Acoustic Ray Tracing Comparisons in the Context of Geodetic Precise off-shore Positioning Experiments

The GNSS-Acoustics (GNSS-A) method couples acoustics with GNSS to allow the precise localization of a seafloor reference in a global frame. This method can extend on-shore GNSS networks and allows the monitoring of hazardous oceanic tectonic phenomena. The goal of this study is to test the influence of both acoustics ray tracing techniques and spatial heterogeneities of acoustic wave speed on positioning accuracy. We test three different ray tracing methods: the eikonal method (3D sound speed field), the Snell-Descartes method (2D sound speed profile), and an equivalent sound speed method. We also compare the processing execution time. The eikonal method is compatible with the Snell-Descartes method (by up to 10 ppm in term of propagation time difference) but takes approximately a thousand times longer to run. We used the 3D eikonal ray tracing to characterize the influence of a lateral sound speed gradient on acoustic ray propagation and positioning accuracy. For a deep water (? 3,000 m) situation, frequent in subduction zones such as the Lesser Antilles, not accounting for lateral sound speed gradients can induce an error of up to 5 cm in the horizontal positioning of a seafloor transponder, even when the GNSS-A measurements are made over the barycenter of a seafloor transponder array.     

Optimal localization of a seafloor transponder in shallow water using acoustic ranging and GPS observations

This study utilized circular and straight-line survey patterns for acoustic ranging to determine the position of a seafloor transponder and mean sound speed of the water column. To reduce the considerable computational burden and eliminate the risk of arriving at a local minimum on least-squares inversion, the position of a seafloor transponder was estimated by utilizing optimization approaches. Based on the implicit function theorem, the Jacobian for this inverse problem was derived to investigate the constraints of employing circular and straight-line survey patterns to estimate the position of a transponder. Both cases, with and without knowledge of the vertical sound speed profile, were considered. A transponder positioning experiment was conducted at sea to collect acoustic and GPS observations. With significant uncertainties inherent in GPS measurements and the use of a commercial acoustic transponder not designed for precise ranging, experimental results indicate that the transponder position can be estimated accurately on the order of decimeters. Moreover, the mean sound speed of the water column estimated by the proposed optimization scheme is in agreement with that derived from conductivity, temperature, and density (CTD) measurements.     

Precise positioning for near-bottom equipment using a relay transponder

The Marine Physical Laboratory of the Scripps Institution of Oceanography has developed an acoustic relay transponder for precise relative positioning of near-bottom instruments and geologic sampling devices. Although specifically designed to position equipment lowered on standard wire ropes without a need to maintain direct electrical contact with the surface ship, the relay transponder may be used to track free vehicles, such as deep submersibles, from the surface. The relay transponder is positioned relative to an array of bottom-anchored acoustic transponders. It is interrogated acoustically from the surface ship; it then sequentially interrogates the bottom transponders which, in turn, reply to the ship. From the measurement of the total travel time (ship to relay transponder to bottom transponder to ship) and assuming, or knowing, the sound velocity of the water, we obtain a relayed range measurement. These relayed ranges, used in conjunction with ship to bottom-transponder ranges, allow us to calculate the position of the relay transponder. A recent application of this technique is described in which several gravity core samples from the crest of the Horizon Guyot were positioned with respect to the detailed bathymetry and the geology within the area. The estimated error in positioning the samples is less than 20 m inside a navigational net extending over 100 km².Contribution of the Scripps Institution of Oceanography, new series.     

A lidar method for determining internal wave characteristics

An analytical model of lidar imaging of pycnoclinic internal waves (IWs) is developed. The IW image is shown to represent a superposition of two images: reflective and shadow. The former reflects perturbations in the profile of the light backscattering coefficient in the IW field, and the latter reflects perturbations in the optical thickness of the water layer, in which the IW disturbed the horizontal uniformity of optical characteristics. Algorithms for reconstructing the IW field from these images are proposed. It is shown that the shadow image, unlike the reflective one, is insensitive to fine details of the profiles of hydrooptical characteristics and can be used for determining IW parameters on the basis of very rough data on optical properties of water. The possibility of determining the mode composition as well as the lengths and amplitudes of IW modes is demonstrated by using the Barents Sea as an example and invoking actual and simultaneously measured profiles of the water density and light attenuation coefficient.     

Centimeter-Level Positioning of Seafloor Acoustic Transponders from a Deeply-Towed Interrogator

An array of three seafloor transponders was acoustically surveyed to centimeter precision with a deeply-towed interrogator. Measurements of two-way acoustic travel time and hydrostatic pressure made as the interrogator was towed above the array were combined in a least-squares adjustment to estimate the interrogator and transponder positions in two surveys spanning two years. No transponder displacements were expected at this site in the interior of the Juan de Fuca Plate (48^?11′ N, 127^?12′ W) due to the lack of active faults. This was confirmed to a precision of ±2 cm by least-squares adjustment. Marginally detectable blunders in the observations were shown to affect the transponder position estimates by no more than 3 mm, demonstrating the geometric strength of the data set. The accumulation of many hundreds of observations resulted in a significant computational burden on the least-squares inversion procedure. The sparseness of the normal matrix was exploited to reduce by a factor of 1000 the number of calculations. The acoustic survey results suggested that the near-bottom sound speed fields during the two surveys were in better agreement than inferred from yearly single-profile conductivity, temperature, and pressure (CTD) measurements.     

基于二次多项式拟合的超短基线声线修正方法

超短基线定位解算中的距离观测值是指换能器与水下应答器之间的直线距离,而海水声速的不均匀分布导致声波在海水中的实际传播路径为连续弯曲的曲线,需要结合实测声速剖面进行声线修正。根据声速在分层介质中的传播特性,本文提出了一种基于二次多项式拟合的声线跟踪算法,采用线性插值方法对声速剖面数据进行合理加密并按等深度进行分层,设定每层声速梯度是不断变化的,用二次多项式拟合声速,基于运动学原理建立了完整的数学解算模型。仿真结果表明,该方法修正后的水下目标分布具有明显的收敛性,且优于等梯度声线跟踪算法和等效声速剖面法,显著提高了超短基线水声定位系统的定位精度。     

Internal wave characteristics at the ASIAEX site in the east China Sea

The characteristics of internal waves (IWs) observed during the Asian Sea International Acoustics Experiment 2001 in the East China Sea are presented in this paper. Temperature data from a 17-element thermistor chain exhibit clear IW features in shallow water. Large-amplitude oscillations, up to 35 m, are noted due to the semi-diurnal internal tides. High-frequency (HF) and narrow-bandwidth IW trains around 6 c/h ride on semi-diurnal IWs. The spectrum of vertical displacement of the IWs, calculated from the thermistor chain data, falls as /spl omega//sup -1.6/ in the frequency band of 0.1-4 c/h. For higher frequencies (>6 c/h), the spectrum falls as /spl omega//sup -3.0/. Vertical coherence of the IWs for both semi-diurnal internal tides and HF IWs is analyzed. Comparisons of our observations with other data, obtained from SWARM95, the Barents Sea, and the Gulf of Mexico, display some common characteristics of shallow-water IWs.     

On generation source sites of internal waves in the Luzon Strait

This effort aims to determine the generation source sites in the Luzon Strait for energetic, long-crest, transbasin internal waves （IW） observed in the northern South China Sea （NSCS）. The roles of islands distributed on eastern side of the strait, Kuroshio, submarine ridges, shoaling thennocline, and strait configuration played in the IW generation are examined using the cruise data analysis, satellite data interpretation, and dynamical analysis. The islands and channels on eastern side of the strait are excluded from a list of possible IW source sites owing to their unmatched horizontal dimensions to the scale of IW crest line length, and the relative low Reynolds number. The Kuroshio has a potential to be a radiator for the long-crest IW disturbances, meanwhile, the Kurosbio west （east） wing absorbs the eastward （westward） propagating IW disturbance. Namely, the Kuroshio blockades the outside west-east propagating IW disturbances. The 3-D configuration of the Luzon Strait is characterized by a sudden, more than one order widening of the cross-section areas at the outlets on both sides, providing a favorable condition for IW type initial disturbance formation. In the Luzon Strait, the thermocline is featured by a westward shoaling all the year around, providing the dynamical conditions for the amplitude growth （declination） to the westward （eastward） propagating IW type disturbance. Thus, the west slope of western submarine ridge at the western outlet of the Luzon Strait is a high possibility source sites for energetic, long-crest, transbasin IWs in the NSCS. The interpretation results of satellite SAR images during a 13 a period from 1995 to 2007 provide the convincing evidence for the conclusions.     

顾及海底地形坡度的应答器互测距能力估计

研究了在海底声速梯度为正的情况下,应答器最大互测距距离的计算方法。对于平坦地形,直接基于声线跟踪的原理计算;对于倾斜地形,首先将实际声线轨迹近似为圆弧,然后利用牛顿法迭代计算初始掠射角,进一步得到两应答器间的水平距离。仿真实验结果表明,应答器的架设高度越高,最大互测距距离越远;同样架设高度,坡度越大的地形互测距距离越短。     

Internal Tides in the Coastal Waters of NE Arabian Sea: Observations and Simulations

Time series of temperature and salinity collected from a station in the NE Arabian Sea during March, April, May, October, and November was utilized to explain the behavior of internal tides. Analysis revealed the existence of semi-diurnal internal tides and high frequency (HF) internal waves (IW). It was observed that the amplitudes of HF IWs were determined by the degree of stratification in the thermocline. Corresponding to an increase in the density gradient in thermocline (0.016 kg/m⁴ in April to 0.14 kg/m⁴ in October), the temperature fluctuations due to internal tides increased from <0.2°C to >1.5°C, respectively. Brunt-Vaiisala frequency also showed similar variations (～10 cph to 22 cph). Within the thermocline, semi-diurnal internal tides caused fluctuations of >10m in the isotherm depths. A linear regression equation was fitted to parameterize the amplitude of HF IWs and its upper frequency limit in terms of thermocline gradient. The IW and one-dimensional models simulated the presence of internal tides and diurnal cycling in the temperature field, respectively. Coupling of these models showed improvement in the simulation of temperature.     

浅海海底控制点水平坐标的简化计算方法

提出了一种在声速剖面未知的条件下计算海底控制点水平坐标的方法,根据流体静力学方程将海底应答器的压力值转化为深度值,并以此深度值作为等效声速剖面法的参考深度,基于等效声速剖面法与船底换能器到海底应答器声波的传播时间计算各历元的测距值,通过圆走航利用距离交会法确定海底控制点水平方向的坐标。松花湖的实验表明,这种方法可以获得较高精度的浅海海底控制点水平方向的坐标。     

Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of /spl plusmn/30cm (2-/spl sigma/) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of /spl plusmn/8 cm (2-/spl sigma/) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to /spl plusmn/10 cm (2-/spl sigma/) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to /spl plusmn/1 m (2-/spl sigma/). Simulations show that /spl plusmn/30 cm (2-/spl sigma/) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicles in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.     

A long‐range and high‐resolution underwater acoustic positioning system

Abstract

It is desired to track the location of an underwater data collecting platform using acoustic range data. A long‐range and high‐resolution acoustic system for underwater locating has been investigated. The system provides continuous and highly accurate tracking of a platform referenced to bottom‐mounted buoys. Each reference buoy contains an acoustic transponder, which is used to obtain ranging data from the transponder to the platform. The transponder has a signal source that is phase‐modulated by a maximal‐length binary sequence and a correlation processing unit to be capable of detecting received acoustic signals with high SNR in a noisy environment or in attenuation due to long‐range propagation, and to identify multipath acoustic signals. The acoustic system has been designed and sea tests tried. The results of that experiment have yielded capability of a submeter underwater acoustic positioning system.     

Interaction between the surface and internal waves: The modulation and maser mechanisms

Two feedback mechanisms are considered which emerge from the interaction between the surface and internal waves. The energy exchange between the wave systems is coupled with the periodic variations of momentum and the losses of the wind wave momentum during their interaction with the internal wave (IW) current field. These mechanisms result in the IW attenuation which is the strongest when IW and wind propagate in the same direction. When attenuating, the IWs give rise to the inertial currents comparable to the Ekman background current.Translated by Mikhail M. Trufanov. UDK 551.466.3: 551.466.8.     

An Improvement of Long Baseline System Using Particle Swarm Optimization to Optimize Effective Sound Speed

Abstract

In long baseline (LBL) positioning system, errors due to uncertain sound speed are the major facts to its positioning accuracy. In this study, the problem is solved by setting acoustic signal travels between the target and different hydrophones with different sound speed and using particle swarm optimization algorithm to solve the multi-parameter optimization problem to obtain the sound speeds. Presented simulation results show that the proposed algorithm can effectively improve the positioning accuracy of the LBL system compared to existing algorithms and its computational efficiency is high enough.     

Characteristics of short-period internal waves in the Kara Sea inferred from satellite SAR data

In this paper we present the results of short-period internal wave (SIW) observations in the Kara Sea on the basis of satellite ENVISAT ASAR data between July and October 2007. Altogether, 248 internal wave (IW) packets and solitons are identified in 89 SAR images. Detailed spatial statistics of IW signatures and their properties in the Kara Sea is presented. The primary regions of IW activity are the areas near the Kara Gates Strait, the southeastern part of the Novaya Zemlya Trough, and in the vicinity of Cape Zhelaniya. We identify the regions where large IW packets are observed with wavelengths up to 2–3 km and the front length exceeding 200 km. The mean interpacket distance for observed IWs is about 20 km, but it may reach 50–60 km. Consequent IW packets are observed to travel up to 500 km from the presumed generation points. The results of satellite observations are compared with results of previous studies.     

Optical flow-based method to estimate internal wave parameters from X-band marine radar images

The velocity and direction of internal waves (IWs) are important parameters of the ocean, however, traditional observation methods can only obtain the average parameters of IWs for a single location or large area. Herein, a new method based on optical flow is proposed to derive the phase velocity vectors of IWs from X-band marine radar images. First, the X-band marine radar image sequence is averaged, and ramp correction is used to reduce the attenuation of gray values with increasing radial range. Second, the average propagation direction of the IWs is determined using the two-dimensional Fourier transform of the radar images; two radial profiles along this direction are selected from two adjacent radar images; and then, the average phase velocity of the IWs is estimated from these radial profiles. Third, the averaged radar images are processed via histogram equalization and binarization to reduce the influence of noise on the radar images. Fourth, a weighting factor is determined using the average phase velocity of a reference point; the phase velocities on the wave crest of the IWs are subsequently estimated via the optical flow method. Finally, the proposed method is validated using X-band marine radar image sequences observed on an oil platform in the South China Sea, and the error of the phase velocity is calculated to be 0.000 3–0.073 8 m/s. The application conditions of the proposed method are also discussed using two different types of IW packets.     

Impact of internal waves on sound propagation off Bhimilipatnam,east coast of India

Internal waves (IW) are identified off Bhimilipatnam, east coast of India, from the time series CTD (hourly interval) and thermistor chain data (2 min interval) collected during 23–25 Feb 2007. The measurements were carried out at 94 m water depth on the continental shelf edge. These data sets are used to describe the characteristics of IW and their impact on acoustic fields. Garrett and Munk (GM) model has been used to predict the characteristics of low frequency (LF) IW with space and time. Active IW are seen in the layers 54 m–94 m with a velocity of 0.548 km h⁻¹ and the wavelengths of the order of 0.03 km–21.8 km. The model could capture the IW features in the thermocline region accurately than at the bottom. This could be due to the limitation of the model which considers linearity. High frequency IW observed at the bottom could be due to the advection of tidal currents over the shallow irregular bottom in the presence of stratification. The study emphasizes linear IW rather than transient non-linear waves induced by tidal interaction with topography.     

The Relationship between Propagation Time and Sound Velocity Profile for Positioning Seafloor Reference Points

This paper focuses on the relationship between the propagation time and the integral of the sound velocity profile (SVP) with respect to depth (SVP area for short) for positioning seafloor reference points. We proved a linear relationship between the propagation time and the SVP area and defined its mathematical expression (ST law). We showed in three simulations of possible variations in SVPs (random errors, internal waves, or a combination of both) that this ST law is verified. Using this new law in a simulated experiment, the ranging residuals due to sound velocity change through time are significantly improved. With this method, monitoring the SVP while acquiring travel times to a submerged transponder can significantly reduce the positioning errors of the transponder.