"Principal component inverse" algorithm for detection in thepresence of reverberation

Detection in the presence of reverberation is often difficult in active sonar, due to the reflection/diffusion/diffraction of the transmitted signal by the ocean surface, ground, and volume. A modelization of reverberation is often used to improve detection because classical algorithms are inefficient. A commonly used reverberation model is colored and nonstationary noise. This model leads to elaborate detection algorithms which normalize and whiten reverberation. In this paper, we focus on a more deterministic model which considers reverberation as a sum of echoes issued from the transmitted signal. The Principal Component Inverse (PCI) algorithm is used with this model to estimate and delete the reverberation echoes. A rank analysis of the observation matrix shows that PCI is efficient in this configuration under some conditions, such as when the transmitted signal is Frequency Modulated. Both methods are validated with real sonar surface reverberation noise. We show that whitening has poor performance when reverberation and target echo have the same properties, while PCI maintains the same performance whatever the reverberation characteristics. Further, we extend the algorithms to spatio-temporal data. We propose a new algorithm for PCI which allows better echo separation. This new method is shown to be more efficient on real spatio-temporal data     

Real- and synthetic-array signal processing of buried targets

Results from two field experiments aimed at investigating the detection and classification of buried targets are presented. In both experiments a 2-16-kHz parametric source was used. In the first experiment, the source was used in combination with a 12-m horizontal line array and in the second with a 1.4-m vertical line array which was displaced horizontally along an underwater rail to form a 10 m /spl times/ 1.4 m two-dimensional synthetic aperture sonar (SAS). To increase the SAS integration time, the parametric source was electronically scanned in azimuth during the displacement along the rail, as in spotlight mode. It is shown that both arrays allow important signal-to-reverberation gains, enhancing the detection of sub-bottom echoes. A new, environmentally adaptive, matched filter which further improves the signal to reverberation ratio while allowing discrimination between proud and buried targets is presented and validated experimentally. The use of resonant scattering for target classification of buried objects is discussed, in the particular case of spherical shells.     

Adaptive noise canceling applied to Sea Beam sidelobe interferencerejection

An adaptive noise cancelling (ANC) technique involving a joint-process deterministic least-squares lattice filter was applied to the Sea Beam bathymetric survey system data. The filtering scheme used in Sea Beam adversely affects the underlying acoustic return and may also lead to bathymetric artifacts. The authors investigate a possible remedy for this sidelobe interference problem offered by ANC coupled with signal preservation, provided both amplitude and phase information. The joint-process deterministic least-squares lattice is the adaptive filter of choice because of its superior transit response in the presence of power discontinuities. A REVGEN (reverberation generator) simulation (R.P. Goddard, 1985) of the Sea Beam system provided support for the proposed filtering technique. A complex data acquisition system was designed and built to record the in-phase and quadrature component of Sea Beam returns. Initial ANC processing of these recorded Sea Beam data provided satisfactory sidelobe interference cancellation with no noticeable degradation of the actual bottom returns     

Mean–Standard Deviation Representation of Sonar Images for Echo Detection: Application to SAS Images

This paper addresses the detection of underwater mines echoes with application to synthetic aperture sonar (SAS) imaging. A detection method based on local first- and second-order statistical properties of the sonar images is proposed. It consists of mapping the data onto the mean-standard deviation plane highlighting these properties. With this representation, an adaptive thresholding of the data enables the separation of the echoes from the reverberation background. The procedure is automated using an entropy criterion (setting of a threshold). Applied on various SAS data sets containing both proud and buried mines, the proposed method positively compares to the conventional amplitude threshold detection method. The performances are evaluated by means of receiver operating characteristic (ROC) curves.     

Active sonar detection in shallow water using the Page test

The use of active sonar in shallow water results in received echoes that may be considerably spread in time compared to the resolution of the transmitted waveform. The duration and structure of the spreading and the time of occurrence of the received echo are unknown without accurate knowledge of the environment and a priori information on the location and reflection properties of the target. A sequential detector based on the Page test is proposed for the detection of time-spread active sonar echoes. The detector also provides estimates of the starting and stopping times of the received echo. This signal segmentation is crucial to allow further processing such as more accurate range and bearing localization, depth localization, or classification. The detector is designed to exploit the time spreading of the received echo and is tuned as a function of range to the expected signal-to-noise ratio (SNR) as determined by the transmitted signal power, transmission loss, approximate target strength, and the estimated noise background level. The theoretical false alarm and detection performance of the proposed detector, the standard Page test, and the conventional thresholded matched filter detector are compared as a function of range, echo duration, SNR, and the mismatch between the actual and assumed SNR. The proposed detector and the standard Page test are seen to perform better than the conventional thresholded matched filter detector as soon as the received echo is minimally spread in time. The use of the proposed detector and the standard Page test in active sonar is illustrated with reverberation data containing target-like echoes from geological features, where it was seen that the proposed detector was able to suppress reverberation generated false alarms that were detected by the standard Page test     

多波束测深海底混响仿真研究

海底混响是海洋混响的重要组成部分,采用模拟仿真进行验证分析,是仿真技术的一项重要应用。采用单元散射模型,研究在单发射阵元下,分布在同一直线上多个接收阵元接收的海底回波,忽略声波传播的相位起伏,只考虑振幅起伏,将传播损失、声吸收系数、海底反射损失、海底沉积层密度等参数带入海底混响仿真数学模型,仿真海底混响,使其更加接近海底的实际情况。     

Morphological and statistical approaches to improve detection in the presence of reverberation

The detection of a target echo in a sonar image is usually a difficult task since the reverberation, consisting of a large number of spurious echoes, generates a lot of false alarms. In this paper, we propose two new detectors derived from image processing algorithms. These detectors are respectively based on a morphological and a statistical contrast. Each detector only requires setting a few parameters. This setting is done using some prior knowledge about the data (shape of the emitted signal and the used antenna, characteristics of the reverberation). Nevertheless, an extensive statistical study of the detection performances proves that the proposed methods are robust and that even an imprecise setting of the parameters leads to satisfactory results. Applied to the real data, these detectors and their sequential combination lead to a significant improvement on the performances: The false alarm rate is drastically reduced while the detection probability is preserved. Based on different contrasts, these detectors have complementary behaviors. Therefore, a further improvement is achieved by a fusion of the different results to classify the remaining echoes as whether spurious or true detection.     

海洋测深中垂直分置海底混响信号的仿真分析

海洋测深中,海底混响信号是测深仪回波信号检测的主要内容。测深仪通常采用信号的相关处理方法对其进行检测,因此在设计测深仪的回波处理单元时,系统地分析海底混响信号的相关特性就显得尤为重要,对混响信号仿真是分析其特性的有效手段。基于单元散射理论,依据海底散射系数的空间相关半径划分散射单元,给出垂直分置海底混响信号的仿真方法。研究结果表明,该模型物理意义明确,计算简单。仿真得到的海底混响信号具有非常好的空间相关性和时间自相关性,与实测的海底混响信号相符,可用于对混响场特性的分析,改善测深仪的设计,从而有效提高测深仪的测量精度。     

ROC evaluation of adaptive beamforming in a simulated shallow waterenvironment

An optimal evaluation of adaptive beamforming techniques in a reverberation-limited shallow water environment is presented. A comprehensive simulation, using the sonar simulation toolset (SST) software in conjunction with the generic sonar model (GSRT) software, is used to create realistic beam data complete with target, noise, and reverberation. Adaptive beamforming techniques from the recursive least squares (RLS) family are applied to enhance detection performance via interference rejection. Two techniques are considered: linearly constrained beamforming using the minimum variance distortionless response (MVOR) beamformer and constrained adaptive noise cancelling (ANC) using the joint process least squares lattice (JPLSL) algorithm. Target detection trials, summarized in the form of receiver operator characteristics (ROC), are used to evaluate performance of the two adaptive beamformers. Results demonstrate mixed performance in reverberation-limited shallow water environments     

A model for numerical simulation of nonstationary sonar reverberation using linear spectral prediction

An innovative approach to the numerical generation of nonstationery reverberation time series is presented and demonstrated. The computer simulated reverberation time series are of high quality, in that they are accurate representations of those which would result from an actual sonar system (transmit/receive and horizontal/ vertical beampatterns; pulse type, shape, length, and power; frequency and sampling rate), platform (speed and depth), and environment (wind speed and direction, backscattering strengths, and propagation loss). Volume, surface, and/or bottom reverberation as seen by a multiple beam sonar on a moving platform is generated. The approach utilizes recent developments in linear spectral prediction research in which the spectra of stochastic processes are modeled as rational functions and algorithms are used to efficiently compute optimal estimates of coefficients which specify the spectra. A two-fold sequence is formulated; first, the expected reverberation spectra for all beams are predicted and, second, the stochastic time series are generated from the expected spectra. The expected spectra are predicted using a numerical implementation, referred to as the REVSPEC (reverberation spectrum) model, of a general formulation of Faure, Ol'shevskii, and Middleton. Given the spectra, the Levinson-Durbin method is used to solve the Yule-Walker equations of the autoregressive formulation of linear spectral prediction. The numerical implementation of the approach, referred to as the REVSIM (reverberation simulation) model, produces nonstationary coherent multiple-beam reverberation time series. The formulation of the REVSIM model is presented and typical results given. A comparison is made between the simulation outputs of the REVSIM model and those of the REVGEN (reverberation generator) model, a standard well-accepted time series simulation model, to demonstrate the validity of the new approach.     

Adaptive port-starboard beamforming of triplet sonar arrays

For a low-frequency active sonar (LFAS) with a triplet receiver array, it is not clear in advance which signal processing techniques optimize its performance. Here, several advanced beamformers are analyzed theoretically, and the results are compared to experimental data obtained in sea trials. Triplet arrays are single line arrays with three hydrophones on a circular section of the array. The triplet structure provides the ability to solve the notorious port-starboard (PS) ambiguity problem of ordinary single-array receivers. More importantly, the PS rejection can be so strong that it allows to unmask targets in the presence of strong coastal reverberation or traffic noise. The theoretical and experimental performance of triplet array beamformers is determined in terms of two performance indicators: array gain and PS rejection. Results are obtained under several typical acoustic environments: sea noise, flow noise, coastal reverberation, and mixtures of these. A new algorithm for (beam space) adaptive triplet beamforming is implemented and tuned. Its results are compared to those of other triplet beamforming techniques (optimum and cardioid beamforming). These beamformers optimize for only one performance indicator, whereas in theory, the adaptive beamformer gives the best overall performance (in any given environment). The different beamformers are applied to data obtained with an LFAS at sea. Analysis shows that adaptive triplet beamforming outperforms conventional beamforming algorithms. Adaptive triplet beamforming provides strong PS rejection, allowing the unmasking of targets in the presence of strong directional reverberation (e.g., from a coast) and at the same time provides positive array gain in most environments.     

Seafloor profiling by a wideband sonar: simulation,frequency-response optimization, and results of a brief sea test

An ahead-looking probe of some kind, optical or acoustic, is critical when one is attempting seafloor exploration from a mobile platform. A single-frequency, split aperture sonar system can be used for this purpose, but a wideband monopulse sonar offers many advantages. It computes a running estimate of the vertical directional cosine of the source of the echo, and can thus reveal the positions of multiple wave scatterers as long as their echoes can still be time resolved. Theoretical studies of its performance have been made previously, but were directly applicable only to extremely simple seafloor geometries. A new time-domain digital simulation that largely circumvents this limitation has been developed. The simulation also provides a means for testing the theory and optimizing system parameters. The reverberation model does not account for some features of acoustic backscattering such as diffraction, but it is believed to be adequate for the investigation of most signal processing aspects of the sonar system. The theory of the simulation is developed and several examples are presented and discussed. In addition, some preliminary results are presented from a sea test that used the air-sea interface as a surrogate seafloor     

Spatio-temporal processing of coherent acoustic communication datain shallow water

Achieving reliable underwater communication in shallow water is a difficult task because of the random time-varying nature of multipath propagation. When the product of Doppler-related signal bandwidth spread and multipath-related time spread of the channel is larger than one, some types of adaptive signal processing may not work very well. In this paper, various methods of coherent space-time processing are compared for a condition of a marginally overspread channel operating at 50 kHz. Various combinations of suboptimal spatially adaptive and time adaptive methods are considered. The coherent path beamformer (CPB) and recursive least squares (RLS) adaptive beamformer, both in combination with RLS time filtering, are analyzed. Also considered in the analysis is the combined RLS space-time optimal adaptive processor. Many experiments using broad-band phase-shift-keyed transmissions in shallow water have been conducted to provide data for testing these various processing methods. Because of the rapid time variation of the multipath, the product of bandwidth spread and time spread at this test site approached unity. In this environment, a suboptimal approach consisting of the adaptive beamformer followed by RLS equalization reduced reverberation and transmission errors     

Signal excess in K-distributed reverberation

Active sonar systems have recently been developed using larger arrays and broad-band sources to counter the detrimental effects of reverberation in shallow-water operational areas. Increasing array size and transmit waveform bandwidth improve the signal-to-noise ratio-and-reverberation power ratio (SNR) after matched filtering and beamforming by reducing the size of the range-bearing resolution cell and, thus, decreasing reverberation power levels. This can also have the adverse effect of increasing the tails of the probability density function (pdf) of the reverberation envelope, resulting in an increase in the probability of a false alarm. Using a recently developed model relating the number of scatterers in a resolution cell to a K-distributed reverberation envelope, the effect of increasing bandwidth (i.e., reducing the resolution cell size) on detection performance is examined for additive nonfluctuating and fluctuating target models. The probability of detection for the two target models is seen to be well approximated by that for a shifted gamma variate with matching moments. The approximations are then used to obtain the SNR required to meet a probability of detection and false-alarm performance specification (i.e., the detection threshold). The required SNR is then used to determine that, as long as the target and scatterers are not over-resolved, decreasing the size of the resolution cell always results in an improvement in performance. Thus, the increase in SNR obtained by increasing bandwidth outweighs the accompanying increase in false alarms resulting from heavier reverberation distribution tails for K-distributed reverberation. The amount of improvement is then quantified by the signal excess, which is seen to be as low as one decibel per doubling of bandwidth when the reverberation is severely non-Rayleigh, as opposed to the expected 3-dB gain when the reverberation is Rayleigh distributed.     

A programmable microcomputer-based sonar echo processor for real-time processing

A programmable microcomputer-based sonar echo processor has been developed and field tested to process echo data in real time. The device was designed for use in fisheries acoustics and to analyze echoes from the sea floor. The instrument simultaneously performs the functions "echo integration" and "echo peak detection' on the sonar signal. The "echo integration" circuitry measures the relative acoustic energy of the echo while the "echo peak detection' electronics measure the probability density function (PDF) of the peak of the echo envelope. Each process is gated so the echoes may be processed in many short time intervals. In fisheries applications, estimates can be made of fish density (fish per unit volume), average backscattering cross section, and a combination of fish size and behavior. When using the device to analyze bottom echoes, it is possible to measure both the returned energy from the bottom sediment interfaces and the microrelief characteristics. The durability, flexibility, computer link, and floppy-disk data-storage features of the system are discussed. Data are presented of the processed echoes from biological organisms and the ocean bottom from a recent research cruise on the Atlantic Ocean near Cape Hatteras, NC. The biological results illustrated the organisms to be clearly divided into two separate spatial distributions-an observation not obvious from a standard echogram which was simultaneously used. The results from the bottom showed both 1) the difference in sub-bottom structure between two locations and 2) changes in microrelief of the water-bottom interface between another pair of locations.     

Adaptive processing of microwave sea echo for the suppression ofstrong reflections from scatterers

Adaptive processing of microwave sea echo is proposed for the suppression of strong reflections from scatterers on the ocean surface. An adaptive method using echoes of pulses at different carrier frequencies is also shown to be effective for this purpose. It is noted that nonadaptive processing of the same data does not suppress these reflections. Results obtained by processing coherent microwave backscatter from the ocean surface are presented     

Simulation of non-Rayleigh reverberation and clutter

The simulation of active sonar reverberation time series has traditionally been done using either a computationally intensive point-scatterer model or a Rayleigh-distributed reverberation-envelope model with a time-varying power level. Although adequate in scenarios where reverberation arises from a multitude of scatterers, the Rayleigh model is not representative of the target-like non-Rayleigh reverberation or clutter commonly observed with modern high-resolution sonar systems operating in shallow-water environments. In this paper, techniques for simulating non-Rayleigh reverberation are developed within the context of the finite-number-of-scatterers representation of K-distributed reverberation, which allows control of the reverberation-envelope statistics as a function of system (beamwidth and bandwidth) and environmental (scatterer density and size) parameters. To avoid the high computational effort of the point-scatterer model, reverberation is simulated at the output of the matched filter and is generated using efficient approximate methods for forming K-distributed random variables. Finite impulse response filters are used to introduce the effects of multipath propagation and the shape of the reverberation power spectrum, the latter of which requires the development of a prewarping of the K distribution parameters to control the reverberation-envelope statistics. The simulation methods presented in this paper will be useful in the testing and evaluation of active sonar signal processing algorithms, as well as for simulation-based research on the effects of the sonar system and environment on the reverberation-envelope probability density function.     

Shallow-water reverberation level: measurement technique and initial reference values

A quality database of reverberation is absolutely essential if one is to understand the shallow-water reverberation problem. However, to get wideband reverberation levels (RL) simultaneously for both short and long ranges at low- and mid-frequencies is a delicate task that can be subject to errors. This paper introduces a simple method to get RL for the Asian Sea International Acoustics Experiment in the East China Sea (ASIAEX01). Special attention is paid to the measurements of the RL at short- and mid-ranges. With this method, one does not need to accurately calibrate hydrophones and measurement systems, or to measure absolute source level (SL). It can avoid signal overflow and saturation problems caused by powerful sound sources. The RL (relative to SL) at 1 s (or at 2 s) after an explosive source is detonated is defined as the initial reference reverberation level (IRRL). The IRRLs from four sites with different sandy sediments and different water depths have been given as a function of frequency in the 150-2500 Hz range. A mathematical model gives a physical explanation of the measured IRRL data. The resultant RL and IRRL may offer some reference values for the design of reverberation measurements or numerical simulations of shallow-water reverberation and bottom scattering.     

Under-ice reverberation rejection

An adaptive joint process structure is described which rejects under-ice reverberation by taking advantage of the spatial separation between acoustic backscatter returning from small elevation angles and transmitted energy reflected off the sea surface.     

Modeling of volume reverberation in the ray approximation

We propose a numerical model for the evaluation of the three-dimensional scattering of sound in the sea. The model is based on the construction of ray patterns both for the primary and secondary (scattered) radiation. The intensity of secondary radiation is expressed via the coefficient of backward volume scattering interpreted as the fraction of backward-scattered acoustic energy per unit length of the primary ray. It is shown that, in the first approximation, it suffices to consider the secondary rays repeating the paths of the primary rays in the opposite direction. The attenuation of the intensity of sound along the paths of the primary and secondary rays is taken into account. The results of numerical analysis of the reverberation signal as a function of time are presented for various conditions (different depths of immersion of the antenna and widths of the directional diagram and the presence of sound-scattering layers). The proposed approach can be used for the purposes of modeling of the surface and bottom reverberation and for the solution of the inverse problems of underwater acoustics. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 50–58, September–October, 2007.