Images Of Sea Surface Research Articles

Estimation of Significant Wave Height From X-Band Marine Radar Images Based on Ensemble Empirical Mode Decomposition

In this letter, an ensemble empirical mode decomposition (EEMD)-based method is proposed to estimate significant wave height (SWH) from the X-band marine radar sea surface images. First, the data sequence in each radial direction of a radar subimage is decomposed by the EEMD into several intrinsic mode functions (IMFs). A normalization scheme is then applied to the IMFs to obtain their amplitude modulation components. Finally, by adopting a linear model, the SWH is estimated from the sum of the amplitudes from the second to the fifth modes. The method is tested using radar and buoy data collected in a sea trial off the east coast of Canada. The root-mean-square differences with respect to the buoy reference for the SWH estimations using the traditional signal-to-noise-based method, a recent shadowing-based method, and the proposed technique are 0.78, 0.48, and 0.36 m, respectively. The result indicates that the proposed technique produces improvement in the SWH measurements.

Facet-Based Investigation on Microwave Backscattering From Sea Surface With Breaking Waves: Sea Spikes and SAR Imaging

A complete facet model for the backscattering from rough sea surface with breaking waves is proposed in this paper. In consideration of the spatial distribution of breaking waves on sea surface, the model is able to give a good interpretation to the “super events” under high sea states, such as sea spikes and high polarization ratios. Based on the proposed model, normalized radar cross section plots of sea surface under backscattering configuration are calculated and compared with measured data. The comparisons show that the proposed model is tractable to estimate the scattering from electrically large ocean surface under high sea states with accuracy and efficiency. In addition, the non-Gaussian statistics and spatial correlation properties of sea clutter are analyzed under different range resolutions and incident angles. The results show that high kurtosis value, which means a sea spike phenomenon, mostly happens in lower grazing angle and higher range resolution cases. The comparisons of simulated SAR images of sea surface with and without breaking waves also reveal that sea spikes and high polarization ratios are caused by breaking waves. All the simulation results prove that our model not only is able to explain the physical mechanism of the scattering but also can be applied to the analyses of statistical properties of sea clutter under high sea states.

Wind Direction Estimation From Rain-Contaminated Marine Radar Data Using the Ensemble Empirical Mode Decomposition Method

Two ensemble empirical mode decomposition (EEMD)-based methods are presented to retrieve wind direction from rain-contaminated X-band nautical radar sea surface images. Each radar image is first decomposed into disparate intrinsic mode function (IMF) components using 1-D EEMD or 2-D EEMD. Then, the standard deviation of one IMF component or the combination of several IMF components as a function of azimuth is least-squares fitted to a harmonic function to determine the wind direction. Tests of the proposed algorithms are conducted by employing radar and anemometer data collected in a sea trial during rain events off the east coast of Canada. The results show that compared with the 1-D discrete-Fourier-transform-based method, both the 1-D- and 2-D-EEMD-based algorithms improve the wind direction results in rain events, showing a reduction of 7.4° and 8.7°, respectively, in the root-mean-square difference with respect to the reference.

Electromagnetic scattering characteristics analysis of freak waves and characteristics identification

Based on the Longuet-Higgins wave model theory, a modified phase modulation method of simulating freak waves is improved in this paper. The method can generate freak waves at assigned time and place, and their waveforms can not only maintain the frequency spectrum structure of the target spectrum and also satisfy the wave series statistics to a great extent. Then, the electromagnetic backscattering model of freak and background wave is established by the finite difference time domain method and the two-scale method. After averaging relative deviation and analyzing the error of the root mean square deviation within the measurement uncertainties, considering the computational efficiency, we use the two-scale model method to calculate the electromagnetic scattering coefficient of freak wave. Numerical results show that the normalized radar cross section (NRCS) of freak wave is much smaller than that of background wave. On the other hand, we analyze the electromagnetic scattering properties of freak waves under the different polarization modes, incident angles and incident frequencies. We find that in the condition of grazing incidence, the backscatter coefficient of freak wave increases with the increase of the incident frequency, but the increase amplitude is reduced, which meets the rough surface scattering theory. When the incident frequency is fixed and the incident〉is small, the backscatter coefficient calculation results of freak wave are similar under the condition of different polarizations VV's and HH's, but the backscatter coefficient of freak wave decreases obviously with the increase of incident angle, which is caused by the radar electromagnetic wave that is parallel to the sea surface and contacts it gradually. In addition, we find that the backscatter coefficient calculation result of freak waves under the VV polarization is much higher than under HH polarization from the two groups of experimental figures. According to the result of datum analysis, a conclusion is drawn that we can determine where the freak wave is when the NRCS difference of synthetic aperture radar (SAR) image is smaller than -11.8 dB. In the practical engineering application, the characteristic parameters are difficult to observe, while the difference in electromagnetic scattering coefficient between freak wave and background wave can be calculated from the SAR image of sea surface. This conclusion provides a reference standard for predicting the freak waves in engineering application, through which we can calculate the characteristic parameters of freak wave, determine its position, and study the electromagnetic scattering characteristics under the different polarization modes, incident angles and incident frequencies in future researches.

Methods for retrieval of sea wave spectra from aerospace image spectra

An approach to the retrieval of sea wave spatial spectra based on satellite optical imagery in linear and nonlinear approximations is described. Physical mechanisms of the formation of disturbed sea surface brightness fields recorded by remote sensing equipment are analyzed. Wave spectra retrieval methods using brightness field formation models that consider linear and nonlinear dependencies on sea surface slopes are suggested. A method for the construction of operators that retrieve the spatial spectra of surface wave slopes and elevations from aerospace imagery and take into account nonlinear modulations of disturbed sea surface brightness fields is developed. This method is based on the numerical simulation of sea surface images and the construction of a retrieving operator with respect to a set of parameters determined by aerospace imaging conditions. Examples of the use of the developed methods are given.

Sea surface imaging simulation and sea wave retrieval method \\based on MIMO radar

Multiple-input-multiple-output (MIMO) radar is an emerging radar system that has significant potential. MIMO radar can provide a higher resolution real-time imaging solution than traditional phased-array radar schemes and is well suited for the imaging of complex sea surfaces. MIMO radar imaging of sea scenes is still in the exploratory stage and several key issues need to be resolved and improved further. With the objective of achieving sea surface monitoring using MIMO radar, some basic research work on MIMO radar imaging simulation of sea surfaces, and MIMO radar sea current and wave information retrieval were conducted in this study. In order to analyze the feasibility and advantages inherent in MIMO radar imaging simulation of sea surfaces, ocean surface imaging based on MIMO radar was carried out via computer simulation. The curvature-modified two-scale method and shadowing modulation were used to build a back scatter section distribution model of a sea surface, which is taken as the object of MIMO radar imaging. In terms of sea current and wave information retrieval from MIMO radar images, the weighted least square method was used to fully explore the advantages of MIMO radar surface imaging. While retrieving the wave spectrum and wave parameters, the modulation transfer function of MIMO radar was determined via imaging simulation experiments. The retrieved significant wave height, main wave direction, and cross-zero cycle from MIMO radar images were found to be more precise than those from navigation radar images. The simulation results obtained also showed that MIMO radar sea surface imaging and parameter retrieval are superior to and more effective than marine radar.

Comparison of Wave Height Measurement Algorithms for Ship-Borne X-Band Nautical Radar

. In this article, a comparison of signal-to-noise ratio (SNR) and shadowing-based algorithms for ocean wave height estimation from ship-borne X-band nautical radar sea surface images is presented. Modifications, including selecting a subarea along the upwind direction and smoothing the edge pixel intensity histogram, are made to the original shadowing algorithm to achieve more accurate wave height measurements. Tests of the algorithms are conducted using radar and buoy data acquired in a sea trial in the North Atlantic Ocean off the east coast of Canada. Compared with the original shadowing algorithm, the modified algorithm improves the wave height estimation with a decrease of about 1.70 m in the root mean square (RMS) difference with respect to in situ measurements. In addition, the result of the comparison shows that the modified shadowing algorithm produces more accurate wave heights than the SNR method with improvements of about 0.05 in the correlation coefficient and about 0.04 m in the RMS difference.

Elimination of the impact of vessels on ocean wave height inversion with X-band wave monitoring radar

Directional wave spectra and integrated wave parameters can be derived from X-band radar sea surface images. A vessel on the sea surface has a significant influence on wave parameter inversions that can be seen as intensive backscatter speckles in X-band wave monitoring radar sea surface images. A novel algorithm to eliminate the interference of vessels in ocean wave height inversions from X-band wave monitoring radar is proposed. This algorithm is based on the characteristics of the interference. The principal components (PCs) of a sea surface image sequence are extracted using empirical orthogonal function (EOF) analysis. The standard deviation of the PCs is then used to identify vessel interference within the image sequence. To mitigate the interference, a suppression method based on a frequency domain geometric model is applied. The algorithm framework has been applied to OSMAR-X, a wave monitoring system developed by Wuhan University, based on nautical X-band radar. Several sea surface images captured on vessels by OSMAR-X are processed using the method proposed in this paper. Inversion schemes are validated by comparisons with data from in situ wave buoys. The root-mean-square error between the significant wave heights (SWH) retrieved from original interference radar images and those measured by the buoy is reduced by 0.25 m. The determinations of surface gravity wave parameters, in particular SWH, confirm the applicability of the proposed method.

Numerical Simulation of Synthetic Aperture Radar Images for Ocean Oil Spill Pollution

ABSTRACTIn order to study the scattering mechanism of oil spill pollution and the impacts on synthetic aperture radar images, both the computational modeling and its inverse problem are explored. The forward–backward iteration method and some optimization techniques are proposed to simulate the electromagnetic scattering from extra-large-scale sea surfaces. The local iteration methodology is first proposed to reduce the computational complexity to the order of rather than in terms of the nearly planar structure of the sea surface; therefore, the proposed method can effectively collect the massive backscattered data from the oily, polluted sea surface. Finally, based on the simulated data, the dual-polarization synthetic aperture radar images of the oil spill sea surface are studied for various conditions of sea states, radar polarizations, frequencies, and incidence angles.

Ocean Surface Observations Using the TanDEM-X Satellite Formation

The TanDEM-X SAR satellite formation permits improved ocean surface observations by means of bistatic along-track interferometry (ATI) when compared to single-satellite systems. The flexible imaging geometry of its two cooperating SAR sensors forms an interferometer that can achieve very high sensitivity to motions of objects on ground. This way, radar imaging of surface currents with unprecedented accuracy, high spatial resolution and wide coverage at the same time becomes possible. We demonstrate the capabilities of the sensor in the contexts of tidal current mapping, measurement of thermohaline and wind-driven ocean currents as well as detection of areas with surface films. We have developed a dedicated postprocessing system for TanDEM-X image products that allows extracting surface current information from the data. By this paper, we address bistatic data acquisition and processing aspects for sea surface imaging with TanDEM-X like interferometric baseline geometry, temporal decorrelation, and phase calibration. We present a variety of examples of data evaluation that clearly demonstrate the application potential of the methodology.

An Automatic Self-Calibration Approach for Wide Baseline Stereo Cameras Using Sea Surface Images

Stereo rig with wide baseline is necessary when accurate depth estimation for distant object is desired. However, in order to make calibration pattern to be viewed from both left and right cameras, the wider the baseline the bigger the calibration pattern is required. In contrast to the traditional stereo calibration method using calibration pattern, we propose a self-calibration approach that can estimate cameras' rotation matrices for stereo rig with wide baseline (3 m). Given images taken from left and right cameras, the relative roll and pitch angles between two cameras are recovered by aligning sea horizon in left and right images. The pitch angle is estimated by making the projections of one point at infinite distance appear at the same location in both images. A photometric minimization is applied to refine the rotation parameters. Compared with conventional checkerboard-based calibration techniques which require extra equipments or personnel, our approach only needs a pair of sea images. Moreover, unlike most self-calibration approaches, feature detection and matching are not required which makes it possible to apply our approach on featureless images. As a result, it is flexible and easy to implement our approach on sea surface images. Real world experiments demonstrate the feasibility of our approach.

Modern approaches to shipborne ocean color remote sensing

In this paper, modernized shipborne procedures are presented to collect and process above-water radiometry for remote sensing applications. A setup of five radiometers and a bidirectional camera system, which provides panoramic sea surface and sky images, is proposed for the collection of high-resolution radiometric quantities. Images from the camera system can be used to determine sky state and potential glint, whitecaps, or foam contamination. A peak in the observed remote sensing reflectance RRS spectra between 750–780 nm was typically found in spectra with relatively high surface reflected glint (SRG), which suggests this waveband could be a useful SRG indicator. Simplified steps for computing uncertainties in SRG corrected RRS are proposed and discussed. The potential of utilizing “unweighted multimodel averaging,” which is the average of four or more common SRG correction models, is examined to determine the best approximation RRS. This best approximation RRS provides an estimate of RRS based on various SRG correction models established using radiative transfer simulations and field investigations. Applying the average RRS provides a measure of the inherent uncertainties or biases that result from a user subjectively choosing any one SRG correction model. Comparisons between inherent and apparent optical property derived observations were used to assess the robustness of the SRG multimodel averaging approach. Correlations among the standard SRG models were completed to determine the degree of association or similarities between the SRG models. Results suggest that the choice of glint models strongly affects derived RRS values and can also influence the blue to green band ratios used for modeling biogeochemical parameters such as for chlorophyll a. The objective here is to present a uniform and traceable methodology for determining shipborne RRS measurements and its associated errors due to glint correction and to ensure the direct comparability of these measurements in future investigations. We encourage the ocean color community to publish radiometric field measurements with matching and complete metadata in open access repositories.

On Detectability of a Ship's Kelvin Wake in Simulated SAR Images of Rough Sea Surface

Synthetic aperture radar (SAR) images of ship wakes frequently exhibit long streaks. Some of the streaks are attributed to the Kelvin V-wave wake, and others are attributed to the turbulent and bright narrow V-wake. Ocean waves reduce the detectability of Kelvin wakes in SAR images. The SAR imaging of Kelvin wakes of two ships with different dimensions moving in rough sea is studied theoretically. The Kelvin wakes are simulated using the Michell theory. The random sea surface is simulated using the Pierson-Moskowitz type wave spectrum. The elevations of the points of the water surface and their velocities and accelerations are considered as a superposition of a ship's coherent waves and wind waves. A two-scale composite model and the velocity-bunching integral are used to simulate the intensity of SAR images of a disturbed sea surface with an embedded Kelvin wake. The fast discrete Radon transform is applied to detect the boundaries of the simulated Kelvin wakes. The probabilities of missing detections (PMD) and false alarms (PFA) are estimated numerically. The influence of ships, wind, and SAR parameters on PMD and PFA is discussed. It is illustrated that PMD/PFA strongly depend on the sea state.

Algorithm for Detection of Current and Water-depth using X-band Marine Radar

The object of this study is to present an algorithm to estimate ambient current and water depth from sea clutter images obtained from X-band marine radar. In the present article, numerically synthetized images are utilized to verify the accuracy of the proposed algorithm. Synthetized sea surface images were analyzed for regular directional waves as well as directional random waves with current and without current. Estimation of currents from sea surface images for cases of known water depth showed good agreement with exact current velocity. However, we have some computational difficulty when prior water depth is unknown.

OIL RESIDUALS IN THE SEA: COMPARISON ITS OPTICAL FEATURES WITH OPTICAL PROPERTIES OF THE SEAWATER

Growing intensity of marine transport results in the increase of the amount of oil pollution released into the sea water. The presence of oil in the sea, which is thought to have come from accidental spills or tanker disaster, is connected mainly with the daily operation of marine engines. Pollution from ships engines, can reach the sea water masses together with ballast or bilge water as the result of human error or a failure of marine equipment. This paper is focused on a comparison of the optical properties of marine waters and oil substances. These include the spectra of absorption coefficient, spectra of scattering coefficient and scattering phase-function (for different wavelength) which describes an angular distribution of scattered light intensity in the sea water. The light scattering phase function depends on the water constituents like mineral suspensions, zooplankton and phytoplankton cells, gas bubbles or so-called yellow substances as well as oil droplets of water pollution. Detection of oil substances in the sea can be effective when it is done remotely, from ships, buoys, inshore or offshore towers, the decks of aircraft or satellites. The knowledge of the listed above properties of natural (clean) seawater is necessary, because they determine the field of light coming out from the sea and allow interpret images of sea surface from point of view of crude oil and the petroleum products detection. In this study we also discuss optical contrast of oil residuals in the sea in various sea regions.

Aperture Filter and Adaptive Filter Response for Reconstruction of Sea Surface Image Based on LIDAR Observation

Aperture Filter and Adaptive Filter Response for Reconstruction of Sea Surface Image Based on LIDAR Observation

Remote sensing technique for near-surface wind by optical images of rough water surface

The study of a new remote-sensing technique for the investigation of near-surface wind fields is an important oceanographic problem. This article is focused on a new method of recording wind fields by the analysis of optical images of sea surface and range–time–intensity images (RTI images) of the sea surface. An RTI image constructed from optical profiles of the sea surface is an optical analogue of a side-looking radar image of the sea surface but has a higher spatial resolution and some possibility for remote sensing of sea roughness. It is possible to form RTI images with a range from some tens of metres to tens of kilometres, depending on the spatial resolution needed. A set of original optical devices for recoding RTI images using linear arrays of CCD-photodiodes was created. An analytical model of sea surface radiance for visible light was developed taking into account the polarization of light and shadowing of surface waves for grazing view geometry. The principle of remote sensing of near-surface winds by its manifestations on a waved surface under grazing angles based on a comparison of measured and modelled surface radiance is discussed. Investigations of near-surface wind field features in internal reservoirs and various regions of the seas during the last few years have been conducted by optical systems. The structure of near-surface wind fields, eddies, wind fronts, and katabatic wind flows for ranges from hundreds of metres to some tens of kilometres was recorded and analysed. Derived data of optical monitoring of water surfaces may serve for future investigations of near-surface wind features.

Study of Wave Group Velocity Estimation From Inhomogeneous Sea-Surface Image Sequences by Spatiotemporal Continuous Wavelet Transform

In this paper, a new image processing technique is introduced for the analysis of consecutive ocean wave images by spatiotemporal continuous wavelet transform (STCWT) using the Morlet wavelet as the mother wavelet. This technique, which has been adapted to give a full time–frequency and spatiofrequency representation of ocean surface waves from remotely captured wave image sequences, provides more detailed information from wave-field measurements than the traditional Fourier transform (FT) method. A series of numerical simulations of wave image sequences was analyzed to justify the algorithm and to present quantitative theoretical results on the propagation of ocean waves for normal incidence, diffraction, reflection, and shoaling in coastal areas. The comparisons of these estimates to simulated conditions for several wave parameters show that the wavelet theory is applicable to the identification of wave spectra from inhomogeneous sea-surface image sequences.

An Algorithm for Estimation of Wave Height From Shadowing in X-Band Radar Sea Surface Images

Directional wave spectra and integrated wave parameters can be derived from X-band radar sea surface images. The calibration of such spectra has until now depended on various empirical and semiempirical methods, which also require an external reference increasing the total expenses of operation. A novel algorithm, which is fully based on theory, producing reasonable estimates of the significant wave height based on shadowing in the images, has been now developed. The method does not require any reference measurements, and with that, enables automatic calibration of spectra from radar sea surface images, as well as the potential for reduced operational cost and increased accuracy of such wave-monitoring systems. Promising results are obtained with data acquired at a test site in the North Sea. Further work is, however, required to refine the algorithm and to test it under varying circumstances at various sites.

Oceanic wave breaking coverage separation techniques for active and maturing whitecaps

Whitecaps on the ocean surface mark localized areas where interactions between the atmosphere and ocean are enhanced. Contemporary methods of quantifying total whitecap coverage rely on converting color sea surface images into their binary equivalent using specific threshold-based automated algorithms. However, there are very few studies that have separated and quantified whitecap coverage into its active (stage-A) and maturing (stage-B) evolutionary stages, which can potentially provide more suitable parameters for use in breaking wave models, air–sea gas transfer, aerosol production, and oceanic albedo studies. Previous active and maturing whitecap studies have used a pixel intensity separation technique, which involves first separating the whitecap and background pixels, and subsequently establishing a second threshold to distinguish between active and maturing whitecaps. In this study, a dataset of more than 64,000 images from the North Atlantic were initially processed to determine the total whitecap coverage using the Automated Whitecap Extraction method. The whitecap pixels of each image were then distinguished as either stage-A or stage-B whitecaps by applying a spatial separation technique which does not rely solely on pixel intensity information but also on the location (relative to the wave crest), visual intensity, texture and shape of each whitecap. The comparison between the spatial separation and pixel intensity separation techniques yielded average relative errors of 34.8% and −44.0% for stage-A and -B coverage, respectively. The pixel intensity method was found to be less suitable when compared to the spatial separation method as it relies on the assumption that the pixel intensity for stage-A is always greater than that for stage-B.