にヘアピン渦を介しておらず,平板上での境界層の 発達過程と異なる. LES 計算の限界流線がより低い条件の実験と一致する傾 向を持つことの原因として,解像度が十分でないことが考え られるため,今後さらに格子解像度を高めた DNS 相当の精 度を持つ計算を実施し検討する予定であるが,このような検 討を通じて LES 大規模計算が乱流遷移を含む流場を正確に 捉えられることを明らかにできれば,将来的には実船尺度に おいて船体・付加物を含めた船舶の性能を正確に推定可能と なることが期待される.

Numerical Evaluation of Observation Algorithm for Sea Surface Remote Sensing by Doppler Radar

Algorithms of sea surface remote sensing are based on changes of Doppler shifts, which are measured by a Doppler radar. Microwave irradiation width on the sea surface and time taken to collect data for frequency analysis influence Doppler spectra. In order to evaluate the influences of these parameters in observing algorithms, a simulation of microwave backscattering from numerical sea surface was done in time domain to obtain Doppler spectra. Doppler spectra have been simulated in the case of various numerical regular waves. In the case of the microwave irradiation width is larger than the wavelength of the numerical regular wave or the Fourier transform time for the frequency analysis is longer than the period of the numerical regular wave, the peak value of Doppler spectra shows the phase velocity of the Bragg resonance wave. The results show the principle of measuring sea surface current. In the case of the microwave irradiation width is smaller than the wavelength of the numerical regular wave or the Fourier transform time is shorter than the period of the numerical regular wave, Doppler spectra vary with the orbital motions of the regular wave. As the result, when the sea surface wavelength is five times or more as long as the microwave irradiation width, the time fluctuations of Doppler velocity which shows a mean value of Doppler spectrum are good agreement with the orbital motions of the numerical regular wave. Also in such condition, the wave height of the sea surface waves can be observed accurately by analyzing the changes of Doppler velocity.Copyright