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.     

Improved time-delay estimates of underwater acoustic signals usingbeamforming and prefiltering techniques

Passive sonar systems that localize broadband sources of acoustic energy estimate the difference in arrival times (or time delays) of an acoustic wavefront at spatially separated hydrophones, The output amplitudes from a given pair of hydrophones are cross-correlated, and an estimate of the time delay is given by the time lag that maximizes the cross correlation function. Often the time-delay estimates are corrupted by the presence of noise. By replacing each of the omnidirectional hydrophones with an array of hydrophones, and then cross-correlating the beamformed outputs of the arrays, the author shows that the effect of noise on the time-delay estimation process is reduced greatly. Both conventional and adaptive beamforming methods are implemented in the frequency domain and the advantages of array beamforming (prior to cross-correlation) are highlighted using both simulated and real noise-field data. Further improvement in the performance of the broadband cross-correlation processor occurs when various prefiltering algorithms are invoked     

New towed-array shape-estimation scheme for real-time sonar systems

In real-time towed-array systems, performance degradation of array gain occurs when a line array that is not straight is assumed straight in the beamforming process. In this paper, a new method is proposed for array shape estimation. The novelty of this method is that it accounts for the variations in the tow ship's speed, which are typical during course alterations. The procedure consists of two steps. First, we solve for the tow-point induced motion in the time domain based on the constraints from the tow-point compass-sensor readings and from a discretized Paidoussis equation. At each time instance, the shape estimate is solved from a linear system of equations. We also show that this solution is equivalent to a previous frequency-domain solution while the new approach is much simpler. In the second step, we use the tail compass-sensor data to adjust the overall array shape. By noting that variations in the ship speed lead to a distortion in the normalized time axis, we first register the predicted tail displacement with the tail sensor readings along the time axis. Then, distortions in the estimated array shape over its length can be compensated accordingly. We also model a slow-changing bias between sensor zeros and remove systematic sensor errors. The effectiveness of the new algorithm is demonstrated with simulations and real sea-trial data.     

The localization accuracy of a horizontal array observing anarrowband target with partial coherence

The Cramer-Rao lower bound is used to assess the potential localization accuracy of a horizontal array observing a narrowband moving target. The narrowband signal received by the array is assumed to have only partial temporal coherence, which is modeled by taking the signal to be completely coherent over a data block but with an unknown absolute phase from block to block. A numerical example for a linear array illustrates the improvement in localization accuracy caused by an increase in the signal coherence time. The effect of target/array geometry is also studied     

Using ocean ambient noise for array self-localization and self-synchronization

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.     

Deterministic three-dimensional analysis of long-range soundpropagation through internal-wave fields

A Munk profile and a set of propagating internal-wave modes are used to construct a three-dimensional time-varying ocean sound-speed model. Three-dimensional ray tracing is employed to simulate long-range sound propagation of a broadband acoustic signal. Methods are developed to convert three-dimensional ray-tracing results to acoustic time-domain amplitude and phase measurements. The ocean sound-speed model is defined deterministically, and the model acoustic receptions are analyzed deterministically. A single internal-wave mode that is “spatially synchronizes” to an arrival can coherently focus and defocus the acoustic energy. These internal waves can cause an arrival's amplitude fluctuation to mimic Rayleigh fading; however, the time-domain phase is stable, in contradiction to the classical Rayleigh fading environment where the received phase is uniformly distributed. For example, the received power attributed to an early arrival propagated over a 750-km range can fluctuate over 40 dB, while the time-domain phase remains within a quarter of a 75 Hz cycle. The characteristics of the time-domain phase are important for establishing coherent integration times at the receiver     

Simultaneous reception of long-range low-frequency backscattered sound by a vertical and a horizontal array

Simultaneous measurements of low-frequency sound generated by an explosive source and backscattered from the seafloor in the eastern Mediterranean were made with two receiver configurations: a towed horizontal array and a vertical array. Images of the scattering features on the beam-time data of the horizontal array were useful in the interpretation of the scattering process and in estimating areas of scatterers received by the vertical array, and permitted scattering strengths to be estimated for both configurations. Images of the vertical array data provided information about the vertical arrival angles at the array from specific scatterers. At long range, the sound from the scattering features was received at grazing angles less than13deg. The scattering strengths for three features varied from - 47 to - 25 dB. The mean frequency dependence over the band 125 to 700 Hz varied from 0 to 2.5 dB/octave with greater variations occurring within smaller bands.     

Joint direction-of-arrival and array-shape tracking for multiplemoving targets

The problem of tracking the directions-of-arrival (DOAs) of multiple moving sonar targets with an array of passive sensors is complicated by sensor movement. An algorithm for the joint tracking of source DOAs and sensor positions is presented to address this problem. Initial maximum-likelihood estimates of source DOAs and sensor positions are refined by Kalman filtering. Spatio-temporally correlated array movement is considered. Source angle dynamics are used to achieve correct data association. The new technique is capable of performing well for the difficult cases of sources that cross in angle as well as for fully coherent sources. Computer simulations show that the approach is robust in the presence of array motion modeling uncertainty and effectively reduces dependence on expensive and possibly unreliable hardware     

基于单矢量水听器四种方位估计方法

单矢量水听器能同时拾取声场的声压和振速信息,可以估计目标方位。根据不同的噪声背景和信号形式,单矢量水听器有多种方位估计方法。平均声强器的处理方法能很好地抗各向同性的非相干干扰;线谱方位估计能有效检测辐射线谱信号的目标,并进行目标方位估计。当宽带信号中存在线谱相干干扰,以及线谱信号被宽带相干噪声干扰时,上述两种方法不能检测目标。为解决这个问题,新提出了直方图和加权直方图两种方法,直方图方位估计法能抗强线谱相干干扰,并能区分含线谱的多目标;加权直方图方位估计法能从宽带相干干扰中检测目标,估计目标方位。并着重介绍了后两种方法的原理,对四种方法进行了计算机仿真,并用海试结果验证了上述结论。     

Optimal modal beamforming of bandpass signals using an undersizedsparse vertical hydrophone array: theory and a shallow-water experiment

Conventional methods for modal beamforming of underwater acoustic signals using a vertical-line hydrophone array (VLA) can suffer significant degradation in resolution when the array is geometrically deficient, i.e., consists of sparsely spaced elements and spans the water column partially or is poorly navigated. Designed for estimating the coefficients of the normal modes, these conventional methods include the direct projection (DP) of the data on the calculated mode shapes and least-squares (LS) fitting of the mode sum to the data. The degradation, in the form of modal cross talk or sidelobes, is a result of an undersampling in depth. This cross talk may be mitigated with the application of proper space-time filter constraints in the case of a pulse transmission. In this paper, a generalized least-squares (GLS) mode beamformer, capable of incorporating physical space-time constraints on the propagation of sound, is presented. The formulation is based on the well-known theorem of Gauss and Markov. Initialized by a model prediction of the basic arrival structure of the normal modes and incorporating, iteratively, refined estimates of the statistics of the modal fluctuations, this GLS technique strives to boost the resolution of a geometrically deficient VLA. The improvement is demonstrated using the VLA data collected during a shallow-water tomography experiment in the Barents Sea. The superiority of the GLS method over the conventional DP and LS methods is evident, providing a high-quality time series of modal arrivals as a function of geophysical time, which, in turn, reveals the dominant time scales of the oceanic processes associated with the Barents Sea Polar Front     

A method minimizing the correlation properties of a noise field andimproving detection of signals embedded in anisotropic noise

This paper deals with the development of a processing technique that improves the signal-to-noise ratio (SNR) at the single sensor for a received signal that is embedded in a partially correlated noise field. The approach of this study is unique in that the noise is treated as being non-white and partially correlated. The concept of the proposed development is based on the time interval over which the temporal coherence or correlation properties of a noise field are defined. For narrowband signals, the associated temporal coherence period is much longer than the correlation time interval of the anisotropic noise field. Thus, a coherent integration of discontinuous segments of received signals will enhance the SNR at the single sensor by lowering the correlation properties of the associated non-white noise. Reconstruction of the narrow-band signal time series, with improved SNR at the sensor will allow the use of the existing high resolution techniques to be utilized more effectively by lowering their threshold values in order to detect very weak signals. The intention here is to integrate the characteristics of the real anisotropic noise field during the preliminary processing stages of the received signals by an array of sensors. Simulations show that the proposed method can be integrated in the signal processing functionality of sonar and radar systems     

Optimization of conventional beamformer shading weights for conformal velocity sonar

A numerical optimization technique that uses sonar array noise measurements is used to determine conventional shading weights that maximize the broadband deflection coefficient at the output of the optimal square-law detector, across a frequency band of interest. This process maintains the structure of the conventional processor while providing performance improvement typical of adaptive techniques. The performance of the optimized time domain delay-and-sum beamformer is compared with that of the traditional beamformer that uses conventionally chosen shading weights. Application of this method to conformal velocity sonar array data is shown to provide large improvements in performance over heuristic designs.     

Depth measurement of remote sources using multipath propagation

A submerged acoustic source radiates narrowband Gaussian noise. Its signal propagates to a remote, large aperture vertical array over a multipath channel whose characteristics may or may not be fully known. The primary concern of this study is the accuracy of source depth estimates obtainable from the array output. Cramer-Rao bounds for the depth estimate are calculated. When the velocity profile is known exactly, the value of the bound is quite insensitive to the precise form of the velocity profile. A bound calculated from a constant velocity profile yields an excellent approximation for many situations likely to be encountered in practice. Introduction of an unknown parameter into the velocity profile has little effect on the Cramer-Rao bound for depth. However, a maximum likelihood estimator of depth working with an inaccurate value of the unknown parameter performs poorly. To obtain satisfactory performance, one must estimate the unknown parameters along with the source depth. Simulations demonstrate the success of this approach     

A time reversal damage imaging method for structure health monitoring using Lamb waves

This paper investigates the Lamb wave imaging method combining time reversal for health monitoring of a metallic plate structure. The temporal focusing effect of the time reversal Lamb waves is investigated theoretically. It demonstrates that the focusing effect is related to the frequency dependency of the time reversal operation. Numerical simulations are conducted to study the time reversal behaviour of Lamb wave modes under broadband and narrowband excitations. The results show that the reconstructed time reversed wave exhibits close similarity to the reversed narrowband tone burst signal validating the theoretical model. To enhance the similarity, the cycle number of the excited signal should be increased. Experiments combining finite element model are then conducted to study the imaging method in the presence of damage like hole in the plate structure. In this work, the time reversal technique is used for the recompression of Lamb wave signals. Damage imaging results with time reversal using broadband and narrowband excitations are compared to those without time reversal. It suggests that the narrowband excitation combined time reversal can locate and determine the size of structural damage more precisely, but the cycle number of the excited signal should be chosen reasonably.     

Multi-Bernoulli Filter for Tracking Multiple Targets Using Sensor Array

This paper presents a multi-Bernoulli filter for tracking the direction of arrival(DOAs) of time-varying number of targets using sensor array. Our method operates directly on the measurements of sensor array and does not require any detection. Firstly, more information is reserved and compared with the after-detection measurements using a finite set of detected points. It can significantly improve the tracking performance, especially in low signal-to-noise ratio. Secondly, it inherits the advantages of the multi-Bernoulli approximation which models each of the targets individually. This allows more accurate multi-target state estimation, especially when targets cross. The proposed filter does not need clustering step and simulation results showcase the improved performance of the proposed filter.     

Forward Scattering Observation With Partially Coherent Spatial Processing of Vertical Array Signals in Shallow Water

In this paper, we address the problem of detecting an inhomogeneity in shallow water by observing changes in the acoustic field as the inhomogeneity passes between an acoustic source and vertical line array of receivers. A signal processing scheme is developed to detect the perturbed field in the presence of the much stronger primary source signal, and to estimate such parameters as the time when the inhomogeneity crosses the source-receiver path, its velocity, and its size. The effectiveness of incoherent, coherent, and partially coherent spatial processing of the array signals is evaluated using models and data obtained from experiments in a lake. The effect of different bottom types is also considered, and it is shown that partially coherent processing can have a significant advantage depending on the bottom type. Estimates of the minimum input signal-to-noise ratios (SNRs) for which the diffracted signal can be observed are presented.     

A Simplified Subspace Fitting Method for Estimating Shape of a Towed Array

Although the existing array shape estimation methods using reference sources demonstrate good performance, most of the methods require at least two reference sources 2RS in known locations and the reference sources must be in far field. This communication proposes a simplified subspace fitting method that uses only a single reference source 1RS. In the proposed method, the horizontal and vertical positions of each sensor are modeled by applying fixed intersensor spacing and sensor angles, so that the unknown parameters of the cost function are reduced. Furthermore, the Cramer–Rao lower bound (CRLB) is investigated for the proposed method. The results of the numerical experiments indicate that the proposed method is practical for estimating the shape of a towed array.      

The Transarctic Acoustic Propagation Experiment and climatemonitoring in the Arctic

In April 1994, coherent acoustic transmissions were propagated across the entire Arctic basin for the first time. This experiment, known as the Transarctic Acoustic Propagation Experiment (TAP), was designed to determine the feasibility of using these signals to monitor changes in Arctic Ocean temperature and changes in sea ice thickness and concentration. CW and maximal length sequences (MLS) were transmitted from the source camp located north of the Svalbard Archipelago 1000 km to a vertical line array in the Lincoln Sea and 2600 km to a two-dimensional horizontal array and a vertical array in the Beaufort Sea. TAP demonstrated that the 19.6-Hz 195-dB (251-W) signals propagated with both sufficiently low loss and high phase stability to support the coherent pulse compression processing of the MLS and the phase detection of the CW signals. These yield time delay measurements an order of magnitude better than what is required to detect the estimated 80-ms/year changes in travel time caused by interannual and longer term changes in Arctic Ocean temperature. The TAP data provided propagation loss measurements to compare with the models to be used for correlating modal scattering losses with sea ice properties for ice monitoring. The travel times measured in TAP indicated a warming of the Atlantic layer in the Arctic of close to 0.4°C, which has been confirmed by direct measurement from icebreakers and submarines, demonstrating the utility of acoustic thermometry in the Arctic. The unique advantages of acoustic thermometry in the Arctic and the importance of climate monitoring in the Arctic are discussed. A four-year program, Arctic Climate Observations using Underwater Sound is underway to carry out the first installations of sources and receivers in the Arctic Ocean     

Efficient Sensor Characteristic Selection for Cost-Effective Distributed Sensor Networks

For distributed sensor technologies whose costs are understood (or which may be estimated in some reasonable manner), we derive a simple analytic means by which to estimate the most cost-effective sensor detection range. Specifically, we consider design of sensor nodes whose purpose is to exploit a set of coherent acoustic array technologies to detect a target with a specified radiated signature in an environment characterized by the sonar equation. We define a simplified calculus of distributed search that exploits simple target motion as a means to enhance spatial coverage for a sparse field of uniformly distributed sensor nodes. We examine this strategy in the context of both area (two-dimensional) and volume (three-dimensional) surveillance coverage under both cylindrical and spherical spreading models. In all situations, cost-effective design guidance is given based on maintaining spatial detection coverage     

Refraction and scattering into a sandy ocean sediment in the 30-40-kHz band

To observe sound penetration into a sandy sediment, a buried acoustic receiving array was insonified by a wide band sound source carried by a remotely operated vehicle. A slanting array design was used to avoid scattering artifacts. This design overcame possible problems in previous experiments, in which scattering artifacts from the array structure could be mistaken for a propagating wave. The experiments took place in a sandy sediment off the West coast of Florida, as part of the sediment acoustics experiment, which is a multidisciplinary effort to study sediment acoustics. A coherent angle, speed, and height estimation process searched through a four-dimensional search space, of source height and elevation angle, wave speed, and propagation delay to find spherical acoustic wave fronts. Three main categories of waves were found: first refracted, dominant nonrefracted and evanescent. Later acoustic arrivals, a fourth category, remain to be analyzed. Their relative intensities effectively characterize the sediment penetrating acoustic energy. The acoustic sound pressure level of penetrating waves below the critical grazing angle was found to be greater than expected for a flat interface.