High Grazing Angles Research Articles

Inferences of seabed characterization in a variable ocean environment

Dimensional reduction methods based on feature extraction are critical for statistical inference application in variable ocean environments. Presented are analyses for selected acoustic measurements made during the Seabed Characterization Experiment in 2021-2022. Broadband sources were deployed on the New England continental slope between the 200 and 400 m depth contours. The method of analysis includes first analyzing short range (higher grazing angles) acoustic measurements provided by both a broadband towed source and explosive sources. Features utilized for short-ranged data include those associated with seabed interferometry. Then, longer range data (lower grazing angles) are utilized to provide improved estimates of such quantities as the attenuations and sound speed gradients. Features utilized for long-ranged data include those associated with the dispersion of signals generated by explosive sources. As a general guide the mid-frequency band is utilized to infer characteristics about the upper portions of the sediment whereas the lower frequencies are utilized to infer the deeper portions of the seabed. [Work supported by Office of Naval Research.]

Surface and Bulk Plasmon Excitation on Aluminum Surface at Small to High Grazing Angles

We present a series of spectra measured by reflected electron energy loss spectroscopy on an aluminum sample using a cylindrical mirror analyzer. The measurements were performed in the energy range between 250 eV and 2000 eV and with various incident angles, including the grazing geometry of an 88° incident angle. The observed spectra were evaluated and decomposed for surface and bulk excitation. The determined surface plasmon excitations were compared to the elastic peak and to the bulk excitation. We found a slight surface plasmon energy shift with altering glancing angles. We show that this shift exists independently from the bulk plasmon interference.

Study on Numerical Simulation and Electromagnetic Scattering of Time-Invariant Freak Waves

The occurrence probability of freak waves is related to the sea wave spectrum. In this paper, different wave spectrums are used to simulate time-invariant three-dimensional freak waves. Freak waves that meet the international standards are generated at fixed time and location by adjusting the energy of the wavelets. We studied the occurrence probability of freak waves under the conditions of different wave spectrums, different wind speeds, and different modulation ratios and optimized the calculation speed of the model. Simulation data show that the difference in the shape of the wave spectrum affects the probability of freak waves occurrence. The model conforms to the Benjamin–Feir index (BFI), and the ratio of wave steepness to spectrum bandwidth is the key. In this paper, the Kirchhoff approximation theory is used to study the electromagnetic scattering (EM scattering) properties of freak waves on the large scale. We ideally calculate the Normalized Radar Cross-Section (NRCS) from the sea surface with freak waves, under different wind speeds and different grazing angles. The NRCS of freak waves is extremely low, and the increase of wind speed and the decrease of the grazing angle will make the detection of freak waves more difficult. The possibility of detection of freak waves is higher at high grazing angles (low incidence angles). The numerical simulations provide reference for engineering.

Performance analysis of empirical models of sea surface reflectivity based on measured data

The empirical models of sea surface reflectivity have many categories and different applicable conditions. In order to solve the problem of how to select the best models in different environments, we make a systematic analysis of the performance of different models based on the measured data of sea surface and the meteorological observation data in the Northwest Pacific. The results show that the accuracy of the empirical models of sea surface reflectivity may be biased when only the sea state is used as the description of sea surface roughness. For low radar grazing angle, NRL and HYB models can obtain better performance. It is recommended to add a fixed offset for NRL model at medium grazing angles. Constant gamma model can get better fit at high radar grazing angle.

Sea-surface acoustic backscattering measurement at 6–25 kHz in the Yellow Sea

Sea-surface acoustic backscattering measurements at moderate to high frequencies were performed in the shallow water of the south Yellow Sea, using omnidirectional spherical sources and omnidirectional hydrophones. Sea-surface backscattering data for frequencies in the 6–25 kHz range and wind speeds of (3.0±0.5) and (4.5±1.0) m/s were obtained from two adjacent experimental sites, respectively. Computation of sea-surface backscattering strength using bistatic transducer is described. Finally, we calculated sea-surface backscattering strengths at grazing angles in the range of 16°–85°. We find that the measured backscattering strengths agree reasonably well with those predicted by using second order small-roughness perturbation approximation method with “PM” roughness spectrum for all frequencies at grazing angles ranged from 40° to 80°. The backscattering strengths varied slightly at grazing angles of 16°–40°, and were much stronger than roughness scattering. It is speculated that scattering from bubbles dominates the backscattering strengths at high wind speeds and small grazing angles. At the same frequencies and moderate to high grazing angles, the results show that the backscattering strengths at a wind speed of (4.5±1.0) m/s were approximately 5 dB higher than those at a wind speed of (3.0±0.5) m/s. However, the discrepancies of backscattering strength at low grazing angles were more than 10 dB. Furthermore the backscattering strengths exhibited no significant frequency dependence at 3 m/s wind speed. At a wind speed of 4.5 m/s, the scattering strengths increased at low grazing angles but decreased at high grazing angles with increasing grazing angle.

An Improved SASS Model for Sea Clutter Reflectivity at Ku-band

The typical models, namely, Hybrid (HYB) model, Technology Service Corporation (TSC) model, and Naval Research Laboratory (NRL) model, reveal a large deviation at medium and high grazing angles that leads to an inaccurate prediction of sea clutter backscattering coefficient, owing to a limitation of the measured Nathanson data. An improved model for sea clutter reflectivity is proposed as a function of grazing angle, azimuth direction, radar polarization and wind speed, based on the SEASAT-A Satellite Scatterometer (SASS) model with sufficient measured data at Ku-band. To deal with the redundant structure of SASS model, the improved SASS model is developed to simplify the calculation of mean backscatter coefficients of sea clutter. In addition, the design algorithm of the improved SASS model, as well as the model usage method is presented. The reliability of the improved SASS model is proved by comparisons with Nathanson models and measured sea clutter data.

Analysis of amplitude statistical and correlation characteristics of high grazing angle sea‐clutter

Research on the characteristics of sea clutter has always been the focus in the problem of sea target detection as an indispensable part. In recent years, the problem of sea clutter characteristics under medium-high grazing angle is also paid more and more attention. Combined with the measured data of sea clutter, the temporal correlation, spatial correlation characteristics, and the amplitude distribution at three different grazing angles are studied. The results show that, when the grazing angle is >59°, the sea clutter becomes less correlative in temporal area and less correlative in spatial area as the grazing angle goes up. The sea clutter fitting result of KR distribution is better than both the K distribution, KK distribution, and Rayleigh distribution at high grazing angle in Ku-band high-resolution situation.

높은 지표각에서 해상 클러터 환경을 고려한 해상 표적 영상 생성 및 탐지

탄도 미사일은 상공에서 자유 낙하하며 표적을 요격하기 때문에 탄도 미사일에 부착되는 탐색기는 높은 지표각에서 해상 클러터 영향을 받게 되며, 그 결과 탐색기의 탐지 성능이 급격히 낮아지게 된다. 이를 해결하기 위해서는 다양한 시나리오 기반의 시뮬레이션들을 통한 해상 표적 탐지 성능 분석이 반드시 필요하다. 따라서 본 논문에서는 실제와 유사한 높은 지표각의 해상 클러터 수신 신호를 모델링한 후, 이를 신호 대 클러터 비에 따라 해상 표적 수신 신호와 합성하여 2차원 레이다 영상을 생성한다. 이후, 레이다 영상에 2차원 CA-CFAR 탐지기를 적용하여 다양한 시나리오에서 해상 표적 탐지 성능을 분석하였다. CAD 모델과 전자기 수치해석 도구를 사용한 시뮬레이션 결과, 지표각과 방위각에 따라 해상 표적의 탐지 여부가 다르게 나타남을 확인할 수 있었다.

Suppression of reflected side lobes in narrow-band X-ray multilayer coatings.

We present an analytical method for the design of narrow-band X-ray multilayer coatings having greatly reduced reflected side-lobe intensity, for the realization of X-ray mirrors that have improved spectral purity. The method uses a specific variation of the individual layer thicknesses as a function of depth in the multilayer stack, derived from Laplace transform analysis of the multilayer's reflectance profile. The design process and mathematical foundations are outlined. Pt/C multilayers with 5 nm d-spacing for hard X-rays are designed, fabricated and measured to demonstrate the validity and effectiveness of the method are presented. As an extrapolation, three additional side lobe suppressed multilayers for soft X-rays and EUVs are also designed and investigated: 1) Cr/Sc multilayer for soft X-rays (4.96 nm wavelength) at high grazing angle (30°), 2) Mo/Si multilayer for EUV (13.5 nm wavelength) at normal incidence angle and 3) SiC/Mg multilayer for EUV (30.4 nm wavelength) at normal incidence angle. The calculated reflectances demonstrate that the presented method is robust for the energy range from X-rays to EUVs.

CFAR Detection of High Grazing Angle Sea-Clutter Based on KR Distribution

Constant false alarm rate (CFAR) is a traditional method for radar target detection. A lot of relevant researches assume that the sea clutter obeys the K distribution. However, with the improvement of the radar resolution and the increase of the grazing angle, the distribution of sea clutter gradually deviates from the K distribution. Although good fitting results can be obtained through the more complicated sea clutter models such as KA distribution, K+Rayleigh distribution and so on, the computational efficiency of the detection algorithm is reduced obviously. For computational complexities are mainly caused by non-closed expression, A hybrid sea-clutter distribution (KK distribution) with closed-form expression for high-resolution and high grazing angle situation is proposed, which fits the measured data. Inspired by this, a new hybrid sea-clutter distribution (KR distribution) with 4 parameters is proposed. A new method of parameter estimation for KR distribution is given, that is, passingparameter estimation method based on hybrid distribution. The results show that the fitting effect of KR distribution is better than that of both K distribution and KK distribution in case of high resolution and large grazing angle. And the CA-CFAR based on KR distribution of passing-parameter estimation performs relatively better in the aspect of target detection.

Kirchhoff approximation for scattering from solid spheres at flat interfaces: Improved description at large grazing angles

The Kirchhoff approximation has previously been used to model scattering from partially exposed elastic spheres breaking through a flat interface, with variable exposure level, grazing angle, and frequency [J. Acoust. Soc. Am. 136, 2087 (2014)]. The limits of the Kirchhoff integral are determined by the boundaries of illumination on the sphere for each scattering path. Recent adaptations to the methods of boundary determination within the Kirchhoff approximation have yielded faster numerical integration algorithms and higher similarity of results when compared to experimental scattering data and the exact solution at half exposure [J. Acoust. Soc. Am. 140, 3582-3592 (2016)]. Additional steps have been taken to account for the partial blocking of the interface by the partially exposed sphere, through inclusion of a new correction term. This correction is largest at high grazing angles, low frequencies, and low target exposures, and drastically improves the Kirchhoff approximation within these limits. [Work supported by ONR.]

Development of an improved minimum order statistic detection process for Pareto distributed clutter

A simple modification of a minimum-based constant false alarm rate detector for Pareto clutter models is examined in this study. This has been motivated by the need to develop improved detection processes to operate in high grazing angle clutter as exemplified by returns acquired from the Defence Science and Technology Organisation's Ingara radar. A minimum-based detector has the greatest resilience to clutter transitions and interfering targets. However, it has been shown recently that a minimum-based detector for the Pareto environment has a tendency to saturate. Using a simple transformation, progress is made towards the development of a useful incoherent detection scheme. Its performance is then analysed in both synthetic and real clutter.

Characterization of High Grazing Angle X-Band Sea-Clutter Doppler Spectra

Collection of radar sea-clutter is typically performed from the top of a cliff looking out to sea, as it is relatively simple and inexpensive. This constrains the radar look direction with respect to the wind and limits the grazing angle. To improve our understanding at high grazing angles in the range 15° to 45°, the Defence Science and Technology Organisation's Ingara airborne X-band fully polarimetric radar has been used to collect 12 days worth of sea-clutter data. It has previously been shown that Walker's mean Doppler spectrum model is not suitable at these grazing angles; hence, a new two-component model is proposed that captures both the slow Bragg component and the fast non-Bragg component of the radar backscatter. A temporal decorrelation model is then presented that can be used to provide realistic performance prediction modeling.

Sea-Spike Detection in High Grazing Angle X-Band Sea-Clutter

Knowledge of sea-clutter phenomenology allows accurate models to be developed for assessing target detection performance. The majority of work in this area has been at low grazing angles from clifftops or wave tanks and does not consider scattering in the high grazing angle region beyond 10 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex> </formula> . To improve our understanding at high grazing angles of 15 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$ </tex></formula> –45 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}$</tex></formula> , the Defence Science and Technology Organisation's Ingara airborne X-band fully polarimetric radar has been used to collect 12 days' worth of sea-clutter data. This paper focuses on understanding the characteristics of sea-spikes as they are often the cause of false detections in a radar processor. Using the Ingara data, a threshold is used to isolate these scatterers in the range/time domain with results verified against the KK probability distribution function. Detections due to discrete and persistent scattering are then isolated to provide more information regarding the underlying cause of sea-spikes and answer the question of whether Walker's three-component mean Doppler spectrum model is suitable at high grazing angles.

High-frequency acoustic backscattering from a sand sediment: Experiments and data/model comparisons

In the Spring of 2012, high-frequency backscattering from a sandy sediment was measured in the Gulf of Mexico at the site of the upcoming, ONR-sponsored reverberation experiment. The measurements were made using an array of sources and receivers that collected data from 200 to 500 kHz and that could be rotated such that the incident grazing angles varied from 10 to 50 degrees. This array was used previously to measure scattering from a sand/mud sediment during the Sediment Acoustics Experiment 2004 (SAX04). To support data/model comparisons, the seabed roughness, sediment shell content, sediment sound speed, and sediment attenuation were also measured. For scattering below the critical grazing angle, sediment roughness is found to be the dominant scattering mechanism while above the critical angle, roughness scattering underpredicts the measured scattering strength. To understand the scattering strength at high grazing angles, scattering from shells and shell hash is considered. The measured scattering strengths and environmental properties at the experiment site are also compared to those made during SAX04. (Work supported by the US Office of Naval Research)

Validity of whitening-matched filter approximation to the Pareto coherent detector

It has been observed that the coherent multilook detector for targets embedded within Pareto intensity clutter can be approximated by the Gaussian optimal detector, or whitening-matched filter, in a number of real data sets. These correspond to clutter returns obtained from the Defence Science and Technology Organisation&apos;s Ingara radar, operating in X-band, at high grazing angles and in a circular spotlight mode. This study will establish conditions under which this can be explained mathematically. The key to this is to apply Stein&apos;s method from probability theory to establish rules of thumb to determine the validity of the approximation.

Range-layover to height mapping in high grazing angle synthetic aperture radar for non-interferometric 3-D image formation

Small unmanned aircraft or other aircraft that operate at short range are likely to need to operate at steep look-down angles. Imaging at high grazing angle causes range-layover which affects synthetic aperture radar (SAR) images in the down-range direction, but not in cross-range. By making a number of images at different look angles relative to the target, the 3-D position of individual scatterers on the target can be calculated, as long as they remain visible over an appreciable angular extent. The range-layover to height mapping algorithm is presented here for forming 3-D images from high grazing angle SAR data. This algorithm only requires data collected from a single antenna and a single pass of the target to form a 3-D image.

Reverberation clutter from combined internal wave refraction and bottom backscatter.

A hypothesis is proposed that one mechanism for clutter observed in shallow water reverberation measurements is due to the combined effect of forward scatter and subsequent backscatter. The forward scatter refracts part of the sound from low grazing angle to high grazing angle, then being backscattered by bottom roughness. The refracted sound impinges onto the bottom at higher grazing angle, resulting in higher backscatter because of elevated bottom scattering cross section as compared to that at lower grazing angles. The resultant reverberation will stand out as a target-like clutter. An example is presented as a non-linear internal wave propagates in a shallow water channel, resembling conditions found on the New Jersey Shelf. The effect on reverberation of the internal wave as clutter is investigated using a time-domain numerical model. The model uses parabolic equation for the two-way propagation and first order perturbation approximation for bottom backscatter. [Work supported by ONR.]

Analysis of the KK-distribution with medium grazing angle sea-clutter

Robust maritime surveillance with radar requires an accurate description of the backscatter from the sea. An estimated probability distribution of the backscatter is commonly used to determine the threshold for separating targets from clutter at a given false alarm rate. Data collected at medium to high grazing angles by the Defence Science Technology Organisation (DSTO) Ingara fully polarimetric X-band radar demonstrate that the commonly used K-distribution is not always adequate for modelling the probability distribution. This is especially the case for the horizontal polarisation and in regions of high backscatter where target detection can be a problem. An alternative proposed as a more accurate model in this region is known as the KK-distribution. The analysis presented in this study describes this model with the addition of multiple looks and a thermal noise component to produce greater accuracy in the mean and underlying shape. The threshold required to achieve a constant false alarm rate is then studied and compared with the K-distribution.

Acoustic mode radiation from the termination of a truncated nonlinear internal gravity wave duct in a shallow ocean area

Horizontal ducting of sound between short-wavelength nonlinear internal gravity waves in coastal environments has been reported in many theoretical and experimental studies. Important consequences arising at the open end of an internal wave duct (the termination) are examined in this paper with three-dimensional normal mode theory and parabolic approximation modeling. For an acoustic source located in such a duct and sufficiently far from the termination, some of the propagating sound may exit the duct by penetrating the waves at high grazing angles, but a fair amount of the sound energy is still trapped in the duct and propagates toward the termination. Analysis here shows that the across-duct sound energy distribution at the termination is unique for each acoustic vertical mode, and as a result the sound radiating from the termination of the duct forms horizontal beams that are different for each mode. In addition to narrowband analysis, a broadband simulation is made for water depths of order 80 m and propagation distances of 24 km. Situations occur with one or more modes absent in the radiated field and with mode multipath in the impulse response. These are both consistent with field observations.