Developing a remote sensing system based on X-band radar technology for coastal morphodynamics study

New data processing techniques are proposed for the assessment of scopes and limitations from radar-derived sea state parameters, coastline evolution and water depth estimates. Most of the raised research is focused on Colombian Caribbean coast and the Western Mediterranean Sea. First, a novel procedure to mitigate shadowing in radar images is proposed. The method compensates distortions introduced by the radar acquisition process and the power decay of the radar signal along range applying image enhancement techniques through a couple of pre-processing steps based on filtering and interpolation. Results reveal that the proposed methodology reproduces with high accuracy the sea state parameters in nearshore areas. The improvement resulting from the proposed method is assessed in a coral reef barrier, introducing a completely novel use for X-Band radar in coastal environments. So far, wave energy dissipation on a coral reef barrier has been studied by a few in-situ sensors placed in a straight line, perpendicular to the coastline, but never been described using marine radars. In this context, marine radar images are used to describe prominent features of coral reefs, including the delineation of reef morphological structure, wave energy dissipation and wave transformation processes in the lagoon of San Andres Island barrier-reef system. Results show that reef attenuates incident waves by approximately 75% due to both frictional and wave breaking dissipation, with an equivalent bottom roughness of 0.20 m and a wave friction factor of 0.18. These parameters are comparable with estimates reported in other shallow coral reef lagoons as well as at meadow canopies, obtained using in-situ measurements of wave parameters.DoctoradoDoctor en Ingeniería Eléctrica y Electrónic

A Marine Radar Wind Sensor

A new method for retrieving the wind vector from radar-image sequences is presented. This method, called WiRAR, uses a marine X-band radar to analyze the backscatter of the ocean surface in space and time with respect to surface winds. Wind direction is found using wind-induced streaks, which are very well aligned with the mean surface wind direction and have a typical spacing above 50 m. Wind speeds are derived using a neural network by parameterizing the relationship between the wind vector and the normalized radar cross section (NRCS). To improve performance, it is also considered how the NRCS depends on sea state and atmospheric parameters such as air–sea temperature and humidity. Since the signal-to-noise ratio in the radar sequences is directly related to the significant wave height, this ratio is used to obtain sea state parameters. All radar datasets were acquired in the German Bight of the North Sea from the research platform FINO-I, which provides environmental data such as wind measurements at different heights, sea state, air–sea temperatures, humidity, and other meteorological and oceanographic parameters. The radar-image sequences were recorded by a marine X-band radar installed aboard FINO-I, which operates at grazing incidence and horizontal polarization in transmit and receive. For validation WiRAR is applied to the radar data and compared to the in situ wind measurements from FINO-I. The comparison of wind directions resulted in a correlation coefficient of 0.99 with a standard deviation of 12.8°, and that of wind speeds resulted in a correlation coefficient of 0.99 with a standard deviation of 0.41 m s^−1. In contrast to traditional offshore wind sensors, the retrieval of the wind vector from the NRCS of the ocean surface makes the system independent of the sensors’ motion and installation height as well as the effects due to platform-induced turbulence

Ocean wind and wave parameter estimation from ship-borne x-band marine radar data

Ocean wind and wave parameters are important for the study of oceanography, on- and off-shore activities, and the safety of ship navigation. Conventionally, such parameters have been measured by in-situ sensors such as anemometers and buoys. During the last three decades, sea surface observation using X-band marine radar has drawn wide attention since marine radars can image both temporal and spatial variations of the sea surface. In this thesis, novel algorithms for wind and wave parameter retrieval from X-band marine radar data are developed and tested using radar, anemometer, and buoy data collected in a sea trial off the east coast of Canada in the North Atlantic Ocean. Rain affects radar backscatter and leads to less reliable wind parameters measurements. In this thesis, algorithms are developed to enable reliable wind parameters measurements under rain conditions. Firstly, wind directions are extracted from raincontaminated radar data using either a 1D or 2D ensemble empirical mode decomposition (EEMD) technique and are seen to compare favourably with an anemometer reference. Secondly, an algorithm based on EEMD and amplitude modulation (AM) analysis to retrieve wind direction and speed from both rain-free and rain-contaminated X-band marine radar images is developed and is shown to be an improvement over an earlier 1D spectral analysis-based method. For wave parameter measurements, an empirical modulation transfer function (MTF) is required for traditional spectral analysis-based techniques. Moreover, the widely used signal-to-noise ratio (SNR)-based method for significant wave height (HS) estimation may not always work well for a ship-borne X-band radar, and it requires external sensors for calibration. In this thesis, two methods are first presented for HS estimation from X-band marine radar data. One is an EEMD-based method, which enables satisfactory HS measurements obtained from a ship-borne radar. The other is a modified shadowingbased method, which enables HS measurements without the inclusion of external sensors. Furthermore, neither method requires the MTF. Finally, an algorithm based on the Radon transform is proposed to estimate wave direction and periods from X-band marine radar images with satisfactory results

Analysis of Sea Surface Features by Using X-Band Radar Data Sets

En este trabajo se recoge el estudio de algunos de los fenómenos que ocurren en el océano debido al oleaje mediante técnicas de teledetección en el rango de las microondas. Estos fenómenos están relacionados con los diferentes mecanismos de formación de la imagen radar en banda X y en condiciones de incidencia tangente. Dichos mecanismos permiten detectar fenómenos en dichas imágenes radar (conocidas como “clutter” marino para propósitos de navegación), como son la relación de dispersión del oleaje, sus armónicos superiores y la contribución espectral conocida en la literatura científica como “group line”. Para el estudio de estos fenómenos se emplean los espectros de las imágenes proporcionadas por diferentes estaciones que utilizan tecnología basadas en radar de navegación en banda X. Los sistemas radar proporcionan una secuencia de imágenes en el dominio del tiempo que, gracias a la descomposición tridimensional de Fourier, permite obtener dichos espectros correspondientes de la secuencia de imágenes radar para su posterior análisis. Así, el espectro de la secuencia de imágenes de radar marino proporciona información sobre la distribución de la energía del oleaje, haciendo visible todos los fenómenos relacionados con el oleaje, el viento local, etc. El estudio del “clutter”, o del ruido de fondo del espectro, también es importante ya que permite la estimación de la altura significativa de las olas. En este trabajo se recoge un estudio detallado de la detección del “group line” y de la relación de dispersión del oleaje en función de la dirección de los diferentes ángulos de azimut que barren la imagen del radar, así como para diferentes alcances a partir de la ubicación del radar, además, de un estudio de la relación señal ruido considerando los fenómenos anteriores, así como de la máscara de iluminación de la superficie del mar, debida al efecto de ensombrecimiento de la antena radar, que también contiene las principales contribuciones del espectro de la imagen. A partir del análisis de las diferentes contribuciones del espectro de la imagen radar, y utilizando diversas técnicas de inteligencia artificial, se desarrollan algoritmos que mejoran la estima de parámetros oceanográficos, como la altura significativa del oleaje y las corrientes superficiales

Satellite observations of mesoscale features in lower Cook Inlet and Shelikof Strait, Gulf of Alaska

The Seasat satellite launched in Summer 1978 carried a synthetic aperture radar (SAR). Although Seasat failed after 105 days in orbit, it provided observations that demonstrate the potential to examine and monitor upper oceanic processes. Seasat made five passes over lower Cook Inlet and Shelikof Strait, Alaska, during Summer 1978. SAR images from the passes show oceanographic features, including a meander in a front, a pair of mesoscale eddies, and internal waves. These features are compared with contemporary and representative images from a satellite-borne Advanced Very High Resolution Radiometer (AVHRR) and Coastal Zone Color Scanner (CZCS), with water property data, and with current observations from moored instruments. The results indicate that SAR data can be used to monitor mesoscale oceanographic features

Study on Real-Time Ocean Wave Analysis Based on X-Band Radar Measurement Data

학위논문(석사) -- 서울대학교대학원 : 공과대학 조선해양공학과, 2023. 2. 김용환.해양 활동의 안전성 및 효율을 향상하기 위해 신뢰도 높은 파랑 정보의 획득이 요구됨에 따라 전 세계적으로 다양한 방식의 파랑 계측이 수행되고 있다. 이 중 해양 X-band 레이더는 넓은 영역의 파랑 정보를 동시에 계측할 수 있으므로 단시간의 계측을 통해 통계적으로 수렴도 높은 해양파 정보를 얻을 수 있다는 장점이 있다. 때문에, 다양한 선박 및 해양 구조물에서 해양 X-band 레이더를 활용하여 파랑 계측을 수행하고 있으며, 계측 기법의 고도화에 대한 다양한 논의가 진행되고 있다. 해양 레이더는 안테나에서 송신된 X-band 마이크로파와 해면상 잔물결 간의 Bragg 공진 현상에 의해 후방 산란되는 전자기파의 세기를 계측한다. 이러한 원격 계측 과정은 그림자, 기울임, 유체동역학적 효과 등 수많은 비물리적 변조 효과를 수반한다. 따라서, 레이더 이미지로부터 파랑 정보를 도출하기 위해서는 이미지 강도에 포함된 비물리적 성분을 제거하고 스펙트럼의 에너지를 유의파고에 따라 조정하는 파랑장 재구성 과정이 요구된다. 본 논문은 고도화된 파랑장 재구성 기법에 대한 연구를 다루고 있다. 제시된 전체 재구성 절차는 그림자 기반 유의파고 추정과 3D-FFT 기반 파랑장 재구성으로 구성되며, 각 해석 과정이 높은 연산 효율을 지니고 있다. 또한, 본 연구에서는 파랑장 재구성 기법의 정확도 향상을 위해 그림자 발생의 공간 통계적 특성을 엄밀하게 고려하였다. 이를 위해, 유의파고 추정 시 해면의 공간상 자기상관함수 및 평균표면경사의 직교성을 고려하였고, 파랑장 재구성 시 그림자 발생의 공간적 특성에 기인하는 불균일한 분산 분포에 대한 보정을 수행하였다. 본 논문에서 제시된 기법의 검증을 위해 합성 레이더 이미지와 실해역 레이더 이미지에 대해 파랑장 재구성을 수행하였다. 먼저, 다양한 해상 상태의 합성 레이더 이미지를 생성하여 해석에 활용하였고, 해상 상태에 따른 재구성 정확도의 의존성을 살펴보았다. 이를 통해, 다양한 해상 상태에서 그림자 효과에 대한 엄밀한 고려를 통해 파랑장 재구성 정확도를 향상할 수 있음을 확인하였다. 다음으로, 이어도 해양과학기지 및 기상 1호에서 계측된 실해역 레이더 이미지에 대한 유의파고 추정을 수행하였다. 그 결과, 실해역 데이터에 대하여 정확도 높은 유의파고 추정이 가능함을 확인하였다.It is required to obtain reliable wave information to improve the safety and efficiency of marine activities. Various methods for wave measurements are being carried out around the world. Among them, the marine X-band radar has the advantage that it can obtain statistically converged wave information based on short-time measurement. This is because the wave radar can simultaneously measure wave elevation data in a large area. Accordingly, marine X-band radars are installed on various ships and marine platforms to perform wave measurements. Diverse discussions on X-band radar-based wave field analysis techniques are also steadily underway. In general, incoherent marine radar measures the backscattered intensity due to Bragg scattering between X-band microwaves transmitted from the antenna and ripples on the sea surface. This remote sensing process entails numerous non-physical modulation effects, such as shadowing, tilting, and hydrodynamic effects. Therefore, a series of post-processing called wave-field reconstruction is required to retrieve wave information from marine radar images. The wave-field reconstruction procedure consists of removing the non-physical components from the measured spectrum, and adjusting the total spectral energy according to the significant wave height (HS). In this study, the advanced wave-field reconstruction technique is presented. The overall reconstruction procedure is comprised of the shadowing-based HS estimation and 3D-FFT-based wave-field reconstruction, and both of each analysis process have high computational efficiency. Thats why it is suitable for real-time wave-field analysis. To enhance the wave analysis, the statistical characteristics of the shadowing effect were rigorously considered. For this purpose, the spatial autocorrelation function of the ocean surface and the orthogonality of the mean surface slope were considered for HS estimation. Moreover, the uneven variance distribution owing to the spatial dependency of the shadowing effect was mitigated during the wave-field reconstruction. Wave-field reconstruction was applied to the synthetic and real radar images to verify the presented technique. The HS estimation and 3D-FFT-based wave-field reconstruction were performed for synthetic radar images corresponding to various states, and the dependence of this technique on the sea state was examined. As a result, it was confirmed that the reconstruction accuracy could be improved through the rigorous consideration of stochastic characteristics of the shadowing effect for all cases. Moreover, HS estimation was performed for real radar images collected from the Ieodo ocean research station and RV Gisang 1. In conclusion, a satisfactorily accurate HS estimation was also achieved.1. 서론 1 1.1 연구 배경 1 1.2 기존 연구 3 1.2.1 3D-FFT 기반 파랑장 재구성 3 1.2.2 유의파고 추정 5 1.3 연구 목표 및 주요 연구 내용 7 2. 파랑장 재구성 9 2.1 위상 분해 파랑장 재구성 문제 9 2.1.1 문제 정의 9 2.1.2 전체 해석 절차 11 2.2 유의파고 추정 13 2.2.1 그림자 영역 구분 13 2.2.2 Smith 함수 기반 표면 경사 추정 14 2.2.3 총표면경사 추정 18 2.2.4 유의파고 계산 19 2.3 3D-FFT 기반 파랑장 재구성 21 2.3.1 Mean-shift 변형 21 2.3.2 에너지 분포 보정 21 2.3.3 3차원 고속 푸리에 변환(3D-FFT) 23 2.3.4 필터링 23 2.3.5 변조 전달 함수(MTF) 24 3. 합성 레이더 이미지 해석 26 3.1 합성 레이더 이미지 생성 26 3.2 유의파고 추정 30 3.2.1 평균표면경사 추정 30 3.2.2 스펙트럼 해석 34 3.2.3 유의파고 추정 결과 36 3.3 파랑장 재구성 38 3.3.1 에너지 분포 보정 38 3.3.2 재구성 결과 40 4. 실해역 레이더 이미지 해석 45 4.1 데이터셋 정의 45 4.1.1 이어도 데이터셋 45 4.1.2 NIMS 데이터셋 46 4.2 유의파고 추정 결과 48 4.2.1 이어도 데이터셋 해석 결과 48 4.2.2 NIMS 데이터셋 해석 결과 53 5. 결론 57 6. 공학적 기여 및 향후 연구 59 참고문헌 61 부록 66 A.1 Smith 함수 유도 66 A.2 해면 평균표면경사의 물리적 특성 71석

Occurrence and Energy Dissipation of Breaking Surface Waves in the Nearshore Studied with Coherent Marine Radar

Wave breaking influences air-sea interactions, wave induced forces on coastal structures, sediment transport and associated coastline changes. A good understanding of the process and a proper incorporation of wave breaking into earth system models is crucial for a solid assessment of the impacts of climate change and human influences on coastal dynamics. However, many aspects are still poorly understood which can be attributed to the fact that wave breaking is difficult to observe and study because it occurs randomly and involves multiple spatial and temporal scales. Within this doctoral work, a nearshore field experiment was planned and conducted on the island of Sylt in the North Sea to investigate the dynamics of wave breaking. The study combines in-situ observations, numerical simulations and remote sensing using shore-based coherent marine radar. The field measurements are used to investigate the coherent microwave backscatter from shoaling and breaking waves. Three major developments result from the study. The first one is a forward model to compute the backscatter intensity and Doppler velocity from known wave kinematics. The second development is a new classification algorithm to identify dominant breakers, whitecaps and radar imaging artifacts within the radar raw data. The algorithm is used to infer the fraction of breaking waves over a sub- and an inter-tidal sandbar as well as whitecap statistics and results are compared to different parameterizations available in literature. The third development is a new method to deduce the energy of the surface roller from the Doppler velocity measured by the radar. The roller energy is related to the dissipation of roller energy by the stress acting at the surface under the roller. From the spatial gradient of roller energy, the transformation of the significant wave height is computed along the entire cross-shore transect. Comparisons to in-situ measurements of the significant wave height from two bottom mounted pressure gauges and a wave rider buoy show a total root-mean-square-error of 0.20 m and a bias of −0.02 m. It is the first time that measurements of the spatio-temporal variation of the bulk wave energy dissipation together with the fraction of breaking waves are achieved in storm conditions over such a large distance of more than one kilometer. The largest dissipation rates (> 300 W/m² ) take place on a short distance of less than one wave length (≈ 50 m) at the inter-tidal sandbar. However, during storm conditions 50 % of the incoming wave energy flux is already dissipated at the sub-tidal sandbar. The simultaneous measurements of the occurrence frequency and the energy dissipation facilitate an assessment of the bulk dissipation of individual breaking waves. For the spilling-type breakers in this area, the observed dissipation rate is about 30 % smaller than the dissipation rate according to the generally used bore analogy. This must be considered within nearshore wave models if accurate predictions of the breaking probability are required

The Effect of Radar Ocean Surface Sampling on Wave Spectrum Estimation Using X-Band Marine Radar

In this paper, the effect of the ocean surface sampling process on the ocean wave spectral estimation using the Cartesian Fourier transform (CFT) method on X-band marine radar data is investigated. Our analysis shows that the ocean surface sampling process involves a spatial averaging process that might be described as a 2-D low pass filter. Furthermore, a filter referred to as the inverse sampling averaging filter (ISAF) is proposed to be integrated with the CFT method in order to mitigate the effect of the sampling process. For validation, the CFT-with-ISAF method as well as the CFT-without-ISAF method were used to estimate ocean wave spectra and sea state parameters from X-band marine radar field data. The estimates from both methods were compared to ground truth estimates generated using TRIAXYS wave buoy data. The results show that the ISAF improves the CFT method in estimating ocean wave spectra. The recorded accuracy improvements in estimating the non-directional wave spectrum, the peak wave period, the mean wave period, the zero-crossing wave period, and the peak wave direction were 11%, 12%, 21%, 17%, and 34%, respectively. The performances of significant wave height estimation using the ISAF method and the standard CFT method were validated against ground truth estimates and found to be comparable

A temporal waterline approach to mapping intertidal areas using X-band marine radar

Mapping the morphology of intertidal areas is a logistically challenging, time consuming and expensive task due to their large expanse and difficulties associated with access. A technique is presented here that uses standard marine navigational radar operating at X-band frequency. The method uses a series of time-exposure radar images over the course of a two-week tidal cycle to identify the elevation of the wetting and drying transitions at each pixel in the radar images, thereby building up a morphological map of the target intertidal area. This “Temporal Waterline” method is applied to a dataset acquired from Hilbre Island at the mouth of the Dee Estuary, UK, spanning March 2006 to January 2007. The radar gathered data with a radial range of 4 km and the resulting elevation maps describe the intertidal regions of that area. The results are compared with airborne LiDAR data surveyed over the same area and within the radar survey time period. The residual differences show good agreement across large areas of beach and sandbanks, with concentrations of poor estimations around points that are shadowed from the radar or likely to suffer from pooling water. This paper presents the theoretical framework of the method and demonstrates its stability and accuracy. The Temporal Waterline radar method is aimed at providing a useful tool for the monitoring and operational management of coastlines