Refractivity estimation using multiple elevation angles

Estimation of the atmospheric refractivity is important for the prediction of radar performance. Surface or elevated trapping layers formed by the outflow of relatively dry and warm air over a cooler body of water often result in the refractive structure-supporting-convergence-zone-like behavior and multimodal effects. The propagation under such conditions can be very sensitive to even small changes in the vertical and horizontal structure of refractivity. Obtaining in situ measurements of sufficient fidelity to estimate where intensifications in the electromagnetic field will occur is difficult. The authors previously have demonstrated the ability to infer refractivity parameters from grazing-incidence radar sea-clutter data. The radar system was the 2.8-GHz space range radar that overlooks the Atlantic Ocean in the vicinity of Wallops Island, VA. The forward modeling consisted of the mapping of an 11-parameter environmental model via an electromagnetic propagation model into the space of the radar clutter observations. A genetic algorithm was employed to optimize the objective function. Ground truth data were atmospheric soundings obtained by a helicopter flying a saw-tooth pattern. The overall result was that the ability to estimate the propagation within the duct itself was comparable to that of in situ measurements. However, the ability to characterize the region above the duct was quite poor. Modern three-dimensional radars, however, have relatively narrow beams. Using these narrow beams at multiple elevations might resolve the ambiguity leading to the poor characterization in the region above the duct. Using radar data from the SPANDAR radar, it is demonstrated that such an approach is feasible and that more-robust estimates can be obtained by using two elevation angles and/or by constraining the solution to contain realistic refractivity profiles.     

顾及声线高度角的水下定位随机模型优化

先验随机模型本身的误差会给参数估计带来随机性影响,水下定位等权随机模型简单却不符合实际。基于这一事实,在水下声纳定位模型和随机模型基础上,讨论了随机模型不完善对参数估计的影响。考虑了影响定位主要误差源与高度角之间的关系,结合统计经验提出了基于声线高度角相关的水下定位随机模型。试验表明,优化后的高度角相关随机模型在定位精度上较等权随机模型有所提高。因此,在水下定位中,应减少低高度角观测值的权重,利用提出的随机模型可以减小高度角观测误差对定位精度的影响。     

用于深海作业的激光陀螺高精度定位定向系统的仿真与实验

定位定向技术是深海作业系统的主要关键技术之一。本文基于激光陀螺提出了一种用于深海作业的高精度定位定向系统研究方案,建立了一套相应的系统分析方法,对该方案的姿态、速度、位置误差进行了详细的仿真分析。进行了72 h静态定位定向实验,结果表明系统的俯仰角和横滚角误差小于0.006°,航向角误差小于0.035°,速度误差小于0.9m?s–1,定位误差小于29.9 km。实验结果与仿真分析结果基本一致,验证了该研究方案的可行性。和采用动力调谐陀螺的定位定向系统相比,本系统具有精度高和可靠性好等优点。     

An oilspill trajectory analysis model with a variable wind deflection angle

The oilspill trajectory movement algorithm consists of a vector sum of the surface drift component due to wind and the surface current component. In the U.S. Geological Survey oilspill trajectory analysis model, the surface drift component is assumed to be 3.5% of the wind speed and is rotated 20 degrees clockwise to account for Coriolis effects in the Northern Hemisphere. Field and laboratory data suggest, however, that the deflection angle of the surface drift current can be highly variable. An empirical formula, based on field observations and theoretical arguments relating wind speed to deflection angle, was used to calculate a new deflection angle at each time step in the model. Comparisons of oilspill contact probabilities to coastal areas calculated for constant and variable deflection angles showed that the model is insensitive to this changing angle at low wind speeds. At high wind speeds, some statistically significant differences in contact probabilities did appear.     

基于地球椭球体等距正反方位投影参数计算

等距方位投影所提供的主要投影参数是大地距离和大地方位角,可以直接服务于多种目的。等距反方位投影是以定点为极,动点至定点的大地距离和大地方位角保持真值。推导出了这两种投影的坐标计算公式,以及远程大地距离与大地方位角的计算与应用,并以实际算例进行了演示验证。     

Geoacoustical parameters estimation with impulsive and boat-noise sources

Localization is a crucial issue in underwater acoustics: when an underwater source is detected, the next step consists of localizing it. To do this, environmental parameters must be known or estimated. How can we estimate these parameters? As underwater sources produce low frequency (<100 Hz), most of the seismic processing tools developed by petroleum research can be adapted to estimate geophysical parameters of the sea and the seafloor. To know which methods or representations are useful to estimate geoacoustical parameters, a study of the propagation between an underwater source and receivers laid on the floor is proposed in the case of a real waveguide. Then, geoacoustical parameters are estimated on real data using transformations such as velocity correction or frequency-wavenumber transform. To validate these methods, two wave-propagation simulations using a finite-difference algorithm are made: the first in an environment similar to the model used to estimate geoacoustical parameters and the second in a more realistic environment (with several layers of sediments and variable water layer depth). Geoacoustical parameters are estimated and compared to the values used in the simulation. Finally, impulsive source is replaced by a boat-noise source to show that it is still possible to estimate geoacoustical parameters using noise sources.     

可扩展孔径阵方位估计误差仿真分析与实验研究

针对可扩展孔径阵方位估计误差在工程使用中受装载平台影响大的问题,开展建模仿真与实验研究.基于一种装载于移动无人平台的双层圆柱形孔径可扩展的多元线列阵开展建模,根据实际使用的场景和安装平台,设计了具备时延补偿功能的波束形成算法和自适应姿态补偿算法.通过仿真分析和实验验证,证明该方法可有效减小该种可扩展孔径阵的方位估计误差...     

Target parameter estimation using measurements acquired with a small number of sensors

A performance prediction procedure is developed and applied to the evaluation of a passive tracking technique intended primarily for the localization of targets in the near field or vicinity of the sensors. The analysis is sufficiently general to be applied to underwater and air acoustics, passive radar, and electromagnetic direction finding systems. Since near field applications are of primary concern, localization parameter identifiability with a single pair of omni-directional sensors is established with the aid of the Fisher Information Matrix (FIM). The Fisher Information Matrix is also used to determine upper bounds on localization performance, and the corresponding uncertainty ellipses associated with target position are evaluated for various tracking scenarios and types of measurements. Emphasis is placed on the use of measurements such as time difference of arrival and frequency difference of arrival obtained with two sensors, and frequency estimates obtained with a single sensor. It is shown that under certain conditions the time difference of arrival measurements yield full localization information, even though the conditioning can be marginal. Additional measurements, such as frequency, are shown to improve localization performance significantly. Bearing measurements obtained with a closely spaced cluster of a few sensors are also considered.     

The continuity equation for longshore current velocity with breaker angle adjusted for a wave-current interaction

The continuity equation for mean longshore current velocity, V = gmT sin 2θ_b, agrees with selected field and laboratory data covering a wide range of conditions. Agreement between continuity equation and data is improved by eliminating those laboratory data which imply deep-water wave crests at angles near or greater than 90 degrees to the shoreline. Agreement between continuity equation and data is further improved by adjusting breaker angles to account for convection of the breaker point by the longshore current. Breaker point convection increases breaker angle by an amount predictable from the analysis developed here. This increase in angle is significant in those laboratory experiments with breaking wave crests at high angles to the shoreline.In the continuity equation, m is bottom slope, T is wave period, and θ_b is breaker angle, but breaker height does not appear. According to radiation stress theory, mean velocity does depend on breaker height, but only weakly. Consistency between the two approaches would require a dimensionless velocity, C_b/gT, to be relatively constant, which it is. (The same dimensionless velocity appears in the analyses of breaker point convection.) The continuity equation is functionally independent of friction and mixing, in keeping with its derivation from simple conservation of mass considerations. The equation has no adjustable coefficients. The degress of agreement with data and the internal consistency of the analysis suggests that it is a good predictor of mean velocity in ordinary longshore currents.     

Local and far-field surface elevation around a vertical cylinder in unidirectional steep wave groups

The problem of diffraction of a unidirectional incident wave group by a bottom-seated cylinder is considered. We assume the amplitude of the incoming wave to be small in comparison with other linear scales of the problem, and develop the corresponding second-order perturbation theory. We use the Fourier transform to treat time variation and separate spatial variables when solving the non-homogeneous second-order problem. The resulting set of non-homogeneous Bessel equations is solved numerically.Solutions for various types of incoming wave spectrum are obtained including the Gaussian spectrum and the Pierson–Moskowitz spectrum. To validate the method, problems with gradually decreasing bandwidth of Gaussian spectrum are solved and it is shown that the corresponding solution approaches that for the monochromatic case. The Pierson–Moskowitz spectrum with a set of realistic physical parameters is used as an example of extreme wave interaction with an offshore structure. The corresponding first- and second-order solutions are obtained and the effect of non-linearity on the solution is discussed with the emphasis on the growth of maximum free-surface elevation on the cylinder’s surface and generation of high frequency free radiated waves.     

Range and depth estimation using a vertical array in a correlatedmultipath environment

The effects of correlation between time-difference-of-arrival (TDOA) estimates on the localization of an underwater source in a direct-surface reflected propagation environment is investigated for a two-element vertical array with omnidirectional sensors. Expressions for the covariance matrix of the noisy TDOAs are derived. The results show that the estimated TDOAs are correlated and that the degree of correlation depends on the relative location of the source. Expressions for variances of range and depth estimators are developed using a least-squares technique. The results show that the expressions for variances of the range and depth estimators can be in error by as much as a factor 3.3 if the correlation among the TDOAs is ignored. The expressions for the covariances of the TDOAs and the expressions for the variances of the range and depth, which incorporate the correlation in the TDOAs, are corroborated with Monte Carlo simulations     

Single-ping target speed and course estimation using a bistatic sonar

This paper focuses on estimating the two-dimensional (2-D) target-speed vector (course and speed) using a multistatic sonar that consists of one monostatic sonar and one bistatic receiver. The speed and course estimates are obtained after a single transmission. The theory on bistatic Doppler and 2-D target-speed vector estimation is first considered and then applied to simulated and real data. The results can be used to improve classification algorithms or to feed speed information to tracking algorithms, for example.     

坎门高程基准的建立过程及其作用

回顾了我国海洋观测示范站——坎门海洋环境监测站的前身坎门验潮所从选址、建站到观测所走过的艰难历程,阐述了作为我国高程系统的奠基——坎门高程基准的建立过程及其在我国高程系统中的地位和作用。     

Sensor position estimation and source ranging in a shallow water environment

Underwater acoustic transient signals are generated mechanically at known positions along a wharf. These signals are received by a wide aperture planar array of four underwater acoustic sensors, whose positions relative to the wharf are unknown. A method is described that enables the positions of the sensors to be estimated from accurate differential time-of-arrival measurements (with 0.1 /spl mu/s precision) as the signal wavefronts traverse the array. A comparison of the estimated positions with the nominal positions of the first three sensors, which form a 20-m-wide aperture horizontal line array, reveals a 2-cm displacement of the middle sensor from the line array axis. This slight bowing of the line array results in overranging (bias error of 3%) when the wavefront curvature method is used with the nominal collinear sensor positions to locate a static source of active sonar transmissions at a range of 59.2 m. The use of the spherical intersection method coupled with the estimated sensor positions of the line array provides an order of magnitude improvement in the range estimate (within 0.3% of the actual value). However, systematic ranging errors are observed when the sound propagation medium becomes nonstationary. Next, the differences in the arrival times of the direct path and boundary-reflected path signals at the middle sensor of the wide aperture line array are estimated using the differential phase residue of the analytic signal at the sensor output. These multipath delays are used to estimate the range and depth of the source. Although the average value of the multipath range estimates is within 0.5% of the actual value, the variance of the range estimates is 50 times larger when compared with the results of the spherical intersection and wavefront curvature methods. The multipath delay data are also processed to provide a reliable estimate of the temporal variation in the water depth enabling the tidal variation to be observed.     

波致海底缓倾角无限坡滑动稳定性计算分析探讨

波浪作用下海底无限坡滑动稳定性计算的极限平衡法中,忽略了坡体水平向应力状态的影响,为此,针对波浪作用下海底缓倾角无限边坡的特点,提出直接基于滑动面处土体应力状态的滑动稳定性计算方法(简称应力状态法),并分析了其适用范围。对具体算例的分析表明,应力状态法计算得出的安全系数大于极限平衡法的安全系数,且随着滑动面深度的增加、土体泊松比以及边坡坡角的增大,两种计算方法得出的安全系数的差异会逐渐增大;对于波浪作用下的海底缓倾角无限边坡,在失稳时极可能沿具有一定厚度的滑动带而不是单一的滑动面而滑动,且波致最大剪应力所在的深度,常常不是斜坡体最易失稳滑移的深度。     

Influence of water-level elevation and tidal range on the sedimentation in a German tidal flat area

The nearshore region of the German Bight is characterised by a large tidal flat area of 7500km², which is a wedge of alluvial material lying in front of the coast. The elevation of a flat depends on the long-term water-level and currents. As a result of the rising water-level, the flats are migrating over an inclined Pleistocene basement. This migration depends on the balance of sedimentation versus erosion which is a function of water depth. Regular measurements of the water-level have been made since the middle of the 19th century. From the observations of the MHW it is estimated that the water-level has risen to approximately 30cm in the past hundred years. Recent investigations show that the change of the half-tide water-level is less than that of MHW. The increasing tidal range also changes the sedimentation processes over the tidal flats. The quantitative influence of the altered tidal range on the sedimentation and the morphology of the Wadden Sea is as important as that of the influence of mean water-level elevation. Changes in the elevation of the coastal plain caused by tectonic forces or subsidence cannot be determined. Extensive investigations were conducted to determine quantitatively the sedimentation processes which have taken place since 1936. The sediment budget of the Flackstrom basin (low macrotidal coast) is presented. With respect to the MHW datum plane, sedimentation of 20 × 10⁶m³ has taken place in this area. This is equivalent to a mean silting-up of 30cm over the past 46 years, an average 0.7cm/year. The sedimentation process is dependent upon the mean elevation of the flat. Most of the sedimentation has taken place above the half-tide water-level. Therefore, the flanks of the flats have risen more than the tops and the sand-ridges, which certainly reflects the very small long-term water-level elevation change of only 3cm which occurred during the period investigated (1936 to 1982). Recent investigations of the present state show a relationship between the tidal range and the height of the main sedimentation-zone. Comparisons of the Flackstrom (tidal range approximately 3.40m) and parts of the Dutch Wadden Sea (tidal range approximately 1.80m) indicate that the main sedimentation-zone moves down-shore from half-tide water-level to mean-low-water with the decrease of the tidal range, due to differences in the general course of the hypsometric curves over the tidal flat areas.     

Numerical investigation of wave elevation and bottom pressure generated by a planing hull in finite-depth water

A numerical investigation of the bottom pressure and wave elevation generated by a planing hull in finite-depth water is presented. While the existing literature addresses the free-surface deformation and pressure field at the seafloor independently, this work proposes a direct comparison between the two hydrodynamic quantities. The dependence of the pressure disturbances at the ocean floor from the waves generated at the free-surface by a planing hull is studied for several values of both the depth and hull Froude numbers. The methodology employed is Smoothed Particle Hydrodynamics (SPH), a numerical technique based on the discretization of the continuum fields of hydrodynamics through mesh-less particles. The SPH code herein chosen is initially validated against experimental data for transom-stern flow. Subsequently, numerical simulations are presented for a planing hull in high-speed regimes. The results show a direct correlation between surface wave dynamics and hydrodynamic pressure disturbances at the seafloor as the value of the Froude number is varied. This is assessed by studying the inverse dependence of the low-pressure wake angle with the Froude number and by comparison of SPH results with similar works in the cited literature.