Radar Imaging Of Ocean Waves Research Articles

On the cross spectrum between individual-look synthetic aperture radar images of ocean waves

Cross spectra of individual-look synthetic aperture radar (SAR) images of the ocean surface are used to retrieve ocean wave spectra. A quasilinear transform is derived that relates ocean wave spectra to SAR image cross spectra. Furthermore, Monte Carlo simulations are also carried out for those cases where quasilinear imaging does not apply. It is shown that, as the time separation between the individual-look SAR images increases (within a limit determined by the Doppler bandwidth of the original single-look complex SAR image), the spectral energy density of the imaginary part of the SAR image cross spectra increases, while the spectral energy density of the real part decreases. The integration time has a small effect on the SAR image cross spectra as long as the integration time is large compared to the scene coherence time. In order to retrieve ocean wave spectra from SAR data by using cross-spectral analysis techniques, the authors suggest calculating two SAR image cross spectra: one with a short time separation and one with a large one between the individual-look SAR images. The real part of the SAR image cross spectra calculated from individual-look SAR images with the short time separation is used for retrieving ocean wave spectra, which have a 180/spl deg/ ambiguity in wave propagation direction. The imaginary part of the SAR image cross spectra calculated from individual-look SAR images with the long time separation is used for removing this 180/spl deg/ ambiguity.

Real aperture radar imaging of ocean waves during SAXON‐FPN: A case of azimuth‐traveling waves

This paper investigates the radar imaging of ocean waves under two separate wind speed and sea state conditions using an X band real aperture radar (RAR). It is shown experimentally that for the case of higher sea state and wind speed (9.1 m/s), the RAR is capable of imaging the waves from all directions including the azimuth. The case for low sea state and wind speed (4.5 m/s), however, shows somewhat stronger imaging contrast in the range direction with reduced contrast for the azimuth‐traveling waves. The results of the aircraft observations are compared with numerical simulations of a RAR imaging model. Simulations show that cross tilt is a viable mechanism for enhancing the azimuthal modulations, but the results are sensitive to the level of upwind/cross‐wind anisotropy of the shortwave spectrum. Simulations also show that the variations in incidence angle create irregularities in the azimuthal angle distribution.

Resolution of a controversy surrounding the focusing mechanisms of synthetic aperture radar images of ocean waves

This paper addresses key problems regarding the focusing of synthetic aperture radar (SAR) images of ocean surface waves, explaining why applying a processor defocus will generally yield an enhanced image, why the same defocus applies to both image modulations brought about by the radar cross section and by the velocity bunching process, and why the effects apply to both single-look and multilook systems independently of look relocation. Two interpretations are given for the case when surface scatterers are stationary, but modulated in reflectivity (radar cross section) by a propagating wavefield. The first interpretation is what will be called a degrade-and-shift model. In it, a processor focusing adjustment degrades a point image. However, the overall image can be enhanced because an appropriate defocus results in a shifting of points in such a way that the image can most closely resemble the image of the time-invariant (or frozen) reflectivity. The second interpretation is a defocus-and-refocus model in which the image of a time-varying reflectivity is defocused and may be refocused to enhance the image. In justifying this defocus-and-refocus model, it is shown that the radar return from stationary scatterers of time-varying reflectivities is identical to that from physically moving scatterers of constant reflectivity. Thus, the two interpretations are not contradictory; they are, fundamentally, equivalent. The models support the use of a processor defocus corresponding to one half the wave phase velocity. Both qualitative and quantitative illustrations of the effects are given. Finally, it is shown that the same defocusing effect applies to image modulations brought about by the velocity bunching process. >

Comments on "On the focusing issue of synthetic aperture radar imaging of ocean waves" by C. Bruning et al

In the above-named work (see ibid., vol.29, p.120-8, 1991) C. Bruning et al. present results that show the application of the velocity bunching model to synthetic aperture radar (SAR) images obtained during the Tower Ocean Wave and Radar Dependence (TOWARD) experiment. The commenters argue that some of the observations presented require classification and discussion. An attempt is made in this comment to clarify the issues and to present some new relevant results obtained from the SAR X-band and Ocean Nonlinearities: Chesapeake Light Tower (SAXON:CLT) experiment.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the focusing issue of synthetic aperture radar imaging of ocean waves

It is widely accepted that the imaging of ocean surface waves by synthetic aperture radar (SAR) can be adequately described by velocity bunching theory in conjunction with the two-scale wave model. However, it has been conjectured that this theory is incapable of explaining why, under certain conditions, the image contrast of airborne SAR imagery of ocean waves can be enhanced by defocusing the SAR processor. In the present study the velocity bunching theory is defended. >

Aircraft‐and satellite‐borne synthetic aperture radar images of ocean waves in orbital motions

AbstractBecause of its all‐weather capability and high resolution, the synthetic aperture radar (SAR) is very effective for remote sensing of the ocean. However, for a moving target such as ocean waves, they affect the SAR images as a phase distortion. This paper considers the orbital motion of the ocean waves. By modeling the waves with an ensemble of the moving points, the SAR image is computed. Previously, the discussions were limited to the case of snapshot observation of the slowly moving ocean waves with a rather long wavelength by a high‐speed SAR such as that on a satellite for simplicity of the model. In this paper, this restriction is removed and the estimate and study of the SAR image become possible including a low‐speed SAR such as the one on an aircraft and waves with a short wavelength. As a result, it is pointed out that the phase velocity of the ocean waves cannot be neglected in a low‐speed SAR so that the wavelength of the SAR image is expanded or compressed and that the SAR image is strongly dependent on the ocean waves and the SAR parameters.

Wind-directional effects on the hydrodynamic modulation of microwave radar images of ocean waves

Abstract A numerical simulation has been constructed of the hydrodynamic modulation transfer function (HMTF) which defines the spatial and spectral properties of swell waves as observed by an imaging radar. While the simulation is based on the Alpers and Hasselmann (1978) two-scale modulation transfer model, it explicitly takes account of directional properties of the short-wave spectrum and does not necessarily assume an isotropic wind-wave spectrum. Several different spreading functions are modelled and it is demonstrated that an anisotropic wind wave field significantly distorts the HMTF. Depending on the wind direction relative to the radar azimuth, the effect can be to shift the direction and the wave number of the peak of the image spectrum relative to the true swell spectrum. The effect of fetch limitation on the wind-wave spectrum is also examined based on the JONSWAP spectrum, but the consequences of this for the HMTF imaging mechanism are found to be minimal.

Multifocus processing of L band synthetic aperture radar images of ocean waves obtained during the Tower Ocean Wave and Radar Dependence Experiment

As part of the Tower Ocean Wave and Radar Dependence Experiment (TOWARD) objectives, the mechanisms of SAR imaging of ocean waves are investigated using L band SAR data over the Naval Ocean Systems Center tower. This paper provides experimental evidence needed to validate the differing hypotheses. Various processing methods are investigated to generate spectra with large degrees of freedom. The results show that waves traveling in the aircraft direction are most detectable at focus settings in the range 10.0–15.0 m/s, which is consistent with the Marine Remote Sensing Experiment (MARSEN) observations reported by Jain and Shemdin (1983). Waves traveling in the direction opposite to the aircraft are most detectable at settings equal to −5.0 to −15.0 m/s. The SAR imaging system acts as a low‐pass filter with the peak of the ocean wave height spectrum occurring at higher wave numbers compared with the peak in the SAR image spectrum.

Synthetic aperture radar imaging of ocean waves from an airborne platform: Focus and tracking issues

There has been a controversial issue of many years standing in airborne Synthetic Aperture Radar (SAR) ocean imaging which this paper addresses and resolves. Investigators have been strongly divided on the reasons for apparent improvement in wave contrast in response to processor focus adjustment. The dispute has centered on two parameters of wave dynamics: orbital velocity and phase velocity. This paper shows that both orbital velocity and phase velocity are of fundamental importance in the SAR wave imaging problem. The first affects the phase of the received signal, leads to velocity bunching, and is scaled by the ratio of sensor altitude to sensor velocity. The second affects the magnitude of the received signal, leads to translation of wave features during image formation (observed as blurring in the image), and is scaled by the ratio of wave phase velocity to sensor velocity, thus becoming significant for airborne radars. This treatment of the phase velocity parameter is new. It is shown that focus adjustment, as a side effect, shifts image position. This explains why experiments have appeared to “prove” that focus adjustment may be optimised for wave movement. The paper shows that better performance is obtained by compensating individual looks for wave movement before look summation, while using nominal perfect focus just as for static scatterers. The work is applicable to a full ocean wave spectrum and does not depend on the details of the scattering mechanism itself.

A Three-Dimensional Formulation for Synthetic Aperture Radar Images of Ocean Waves in Orbital Motions

An analytic model of the ocean surface SAR images with a three-dimensional framework is developed following the formalism presented by Swift and Wilson (1979); a trochoidal swell propagates through a uniform field of Bragg-type distributed scatterers. Two- dimensional SAR images are calculated for the interpretation and prediction of actual SAR images of the ocean surface as a function of ocean wave amplitude, wave frequency, propagation direction, and radar frequency, off-nadir angle of the antenna, and spatial resolutions.

Removal of ambiguity of two‐dimensional power spectra obtained by processing ship radar images of ocean waves

The usual spatial power spectra of ocean wave images taken from PPI of a ship radar show an ambiguity in the wave propagation direction. A simple numerical procedure is suggested and realized to remove this ambiguity. It makes use of the data from two consecutive turns of the radar antenna as well as of the wave dispersion law. The results obtained show a good agreement with the ground truth.

“Layover” in satellite radar images of ocean waves

Present models explaining radar imaging of ocean surface waves have considered variations in radar cross section of the tilted or roughened surface and velocity bunching effects caused by the doppler shifts of moving scatterers. It is pointed out here that simple “layover” effects will cause an additional, significant modulation of the image brightness that increases the visibility of range‐traveling waves.

Synthetic aperture radar imaging of ocean waves during the marineland experiment

X - and L -band simultaneously obtained synthetic aperture radar (SAR) data of ocean gravity waves collected during the Marineland Experiment were analyzed using wave contrast measurements. The Marineland data collected in 1975 represents a unique historical data set for testing still-evolving theoretical models of the SAR ocean wave imaging process. The wave contrast measurements referred to are direct measurements of the backscatter variation between wave crests and troughs. These modulation depth measurements, which are indicators of wave detectability, were made as a function of: a) the settings used in processing the SAR signal histories to partially account for wave motion; b) wave propagation direction with respect to radar look direction for both X - and L -band SAR data; c) SAR resolution; and d) number of coherent looks. The contrast measurements indicated that ocean waves imaged by a SAR are most discernible when X -band frequency is used (as compared to L -band), and when the ocean waves are traveling in the range direction. Ocean waves can be detected by both X - and L -band SAR, provided that the radar surface resolution is small compared to the ocean wavelength (at least 1/4 of the ocean wavelength is indicated by this work). Finally, wave detection with L -band SAR can be improved by adjusting the focal distance and rotation of the cylindrical telescope in the SAR optical processor to account for wave motion. The latter adjustments are found to be proportional to a value that is near the wave phase velocity.

Synthetic aperture radar imaging of ocean waves: Comparison with wave measurements

Synthetic aperture radar images of ocean waves were obtained in conjunction with reference wave data near Marineland, Florida, December 14, 1975. Each of the various types of measurements were processed into a form that allowed direct comparisons with the others. Maxima of radar spectra occurred at the same frequencies as the maxima of reference wave height spectra. In a comparison of a radar spectrum with observed spectra of wave height, wave orbital velocity, and surface slope the high‐frequency portion of the radar spectrum lay near and between the wave height and the orbital velocity spectra but differed significantly from the surface slope spectrum. The radar‐derived mean directions and model‐fitted directional spreads of wave energy were close to the values from a directional wave buoy and indicated the accuracy of radar measurements of wave direction. However, a directional plot of a radar spectrum near shore at the frequency of the maximum showed a sharper peak than such a plot of a fitted spectrum derived from reference data. The high directional resolution of the radar, in addition to its making observations at different locations, allowed radar images to provide information about ocean waves not available from the other instruments. As a swell propagated across the continental shelf, it was scattered in direction, apparently by the irregularities of the bottom, and very little of its energy reached shore. The shorter sea waves had a narrow directional distribution when first observed offshore that may have been sharpened by interaction with the Gulf Stream. Radar images showed effects of bottom refraction on the sea waves as they moved into progressively shallower water.

Characteristics of synthetic-aperture radar imaging of ocean waves

Characteristics of synthetic-aperture radar imaging of ocean waves

Synthetic aperture radar imaging of moving ocean waves

A theory for the radar imaging of ocean waves is presented under the assumptions that a swell propagates through an ensemble of Bragg scatterers and that the integration time of the synthetic aperture radar (SAR) is small compared to the angular velocity of the swell. Results are prsented which show image development and distortions caused by the radial velocities and accelerations of the swell. Neglecting small wave bunching and tilts due to the longer underlying waves, and considering only one-dimensional geometries, the mechanism of wave motions are considered and their efforts on the production of the usual intensity Pattern representing the wave image are studied. The analysis shows that in certain situations a processed image can appear which has twice the spatial period of the actual long wave on the ocean, which can confuse the interpretation of ocean wave analysis.

Focusing effects in the synthetic aperture radar imaging of ocean waves

We derive the properties of the image obtained for an ocean wave whose cross-section may be given byσ w(x,y,y) and surface profile byh(x,y,t).σ w andh are functions representing the wave phenomena, but whose exact properties are determined by the ocean wave surface properties, for an ocean wavelength ofλ w, heightH, and orbital frequency ω. We calculate the effect of defocusing of the wave image due to its temporal motion, and derive both the resolution of the radar system if no focus compensation is provided in the processor and the necessary distance the azimuth telescope has to be moved to provide diffraction-limited imaging. We illustrate these results for data taken by the JPL synthetic aperture radar over Hurricane Gloria on September 30, 1976, and the ERIM radar over Marineland, Florida, on December 15, 1975.