Cover photograph Image from GOES-E spacecraft

Azimuthally travelling ocean waves are seldom well imaged by microwave real aperture radar (RAR) operating with conventional HH or VV polarizations. Attenuation of image intensity modulation in the azimuthal direction implies that ocean wave spectra derived from such images also will not be accurate. Real aperture radar cross-section modulation by long ocean waves is normally attributed to two principal sources, tilt modulation and hydrodynamic modulation. In ocean radar images both of these modulation sources are significantly attenuated in the azimuthal direction. Therefore, complete two-dimensional k-space wave spectra derived from microwave data often are quite different than physical ocean spectra. This paper uses fully-polarimetric radar measurements of ocean backscatter to identify a new source of backscatter modulation that is strongest in the azimuthal direction. This modulation source has potential for augmenting tilt and hydrodynamic modulation sources in the azimuthal direction where their weakness causes poor wave visibility. The predicted improvement in the measurement of ocean wave spectra using an optimized polarization is investigated by means of a RAR ocean imaging model. Fully-polarimetric, and conventional, radar spectrometers are proposed which are specifically optimized to sense wave-tilts in the azimuthal direction.     

Seasat detection of waves,currents and inlet discharge

Abstract

A new era of remote sensing for coastal and oceanographic monitoring was born on 26 June 1978 with the launch of Seasat. Duck-X was a 2 month experiment conducted during August to October 1978 off the east coast of the U.S.A. for the validation of the Seasat synthetic aperture radar (SAR), During this field experiment, various oceanographic phenomena were monitored. Ground truth observations of these phenomena have been correlated with Seasat SAR imagery. The ground truth sensors included airborne photographic and radar imagery, meteorological satellite imagery, land based radars, and conventional wave gauges. This paper focuses on ocean surface waves, ocean currents and coastal inlet discharge

Specifically, the direction and length of the principal ocean wave trains are compared for the periods of Seasat overflight of the Duck-X area. During these overflights significant wave heights were 1.5 m and less and the maximum wave period was 14 s. The current correlations concentrate on the western boundary of the Gulf Stream and its associated eddy structure. Inlet outflow is shown for inlets on the east coast of the U.S.A

This ground truth study has indicated that the SAR imagery contains an unanticipated abundance of information on a variety of oceanographic and coastal phenomena.     

Airborne microwave Doppler measurements of ocean wave directional spectra

Abstract

A technique is presented for measuring ocean wave directional spectra from aircraft using microwave Dopper radar. The technique involves backscattering coherent microwave radiation from a patch of sea surface which is small compared to dominant ocean wavelengths in the antenna look direction, and large compared to these lengths in the perpendicular (azimuthal) direction. The mean Doppler shift of the return signal measured over short time intervals is proportional to the mean sea surface velocity of the illuminated patch. Variable sea surface velocities induced by wave motion therefore produce time-varying Doppler shifts in the received signal. The large azimuthal dimension of the patch implies that these variations must be produced by surface waves travelling near the horizontal antenna look direction thus allowing determination of the direction of wave travel. Linear wave theory is used to convert the measured velocities into ocean wave spectral densities. Spectra measured simultaneously with this technique and two laser profilometers, and nearly simultaneously with a surface buoy, are presented. Applications and limitations of this airborne Doppler technique are discussed.     

Interferometric SAR imagery of a monochromatic ocean wave in the presence of the real aperture radar modulation

Abstract

The model which allows one to simulate the Synthetic Aperture Radar (SAR) and the along track Interferometric SAR (INSAR) imagery of a monochromatic ocean wave is presented. The model accounts for the effects of the real aperture radar (RAR) modulation of the radar backscatter cross-section. The comparison of the SAR and the INSAR imagery of the wave system under consideration is carried out and the performance of the two imaging systems is studied under various operational conditions. The results of the study indicate that the interferometric SAR is less sensitive to the RAR modulation as compared lo the regular SAR. This fact further augments the assertion that INSAR has considerable advantages over SAR in its potential of providing quantitative information about the ocean wave lengths, directions and amplitudes     

Ocean wave parameter measurement using a dual-radar system A simulation study

Abstract

The accurate measurement of ocean wave parameters using a single narrow-beam HF radar has been shown to be limited to particular wave field/radar beam configurations. In addition, because of symmetries in the scattering process, it is not possible to determine whether estimated wave directions are to the left or right of the radar beam. Measurement of the long-wave directional spectrum has also been limited by a requirement for at least 50 dB dynamic range in the Doppler spectrum of the backscattered radar signal. This paper describes the application of a model-fitting technique to a dual-radar system which overcomes the limitations found for a single system.     

Radar measurement of directional ocean wave spectra at low incidence angles†

Abstract

There exist several techniques for the measurement of directional ocean wave spectra. The most conventional technique is the employment of pitch-roll buoys but a disadvantage of this technique is that it measures at a fixed point. Another promising technique is the use of airborne or spaceborne radars. We present here results of a comparison between the data from an airborne radar, which is measuring near vertical incidence, and measurements of directional wave spectra obtained by means of a pitch-roll buoy and processed by using the Long-Hasselmann iterative algorithm. Although preliminary, these results constitute a step towards the employment of the airborne radar based on the Radar Ocean Wave Spectrometer (ROWS) principle as defined by Jackson (1981), as a validation tool for spaceborne synthetic aperture radars.     

Comparison of a new radar ocean imaging model with SARSEX internal wave image data

Abstract

Good agreement is demonstrated between both L-band and X-band SARSEX internal wave image data and the predictions of a new radar ocean imaging model that incorporates Bragg, specular, and composite scattering effects. It then follows that a two-step hydrodynamic modulation process, as hypothesized by Hughes and Gower, Thompson and Gasparovic, and Watson, does not appear lo be required to explain why the SARSEX L-band and X-band internal wave image modulations are comparable.     

High-resolution satellite measurements of coastal wind field and sea state

Methods to derive wind speed and sea state by simple empirical models from synthetic aperture radar (SAR) data are presented and applied for use in high-resolution numerical modelling for coastal applications. The new radar satellite, TerraSAR-X (TS-X), images the surface of the sea with a high resolution up to 1 m. Therefore, not only wind information and integrated sea state parameters but also individual ocean waves with wavelengths down to 30 m are detectable. Two-dimensional information on the ocean surface retrieved using TS-X data is validated for different oceanographic applications: derivation of finely resolved wind fields (XMOD algorithm) and integrated sea state parameters (XWAVE algorithm). Both algorithms are capable of taking into account fine-scale effects in coastal areas. Wind and sea state information retrieved from SAR data are applied as the input for a wave numerical spectral model (wind forcing and boundary condition) running at a fine spatial horizontal resolution of 100 m. Results are compared to collocated buoy measurements. Studies are carried out for varying wind speeds and comparisons against wave height, simulated using original TS-X-derived wind data, showing the sensitivity of waves to local wind variation and thus the importance of local wind effects on wave behaviour in coastal areas. Examples for the German Bight (North Sea) are shown. The TS-X satellite scenes render well-developed ocean wave patterns of developed swell at the sea surface. Refraction of individual long swell waves at a water depth shallower than about 70 m, caused by the influence of underwater topography in coastal areas, is imaged on the radar scenes. A technique is developed for tracking wave rays depending on changes in swell wavelength and direction. We estimate the wave energy flux along wave tracks from deep water to the coastline based on SAR information: wave height and wavelength are derived from TS-X data.     

Signatures of resonant and non-resonant scattering mechanisms on radar images of internal waves

The results of two polarization airborne radar imagery tests of the ocean surface obtained during the JUSREX'92 experiment are presented. It is shown that the traditional composite surface model with small-scale 'Bragg' waves superposed over larger gravity waves can not explain either the contrasts of internal wave surface manifestations in conditions of a stable atmospheric boundary layer at low grazing angles (LGA), or the apparent difference between the images obtained at different polarizations in unstable atmospheric conditions. We attribute this discrepancy to the presence of mesoscale steep waves, which produce non-resonant scattering and make different relative contributions to the total cross sections for the two polarizations. The possibility of distinguishing between surface manifestations of atmospheric and oceanic origin is also discussed.     

Predicting ocean surface currents using numerical weather prediction model and Kohonen neural network: a northern Adriatic study

The paper documents a concept of ocean forecasting system for ocean surface currents based on self-organizing map (SOM) trained by high-resolution numerical weather prediction (NWP) model and high-frequency (HF) radar data. Wind and surface currents data from the northern Adriatic coastal area were used in a 6-month long training phase to obtain SOM patterns. Very high correlation between current and joined current and wind SOM patterns indicated the strong relationship between winds and currents and allowed for creation of a prediction system. Increasing SOM dimensions did not increase reliability of the forecasting system, being limited by the amount of the data used for training and achieving the lowest errors for 4 × 4 SOM matrix. As the HF radars and high-resolution NWP models are strongly expanding in coastal oceans, providing reliable and long-term datasets, the applicability of the proposed SOM-based forecasting system is expected to be high.

     

Comparison of wave parameters determined from SILAR images and a pitch and roll buoy

Abstract

The nature of the data recorded by Side-Looking Airborne Radar (SLAR) and that by a pitch and roll buoy differ completely. The radar records a spatial data set at one instant in time, whereas the buoy records a time series on a given point on the sea surface, The spectrum in wave number space of the radar image is transformed to the frequency domain by using the dispersion relation for shallow water waves. With further processing, the radar derived data can be presented in the traditional form of pitch and roll buoy displays a (relative scale) waveheight spectrum, a directional distribution and the directional spreading, all as a function of frequency. Limitations of the method and differences in the results of the sensors are discussed.     

Non-linear features related to the dual-frequency technique in maritime applications

Abstract

Field measurements of non-linear characteristics, related to the dual-frequency technique applied to the ocean surface, were carried out in the North Sea. The data observations were made with a CW multi-frequency C-band radar onboard the offshore research platform ‘Forschungsplattform Nordsee’. It is verified that the cross-product spectrum contains additional information on distinct non-linear features, when compared to existing theories previously published in the literature. In particular, a relatively sharp second order Bragg resonance line is found on the right-hand side of the pronounced first order peak. This Doppler line occurs, within error bars, in the spectrum at √2 times the first order resonance frequency

To substantiate this observational feature two alternative models are presented which separately describe non-linear hydrodynamical and scattering mechanisms. Both models are based on a locally homogeneous and stationary ripple wave field. The results from the field experiment have been examined and found to provide firm evidence supporting both proposed models. Furthermore, for longer matched wavelengths (typically > 17 m) energetic lines with very small Doppler shift (<0·1 Hz) were observed. This slowly propagating phenomenon is empirically deduced to be associated with wave group structures.     

Monte-Carlo simulation studies of the nonlinear imaging of a two dimensional surface wave field by a synthetic aperture radar

Abstract

The imaging of ocean surface waves by synthetic aperture radar (SAR) is investigated using two-dimensional Monte-Carlo simulations. The properties of the SAR imaging mechanism for windseas and swell in the Bragg scattering regime are discussed as a function of a few governing non-dimensional parameters formed from a combination of SAR and ocean wave parameters. The parameter ranges may be classified into three regimes corresponding to linear and weakly nonlinear, medium nonlinear and strongly nonlinear imaging. The nonlinearities are induced by motion effects (velocity bunching, velocity spread and acceleration smearing), while the real aperture radar (RAR) tilt and hydrodynamic modulation processes are regarded as linear. In the strongly nonlinear imaging regime, the velocity bunching mechanism causes a rotation of the spectral peak towards the range direction and a stretching of the peak wavelength. In addition, the azimuthal resolution is degraded through the Doppler spreading arising from the different facet velocities within a SAR resolution cell. The imaging properties in this regime are largely governed by two non-dimensional parameters, the velocity bunching and velocity smearing parameter. The nonlinear imaging distortions are strongest for broad spectra (windseas) and are significantly weaker for narrow-band swell. In the linear and weakly nonlinear imaging regime, the superposition of the hydrodynamic and tilt cross-section modulation and the velocity bunching transfer function normally produces a rotation of the spectral peak towards the azimuthal direction. The interference characteristics of these different modulation mechanisms depends on the wave propagation direction and can lead to a significant distortion of the image. This is often seen in large differences in the image modulation depths of waves propagating parallel and anti-parallel to the flight direction.     

极化合成孔径雷达测量海浪的研究

描述了极化合成孔径雷达海浪成像过程。提出了一种极化雷达反演海浪方向谱的方法,该方法利用合成孔径雷达图像谱与海浪方向谱之间简单的线性关系和最优方法反演得到海浪方向谱,并将结果与现场资料进行了比较。经过对同一海区机载雷达P、L、C 3个波段的图像进行反演,其方向谱相当一致,而且有效波高和波向与实测数据吻合相当好。     

高频海态雷达软件无线电接收机的设计

针对现有高频海态雷达均为探测不同距离海域而专门设计，功能单一、通用性差的特点，提出了应用软件无线电技术开发新一代高频地波雷达接收机的思想，即在一通用硬件平台上，尽量通过软件方式来实现不同距离海域探测。结合高频地波雷达的波形体制扣当前器件发展水平，提出了基于VXI总线、“一次混频，高中频采样”的多通道数字接收机结构，详细介绍了其中部分关键技术，包括模拟前端、A／D带通采样、数字下变频等的器件选型以及参数设计等。实践表明，发展新的软件化高频地波雷达的理论扣技术条件均已成熟，它所具有的开放性扣灵活性，能更好地满足21世纪海洋环境动力学参数研究的需要。     

Applications of dual-doppler HF radar measurements of ocean surface currents

HF Doppler radar measurements of near-shore sea-surface currents is a relatively new technique. It has the advantage of covering a wide area of the sea surface simultaneously while being shore-based. In this paper, we compute the M₂, K₁, and mean currents over an area of the easter Strait of Juan de Fuca from HF radar measurements made during the summer of 1979. From these components, we computed trajectories of simulated continuous leaks from a proposed oil pipeline in this area. By computing the trajectory over one day, we estimated the shoreline locations that would be impacted by the oil.     

基于机载X-SAR海浪监测系统的设计

基于机载X波段合成孔径雷达设计了一套低空机载海浪监测系统.首先在对海浪微波散射和机栽合成孔径雷达成像原理进行分析的基础上,设计出海浪监测系统的总体架构;其次研究合成孔径雷达的成像算法,通过建立仿真模型,反演出海浪谱,最后测量出海浪浪高、海浪波长等主要信息,提供给飞行员用来判断飞机是否具备在海面上降落的安全条件.同时,对...     

干涉合成孔径雷达海浪遥感研究

海浪是重要的海洋环境参数, 它对港口建设、海上航行、油气田开发等具有重要的意义。合成孔径雷达(Syn thet ic Apertu re Radar, SAR ) 是目前大范围测量海浪的主要工具, 用它来估计海浪主波系统的波长和传播方向比较好, 但要定量的提供海浪的振幅以及SAR 图像强度与实际海面的关系是相当困难的, 而且由SAR 图像获取海浪方向谱也比较复杂。沿航迹干涉合成孔径雷达(Along-Track Interferometric Synthet ic Aperture Radar,ATI-SAR ) 是在平台飞行方向上安置两根天线的双天线合成孔径雷达。相对于合成孔径雷达, 真实孔径雷达调制传递函数对沿航迹干涉合成孔径雷达相位图像的影响很小, 因此沿航迹干涉合成孔径雷达更适合测量海浪。详细介绍了沿航迹干涉合成孔径雷达海浪遥感的成像机制, 为进一步反演海浪方向谱奠定了基础, 最后针对目前研究存在的问题以及今后有待研究的方向进行了探讨。     

Cumulative probability noise analysis in geophysical spectral records

The technique of rank ordering of spectral energy density samples was applied to high frequency (HF) ocean backscatter radar spectra to examine the properties of signal and noise in the record. HF ocean backscatter spectra have characteristic Bragg scatter lines and bands of energy with random phasing, and it is known that spiky noise from sferics and anthropogenic sources contribute to the observed energy density. Rank ordering of the spectral density data revealed three main features: (i) first-order Bragg scattering; (ii) continuum of randomly phased energy; and (iii) the underlying noise floor of the system. It is shown that the technique of rank ordering of spectral amplitude samples provides better insights and an improved algorithm for differentiating between bands of randomly phased sea echoes and system noise. The analysis reveals a constant offset in the spectral energy density data from the Ocean Surface Current Radar (OSCR) HF radar system, which should be removed before algorithms are applied to extract significant wave height information. The rank ordering of spectral energy density samples is a useful tool for differentiating between system noise and wide-band incoherent energy often obtained in a wide range of geophysical remote sensing.     

On SAR focusing of ocean waves

Abstract

The effect of the focus setting on the image of a long ocean wave propagating (in an arbitrary direction)with its phase speed, and the effect on the (stationary) SAR impulse, response is examined. The analysisis carried out in the two limiting cases of vanishingly small resolutionor correlation length. The exact solution is obtained for an idealized periodic reflectivity pattern, thus obtaining quantitative statements on the focus effect in relation to system and ocean parameters.