Alternate schemes in synthetic aperture sonar processing

Suboptimal processing schemes, the application of which is not widespread in synthetic aperture sonar processing, are described with reference to seafloor imaging. It is shown that their application can result in a significant increase of the azimuth resolution of the sonar system with respect to the resolution due to its physical beamwidth, without imposing unreasonable constraints on the sonar platform trajectory.     

Azimuthal ambiguities in synthetic aperture sonar and syntheticaperture radar imagery

It is shown that azimuthal ambiguities are not eliminated by the nulls of a sonar (or radar) beam pattern and have a definite influence on image quality. In synthetic aperture systems that are strongly limited in spatial sampling, particularly in ocean borne synthetic aperture sonar (SAS) and spaceborne synthetic aperture radar (SAR), azimuthal ambiguities will corrupt the images unless special measures are taken. These azimuthal aliases may be reduced by emphasizing the centermost portion of the available synthetic aperture length, and deemphasizing the endmost portions. This minimizes the effects from synthetic array elements that most strongly contribute to aliases     

Detection of buried targets using a synthetic aperture sonar

This paper presents observations of buried target detections made using a 20-kHz synthetic aperture sonar. At grazing angles below the critical angle, surprisingly high signal-to-noise detections were made of cylindrical targets buried at depths of 15 and 50 cm. During a separate set of measurements, buried spheres were clearly seen at steep grazing angles, but were generally not seen below the critical angle. Since scattering from wave-generated sand ripples may contribute to detections at grazing angles below critical angle, the information available on the ripple fields is discussed and used in acoustic backscatter simulations for the buried spheres. Lack of information on the ripple height precludes a definitive explanation for the absence of buried sphere detections at subcritical grazing angles.     

Theoretical accuracy of synthetic aperture sonar micronavigation using a displaced phase-center antenna

The Cramer-Rao lower bounds on the cross-track translation and rotation of a displaced phase-center antenna (DPCA) in the slant range plane between two successive pings (known as DPCA sway and yaw in what follows) are computed, assuming statistically homogeneous backscatter. These bounds are validated using experimental data from a 118-182-kHz sonar, showing an accuracy of the order of 20 microns on the ping-to-ping cross-track displacements. Next, the accuracy required on the DPCA sway and yaw in order to achieve a given synthetic aperture sonar (SAS) beampattern specification, specified by the expected SAS array gain, is computed as a function of the number P of pings in the SAS. Higher accuracy is required when P increases to counter the accumulation of errors during the integration of the elementary ping-to-ping estimates: the standard deviation must decrease as P/sup -1/2/ for the DPCA sway and P/sup -3/2/ for the yaw. Finally, by combining the above results, the lower bounds on DPCA micronavigation accuracy are established. These bounds set an upper limit to the SAS length achievable in practice. The maximum gain Q in cross-range resolution achievable by a DPCA micronavigated SAS is computed as a function of the key SAS parameters. These theoretical predictions are compared with simulations and experimental results.     

An experimental study in forming a long synthetic aperture at sea

The testing of a synthetic aperture technique, the ETAM algorithm, is extended and its performance for CW pseudorandom signals and broadband ship noise is examined. The results show the limitations of the technique and are of special interest for operational systems development. In the CW experiments, the transmitted signal was generated with high temporal coherence, and loss of the spatial and temporal coherence of the received signal was introduced only by the medium and the stability of the towed array. In the experiments that included the pseudorandom signal and the ship noise, the temporal coherence of the transmitted signals was deliberately chosen to be poor in order to study the effects and the performance of this algorithm with broadband signals. The related experimental results show that for received signals, which have their segments over the synthesizing period highly cross-correlated, a synthetic aperture array gain was achieved that corresponds to the length of an equivalent fully populated array     

Inversion for the compressional wave speed profile of the bottomfrom synthetic aperture experiments conducted in the Hudson Canyon area

In September 1988, a series of acoustic propagation experiments were conducted in the Hudson Canyon area. These included synthetic aperture experiments in which a source transmitting a set of four pure tones was towed toward/away from a vertical array of 24 receivers. Data obtained at 50 Hz during one of the synthetic aperture experiments are used to obtain a model for the compressional wave speed profile in the bottom using a modal inverse method. This model is further refined using 175 Hz data. The ability of the inferred model to predict the field at 50 Hz and higher frequencies is examined     

A versatile sonar system for ocean research and fisheries applications

An echo sounder has been developed with features ideally suited to oceanographic and fisheries research. Instruments commonly used for such research are inaccurate, limited in dynamic range, unstable, and generally inflexible. An effort has been made to overcome these deficiencies with the sonar system discussed here. The echo sounder to be described has a time-varied-gain receiver (20 log_{10} Ror40 log_{10} R + 2alphaR) accurate to withinpm0.5dB over a 100-dB range. The equivalent dynamic range is 140 dB (the ratio of the maximum signal at minimum gain to the equivalent input noise at maximum gain in a 4-kHz bandwidth). The temperature stability ispm0.5dB from10degto35degC at any range. Operating parameters, including frequency, can be easily altered to accommodate a variety of needs.     

A potentiometric study of ferric ion complexes in synthetic media and seawater

An investigation of ferric ion complexing has been conducted in synthetic media and seawater at 25°C. Formation constants were potentiometrically determined for the species FeCl²⁺, FeCl₂⁺, FeOH²⁺, and Fe(OH)₂⁺ at an ionic strength of 0.68 m. Formation constants for the ferric chloride complexes were determined as _Clβ₁ = 2.76 and _Clβ₂ = 0.44. In a study of the reaction Fe³⁺ + nH₂O ? Fe(OH)_n^(3?n)+ + nH⁺ in NaClO₄, NaNO₃ and NaCl the formation constants ${}^1\text{β}{}_1\text{and}{}^1\text{β}{}_2$ were shown to be relatively independent of medium when the effects of nitrate and chloride complexing were taken into account. The average values obtained for these constants are ${}^1\text{β}{}_1\text{= 1.93 · 10}{}^{\mathrm{?3}}\text{and}{}^1\text{β}{}_2\text{= 8.6 · 10}{}^{\mathrm{?8}}$. Reasonable agreement with these values was obtained when these constants were determined in seawater by accounting for the effects of chloride, fluoride and sulfate complexing.     

一种新的用于极化SAR图像船只检测的散射相似性测度

本文提出了一种新的用于极化合成孔径雷达（SAR）海上船只目标检测的测度。首先利用散射相似性参数研究船只与海杂波散射机制的差异。基于这些差异,提出了一种新的船只检测测度,该测度能够有效区分船只目标和海杂波。然后提出了利用核密度估计（KDE）方法对该测度进行建模的方法。基于统计模型,实现了自适应恒虚警率（CFAR）的检测方案。本文利用多景C波段RADARSAT-2极化SAR（Pol-SAR）数据上进行实验,系统分析了所提出测度的海杂波拟合性能与船只检测性能,并与两个经典的极化测度进行了比较,实验和比较结果证明了所提的测度的可行性。     

Clay migration and entrapment in synthetic porous media

Water-sensitivity in sandstones is commonly ascribed to mobilization, transport and recapture of clays in downstream pore throats. This is based mainly on knowledge of the clay mineral content and its electrochemistry. This paper describes a method for directly observing water-sensitivity mechanisms under the microscope using pore networks etched in glass, with clays introduced into the networks. This allows direct identification of colloidal and transport factors, which is not possible in sandstone cores. The migration behaviour has been investigated with variations in flow rate, acceleration forces and salinity. Migration was observed to occur in two distinct modes: (1) as flocs, especially in concentrated brines and at high flow rates, (2) as deflocculated individual particles, especially in distilled water. Recapture of clays occurs by the formation of ‘particle briges’. The upstream pressure indicates that particle recapture is related to decreasing permeability.     

Bistatic synthetic aperture imaging of proud and buried targets from an AUV

The use of autonomous underwater vehicles (AUVs) for the detection of buried mines is an area of current interest to the Mine CounterMeasures (MCM) community. AUVs offer the advantages of lower cost, stealth, reduced operator risk, and potentially improved coverage rates over more traditional mine hunters. However, AUVs also come with their own set of difficulties, including significant error in navigation and low communication rates with the mother platform and each other. In the case of bistatic detection scenarios, AUVs will therefore have difficulty knowing where exactly in space they are and the trigger time of sources on other platforms, be they ships or other AUVs. However, the potential improvement in detection and coverage rates offered by bistatic sonar concepts makes resolution of these issues a high priority. In this paper, the problems of inaccurate navigation and source timing information are addressed for the Generic Oceanographic Array Technology data set. In this experiment, conducted off Marciana Marina during June 1998, a MIT AUV with a SACLANTCEN acoustic array and acquisition system was used together with a TOPAS parametric sonar to explore issues of buried target detection using AUVs. In this paper, solutions to the navigation and timing problems are proposed which enable the effective use of bistatic synthetic aperture sonar (SAS) concepts for the detection of buried objects in the mid-frequency regime of 2-20 kHz.     

侧扫声呐探测数据管理系统设计与实现

侧扫声呐调查获得的数据是一类格式特殊、类型多样、结构复杂、数据量庞大、包含信息丰富的数据,设计和建屯侧扫声呐数据管理系统是数据深层次应用和共享的前提.针对侧扫声呐数据的特点,本文将侧扫声呐获得的完整数据体系规划为原始数据层、标准化数据层、目标数据层和元数据层4个层次,设计并开发了侧扫声呐数据管理系统,整个管理系统包括原...     

全极化SAR图像中溢油极化特征研究

相比于单极化SAR图像,全极化SAR图像不仅能体现海面目标的几何特征、后向散射特征,还能体现目标的极化特征。因此,在溢油检测方面,极化SAR更具优势。特征提取作为溢油检测的关键步骤,直接影响到溢油检测的精度。在本文中,我们分析了全极化SAR图像中海面溢油的极化特征,如极化散射熵、平均散射角等。并提出了新的极化特征P,该特征参数能够反映海面目标电磁散射过程中布拉格散射机制和镜面散射机制的比例。为了研究极化特征溢油检测的能力,本文基于SIR-C/X-SAR和Radarsat-2全极化SAR图像开展了相关实验,并对比分析了溢油的多种极化特征。实验结果显示,在中低风速情况下,C波段溢油探测效果优于L波段;本文提出的极化特征P对海面散射机制敏感;基准高度和特征参数P在C波段比其他极化特征更适于溢油检测。     

Phase and doppler errors in a spaceborne synthetic aperture radar imaging the ocean surface

Data processing in a spaceborne synthetic aperture radar (SAR) imaging the ocean surface is affected by earth rotation, orbit eccentricity, and wave motion. Without compensation these sources will cause the images to shift in range and in-track positions and also cause defocusing. Ionospheric granularities may degrade image quality. Calculations of the magnitudes of these effects are presented.     

海洋溢油合成孔径雷达图像特征提取及其关键度分析

就SAR图像溢油检测的方法论而言,用于识别溢油和疑似现象的定性或定量的统计特征量选择,通常是任意的。对于不同的分类模型,所选用的特征量也不尽相同。主要是进行海洋SAR图像特征提取及其关键度分析。其目的是将"最小距离"判别法应用于海上溢油和疑似溢油的识别研究。首先,针对海洋SAR图像溢油检测常用的特征量,进行冗余处理;然后,引入关键系数,定量地研究特征量的关键度,提取显著特征量;藉以构造一个多维的特征矢量空间,以适于最小距离判别法在特征矢量空间中进行溢油和疑似溢油的识别研究。     

Theoretical prediction of running attitude of a semi-displacement round bilge vessel at high speed

Running attitudes of semi-displacement vessels are significantly changed at high speed and thus have an effect on resistance performance and stability of the vessel. There have been many theoretical approaches about the prediction of running attitudes of high-speed vessels in calm water. Most of them proposed theoretical formulations for the prismatic hard-chine planing hull. In this paper, running attitudes of a semi-displacement round bilge vessel are theoretically predicted and verified by high-speed model tests. Previous calculation methods for hard-chine planing vessels are extended to be applied to semi-displacement round bilge vessels. Force and moment components acting on the vessel are estimated in the present iteration program. Hydrodynamic forces are calculated by ‘added mass planing theory’, and near-transom correction function is modified to be suitable to a semi-displacement vessel. Next, ‘plate pressure distribution method’ is proposed as a new hydrodynamic force calculation method. Theoretical pressure model of the 2-dimensional flat plate is distributed on the instantaneous waterplane corresponding to the attitude of the vessel, and hydrodynamic force and moment are estimated by integration of those pressures. Calculations by two methods show good agreements with experimental results.     

基于一维综合孔径微波辐射计的大气海洋环境参数敏感性分析

相比于实孔径微波辐射计,一维综合孔径微波辐射计具有高空间分辨率和多入射角观测特点.本文提出采用观测频率为6.9,10.65,18.7,23.8和36.5 GHz,且入射角范围为0°～65°的一维综合孔径微波辐射计遥感大气海洋环境要素.基于构建的微波大气海洋辐射传输正演模型,分析了辐射计亮温对大气海洋环境要素的敏感性,为...     

国内外侧扫声纳测量标准比较研究

由于侧扫声纳应用领域不断扩大,用户要求不断提高,国际海道测量组织和世界测量技术发达国家相继发布了各自的新版测量规范和标准。本文将我国侧扫声纳现行测量标准与国际相关标准在系统探测能力、测线布设、数据采集、质量控制等方面进行比较详细的分析比较,指出了我国侧扫声纳测量规范在测线布设、数据采集要求等方面的不足,旨在推进我国测量标准尽快实现与国际接轨。