Stergios Stergiopoulos

Extended towed array processing by an overlap correlator

A method of extending towed array measurements that provides an aperture greater than that of the physical array is presented. Such a technique can be used by matched‐field estimators to obtain information about the range and depth of a source and in other towed array applications requiring a very large aperture. The approach is to combine coherently the acoustic signals arriving at a moving array of hydrophones by making proper compensation through a factor that corrects for considerable fluctuations in phase irregularities in the tow path of the physical array as well as fluctuations in amplitude experienced during the coherent integration time. In this manner, the finite aperture of the physical array is exploited in a process that synthesizes the extended aperture of the method. The concept is based on an algorithm that we call an ‘‘overlap correlator,’’ which provides the phase correction factor by correlating overlapping space samples of the acoustic signal received at successive moments by the movi...

A new passive synthetic aperture technique for towed arrays

An algorithm that synthesizes apertures in the beam domain using FFT transformations and performs coherent processing of subaperture signals at successive time intervals is presented. Experimental tests of the algorithm show that for ocean environments with spatial coherence longer than the synthetic aperture length and for signals with temporal coherence longer than the required acquisition time, a synthetic array gain is achieved which roughly corresponds to the length of an equivalent fully populated array. In the experiments, transducer generated CW with phase stability and pseudorandom signals were used. Limitations on the spatial and temporal coherence were introduced only by the medium, the temporal coherence of the pseudorandom signal, and the shape and stability of the line array used. >

Optimum bearing resolution for a moving towed array and extension of its physical aperture

This paper examines the limits of the angular resolution capability of a moving towed array (MTA) by finding the Cramer–Rao lower bounds (CRLB) and provides algorithms that extend the physical aperture of an MTA. The model that is considered for the CRLB estimates assumes that an N‐hydrophone towed array is moving at a known constant speed and that in the received signal unknowns are all the parameters for two sources. The estimated CRLBs for this model indicated that an N‐hydrophone MTA provides very high angular resolution when the duration T of the received signal is very long. This ability of the moving array to resolve two closely spaced sources is related to the fact that the physical aperture has been extended by the distance traveled by the array during the T seconds of the observation period. Computer‐simulation examples using a maximum‐likelihood estimator and an extended towed array algorithm to find the bearing of sources are presented. The results of these simulations agree with the CRLB if t...

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 ships 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.

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. >

An experimental evaluation of split-beam processing as a broadband bearing estimator for line array sonar systems

This paper examines the potential improvements in bearing estimation performance of split-beam processing over full-aperture beamforming for broadband signals. It presents theoretical results in order to define the details of the signal flow in a split-beam processor, and it provides theoretical performance predictions by using Cramer–Rao lower bound (CRLB) analysis. The split-beam processing scheme was implemented in a real-time line array system. Good agreement between the theoretical performance predictions and the associated experimental broadband results were obtained. The experimental results indicate that the advantage of improved bearing estimation performance of the split-beamformer over the conventional full-aperture beamformer may be practically insignificant for passive line array applications because of the split-beamformer’s poor performance in detecting very weak broadband signals.

Nondestructive imaging of shallow buried objects using acoustic computed tomography

The nondestructive three-dimensional acoustic tomography concept of the present investigation combines computerized tomography image reconstruction algorithms using acoustic diffracting waves together with depth information to produce a three-dimensional (3D) image of an underground section. The approach illuminates the underground area of interest with acoustic plane waves of frequencies 200-3000 Hz. For each transmitted pulse, the reflected-refracted signals are received by a line array of acoustic sensors located at a diametrically opposite point from the acoustic source line array. For a stratified underground medium and for a given depth, which is represented by a time delay in the received signal, a horizontal tomographic 2D image is reconstructed from the received projections. Integration of the depth dependent sequence of cross-sectional reconstructed images provides a complete three-dimensional overview of the inspected terrain. The method has been tested with an experimental system that consists of a line array of four-acoustic sources, providing plane waves, and a receiving line array of 32-acoustic sensors. The results indicate both the potential and the challenges facing the new methodology. Suggestions are made for improved performance, including an adaptive noise cancellation scheme and a numerical interpolation technique.

Limitations on towed‐array gain imposed by a nonisotropic ocean

This study is concerned with the limitations imposed by the characteristics of the ocean on the gain of hydrophone arrays and on the process of creating a synthetic aperture by using towed‐array measurements. The multielement synthetic or physical aperture of a passive array operates in a nonisotropic noise field and in a medium of limited spatial coherence length. In particular, it is assumed that, due to distant shipping, the noise field is partially directive and imposed upon an isotropic noise background, and in the rough‐bounded, transmitted medium, many multipaths exist which are closely spaced in arrival time and arrival angle. In the theoretical development of this study, a processing scheme is suggested for the determination of the array gain from the received signal and the directivity pattern of the noise field, which is assumed to be known. Results from simulations of this processing scheme have indicated that a comparison of the experimental estimates of towed‐array gains with the above array...

An experimental study in forming a long synthetic aperture in a nonisotropic ocean

The formation of a passive synthetic aperture in a nonisotropic ocean is a complex problem that includes two main areas of investigation. The first one, which is discussed in another study, is related to the limitations imposed by the temporal and spatial coherence properties of the underwater medium on the size of an effective synthetic aperture. The second area of investigation is related to the development of an algorithm that will synthesize coherently the successive spatial information of a small moving line array into an extended aperture. The synthetic aperture techniques, that have been discussed in the open literature, however, have not been fully tested with experimental results including many targets. The present study is concerned with an experimental attempt in forming a long (i.e., 100 λ) synthetic aperture in an ocean where the spatial and temporal coherence properties of the acoustic field are known. For a multitarget environment, results indicated that an optimum synthetic aperture proces...

Acoustic 3D computed tomography for demining and archeological applications

The 3D acoustic tomography concept of this paper combines computerized tomography image reconstruction algorithms using acoustic diffracting waves together with depth information to produce a novel, three‐dimensional image of an underground section. The new method illuminates the ground with an array of acoustic sources along equally spaced points on the circumference of a surface of interest. For each transmitted pulse, the reflected–refracted signals are received by a line array of acoustic sensors located at a diametrically opposite point from the acoustic source line array. For a given depth, which is represented by a time delay in the received signal, a horizontal tomographic 2D image is reconstructed from the received projections. Integration of the depth dependent sequence of cross‐sectional reconstructed images provides a complete three‐dimensional overview of the inspected terrain. The method has been tested with an experimental system that consists of a line array of 4 acoustic sources, providin...