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Both copies are missing page 36.

Both copies are missing page 36.

It is the intent of this paper to outline the philosophy which undergirds the field of signal processing as it relates to instrumentation in underwater acoustics and in particular to the most prevalent application of underwater acoustics, sonar. The term signal processing as used here refers to the real time transformations carried out in sonar equipments to maximize the performance of the equipment within the limitations imposed by the ocean and its boundaries. Emphasis is placed on the limitation of the ocean and the influence of these limitations on equipment design rather than on a detailed description of the signal processing instrumentation which has been developed over the past several years.

The possibility of employing optical techniques in sonar signal processing is explored. The virtues and weaknesses of optical systems are examined, and significant developments in electronic systems are noted. State- of-the-art inadequacies of the optical system are discussed. Indications are that at the present and for the immediate future, electronic techniques remain the preferred approach to solving sonar signal processing problems. However, technological advances in the state-of-the-art in optical techniques may justify reconsideration of optics in the future.

We seek to address two important signal processing aspects of multistatic active sonar systems, namely enhanced range-Doppler imaging and improved target parameter estimation. For the former, a hybrid dense-sparse method is considered to generate range-Doppler images with both low sidelobe levels and high accuracy. For the latter, a generalized K-Means clustering (GKC) method for target association is developed to associate the range measurements from different transmitter-receiver pairs, which is actually a range fitting procedure. Moreover, the extended invariance principle-based weighted least-squares (EXIP-WLS) method is developed for accurate target position and velocity estimation. The effectiveness of the considered multistatic active sonar signal processing techniques is verified using numerical examples.

Our goal is to address fundamental signal processing research issues for enhanced multistatic active sonar systems. To effectively mitigate the reverberation problems encountered in shallow water, both probing waveform synthesis and receive filter design need to be optimized. In this report, CAN (cyclic algorithm-new) is employed to synthesize probing sequences with good aperiodic auto-correlation properties. The performance of the CAN sequences will be compared with those of pseudo random noise and random phase sequences. Two adaptive receiver designs, namely the iterative adaptive approach (IAA) and the sparse learning via iterative minimization (SLIM) method, will also be considered. IAA and SLIM will be compared with the conventional matched filter method. In addition, a fast implementation of the SLIM algorithm is presented by taking advantage of the conjugate gradient method and the fast Fourier transform.

The purpose of this note is to present a conceptual approach for using the LMSC 'Barber Pole' optical correlator to simultaneously process multichannel signals from large acoustic arrays. The principal of operation of the LMSC 'Barber Pole' correlator, the description of the experimental model, and the description of test results are reported in reference (1). Briefly, the LMSC optical correlator is a non-coherent-optical correlation detector of the stored reference type. This correlator performs real time correlation of received signals with the stored reference without a priori knowledge of the signal delays. The use of real time integration in the correlation operation avoids the problems of recording received signals on film prior to correlation with the reference. The first experimental model achieved a processing gain of 20 db for a reference record that was theoretically capable of providing a processing gain of 23 db. A substantial increase in processing gain can undoubtedly be obtained by using more densely recorded reference records and by further refining the correlator. Consideration is given here to the conceptual design of a sonar signal processing unit based on the principle of the LMSC correlator. The conceptual design is intended to be compatible with the requirements of both active and passive sonar signal processing.

The purpose of this note is to present a conceptual approach for using the LMSC 'Barber Pole' optical correlator to simultaneously process multichannel signals from large acoustic arrays. The principal of operation of the LMSC 'Barber Pole' correlator, the description of the experimental model, and the description of test results are reported in reference (1). Briefly, the LMSC optical correlator is a non-coherent-optical correlation detector of the stored reference type. This correlator performs real time correlation of received signals with the stored reference without a priori knowledge of the signal delays. The use of real time integration in the correlation operation avoids the problems of recording received signals on film prior to correlation with the reference. The first experimental model achieved a processing gain of 20 db for a reference record that was theoretically capable of providing a processing gain of 23 db. A substantial increase in processing gain can undoubtedly be obtained by using more densely recorded reference records and by further refining the correlator. Consideration is given here to the conceptual design of a sonar signal processing unit based on the principle of the LMSC correlator. The conceptual design is intended to be compatible with the requirements of both active and passive sonar signal processing.

Partial Contents: Receiver Structures and Performance Characteristics of Optimized Array and Diversity Receiving Systems; Optimization of the Detectability of Array and Diversity Receiving Systems; Optimum Demodulation and Information Extraction for Continuous Time Functions under Broad Channel Impairment Conditions; and Optimal Signal Design for Fading Incoherent Channels.

Multistatic active sonar systems involve the transmission and reception of one or more probing sequences, which provide a basis for extraction of target information in a region of interest. The probing sequences at the transmitter and signal processing at the receiver play crucial roles in the overall system performance. CAN (cyclic algorithm-new) is employed to synthesize probing sequences with good aperiodic autocorrelation properties. The performance of the CAN sequences is compared with those of pseudo random noise and random phase sequences. Two adaptive receiver designs, namely the iterative adaptive approach (IAA) and the sparse learning via iterative minimization (SLIM) method, are also considered. IAA and SLIM are compared with the conventional matched filter method. The performances of the algorithms are illustrated via numerical examples, which show that CAN, IAA, and SLIM can contribute to the overall performance improvement of the active sonar systems.

Brief summaries are given of time-varying linear system theory in both the conventional frequency-shift version and the novel frequency-shift version. The latter version of the theory is utilized to obtain the matched filter output for the bistatic echo from a moving, turning line target for a linear FM pulse in a non-dispersive medium.

The bottlenose dolphin has evolved a unique system of biosonar, or echolocation, that allows it to exploit a visually limited littoral niche. The effectiveness of dolphin echolocation at finding and identifying submerged objects is unsurpassed by man-made hardware systems built for similar tasks. It has become a model system from which to draw hardware and signal processing design concepts and is the basis for the development of biomimetic mine-hunting systems of the shallow water (SW) and very shallow water (VSW) zones. The Dolphin Based Sonar (DBS) system is a proof-of-concept sonar system designed for operations in the SW/VSW environments that incorporates characteristics of dolphin sonar transmission and structural elements of dolphin auditory anatomy. Specifically, the system is designed to reproduce dolphin signal types and source levels and to match the animal's transmission and reception beam patterns, directivity indices, receive sensitivity, and auditory filtering capabilities.

Computer simulation studies of two frequency domain adaptive beamforming algorithms are presented. The two algorithms are the frequency domain adaptive beamforming modified least-mean-square algorithm, and the proposed new frequency domain adaptive beamforming algorithm based on modified adaptive linear prediction-error filtering. The simulation studies were conducted to determine the multiple broadband target localization capability and the full angular coverage capability of the two algorithms. The number of iterations that the adaptive algorithms took to reach a minimum estimation error was determined. The algorithms were evaluated at several signal-to-noise ratios. Finally, using the results of the simulation studies a comparison between the two algorithms is performed. (RH)

A biologically inspired acoustic imaging process is obtained by considering the constraints (e.g., limited array/aperture size), and advantages (e.g., unrestricted motion, wide bandwidth) of biological sonar systems. The resulting system is easy to implement compared with conventional synthetic aperture sonar. Images are formed sequentially; an image is updated on the basis of each new echo. The processor is Doppler tolerant and is tolerant of target-induced phase shifts that occur as the result of aspect changes. The simplicity of the process leads to generalized images that can distinguish between smooth and rough surfaces and that can detect and classify low-reflectivity targets in strong volume clutter. Accuracy is improved via an associative gradient descent algorithm that can converge rapidly to a global minimum by utilizing prior information, extra acoustic information that is not directly used for imaging (e.g., resonance phenomena), and non-acoustic cues. An image-based tracker exploits sequential image formation and parallel processing to compensate for motion perturbations and to model simultaneous imaging and pursuit of prey in animal sonar systems. The imaging system is remarkably robust with respect to sparse angular sampling; image quality degrades gracefully as angular sampling intervals are increased well beyond conventional limits. Insights into neurophysiology and cognition include the utilization of top-down, bottom-up processing to obtain high resolution images from low resolution representations, as in the superior colliculus.

A biologically inspired acoustic imaging process is obtained by considering the constraints (e.g., limited array/aperture size), and advantages (e.g., unrestricted motion, wide bandwidth) of biological sonar systems. The resulting system is easy to implement compared with conventional synthetic aperture sonar. Images are formed sequentially; an image is updated on the basis of each new echo. The processor is Doppler tolerant and is tolerant of target-induced phase shifts that occur as the result of aspect changes. The simplicity of the process leads to generalized images that can distinguish between smooth and rough surfaces and that can detect and classify low-reflectivity targets in strong volume clutter. Accuracy is improved via an associative gradient descent algorithm that can converge rapidly to a global minimum by utilizing prior information, extra acoustic information that is not directly used for imaging (e.g., resonance phenomena), and non-acoustic cues. An image-based tracker exploits sequential image formation and parallel processing to compensate for motion perturbations and to model simultaneous imaging and pursuit of prey in animal sonar systems. The imaging system is remarkably robust with respect to sparse angular sampling; image quality degrades gracefully as angular sampling intervals are increased well beyond conventional limits. Insights into neurophysiology and cognition include the utilization of top-down, bottom-up processing to obtain high resolution images from low resolution representations, as in the superior colliculus.

Pulsed active sonar (PAS) and continuous active sonar (CAS) in the presence of strong direct blast are studied for the Doppler-tolerant linear frequency modulation waveform. A comparative analysis is done between PAS and CAS based on the range-Doppler and range-compression images obtained. Further, a generalized likelihood ratio (GLR) based criterion is developed for moving target detection. CAS is further studied with a Doppler-sensitive SHAPE waveform and a weighted range-Doppler processing for CAS-SHAPE system is introduced. In-water experimentation results are provided, aimed at highlighting the merits and limitations of PAS and CAS, as well as Doppler-tolerant and Doppler-sensitive waveforms in the presence of a strong delay and Doppler-spread direct blast.

Thesis advisor(s): Ziomek, Lawrence J

This report summarizes linear time-varying systems theory and its application to active sonar signal processing when certain of the parameters are random variables. To keep the analysis within reasonable bounds the random parameters are described by their first- and second-order moments only. The report starts by outlining the theory and follows by describing two possible applications: one for the derivation of the scattering function of a moving, turning line target; the other in the use of pulse trains to measure scattering functions. (Author)

Continuous active sonar (CAS) in the presence of strong direct blast is studied for the Doppler-tolerant linear frequency modulation waveform. A receiver design which is capable of detecting slowly moving, stationary, or tangentially moving targets, while simulatenously providing low false alarm rate is developed. This receiver uses an adpative beamformer to suppress the direct blast signal to a managable level, so that effective target detection with acceptable false alarm performance is achieved. In fact, the developed receiver is capable of detecting the target signal without false alarms caused by the direct blast using only a single-ping of received data. The performance of the system with either a DAS beamformer, or one of three Capon beamformer variants is examined.

Our objectives are (1) to consider multistatic continuous active sonar (MCAS) systems, aka high duty cycle systems, that involve the transmission and reception of multiple continuous probing sequences or long duty cycle sequences; (2) to investigate the merits of MCAS on its ability to achieve significantly enhanced target detection and parameter estimation performance through exploiting the advantages of continuous illumination and spatial diversity; (3) to develop innovative techniques to synthesize spectrally contained continuous sequence sets with low correlation sidelobe levels for the MCAS transmission, so that the so-generated sequences meet the spectral containment restrictions and the weak correlations among the received echoes can be exploited to improve the target detection performance, and (4) to apply robust, computationally efficient, and data-adaptive receiver signal processing algorithms to effectively mitigate the mutual interferences among multiple transmitters and suppress direct blasts for MCAS systems

Synthetic Aperture Sonar (SAS), the underwater sound application of the aperture synthesis method borrowed from Synthetic Aperture Radar (SAR), is investigated by means of ocean medium effects, platform motion effects, and signal processing effects. Synthetic aperture methods, both in radar and in sonar, allow large synthesized apertures, or equivalently synthesized arrays, to be formed from the combined use of signal processing and a small, real aperture transducer (e.g. a radar or a sonar transducer). Large synthetic apertures give rise to relatively narrow synthesized beams, which in turn allow very high resolution focused images to be formed from radar or sonar data. The target in the synthetic aperture sonar case could be the ocean bottom, and the objective could be, for example, ocean-bottom mapping or searching for a sunken vessel.... Synthetic Aperture Sonar(SAS) , Synthetic aperture radar, Acoustical imaging, Radar imaging

Computer simulation studies of two frequency domain adaptive beamforming algorithms are presented. The two algorithms are the frequency domain adaptive beamforming modified least-mean-square algorithm, and the proposed new frequency domain adaptive beamforming algorithm based on modified adaptive linear prediction-error filtering. The simulation studies were conducted to determine the multiple broadband target localization capability and the full angular coverage capability of the two algorithms. The number of iterations that the adaptive algorithms took to reach a minimum estimation error was determined. The algorithms were evaluated at several signal-to-noise ratios. Finally, using the results of the simulation studies a comparison between the two algorithms is performed.

The purpose of this paper is to delineate the signal processing gain which can be achieved at sea with the AN/SQS 26 sonar system. This is done by describing partial results of a program of work in which signal processing is analyzed by simuating the processing technique to be investigated on a digital computer. Specifically, we consider the processing gains which can be obtained by using as a transmitted pulse a 0.5 second duration linear FM slide with a bandwidth of 100 cps at a center frequency of 3.4 kc and a system bandwidth of about 130 cps.

This work has involved the use of the Naval Postgraduate School's TRITECH ST 725 high frequency mechanically scanned sonar system to acquire sonar images of simulated surface roughness on an aluminum plate. Signal post processing for such image data is reviewed, and post processed data is analyzed and compared to the known roughness locations on the plate. The simulated roughness pattern of one half inch steel nuts is used as a preliminary experiment in the development of a sonar detection system for marine growth on ship hull plating. Such a sonar system will be an integral part of any ship hull cleaning robot (SHACR). Contained in this report is a description of the experimental arrangement, typical sonar returns, a summary of image processing techniques, and results of processed data. The algorithms presented here will ultimately lead to a real time processing capability for the specification of location and extent of roughness as needed for the automatic direction of the robot's motion. Sonar imaging, Robotics, Hull cleaning, Autonomous systems.

A previous report entitled 'The First Cut Signal Processor for Conformal/Planar Array Sonar System', in which a feasible signal processing sub-system for the C/PAS System was recommended, contains much of the background material for the work reported here. In the course of this work various techniques were studied under the requirements imposed by the C/PAS System concept. It was deemed appropriate to review the technique studies that were previously made and to determine their range of applicability in terms of signal bandwidth and number of beams to be processed. Therefore, the purpose of the work reported here is to: (1) Review the technique studies previously conducted to determine feasibility of these techniques for various combinations of signal bandwidth and number of beams. Specifically, bandwidths in the range of 50 to 200 cps for the wideband signals, 1 to 4 cps for the narrow band signal, and number of beams in the range of 100 to 1000 were assumed. (2) Extend the studies previously conducted to include cost and size estimates for hardware implementations of the First Cut Signal Processing Sub-System, as well as for other techniques studied, as a function of signal bandwidth and number of beams. (3) Identify critical components of the most promising techniques which require further detailed study and/or laboratory breadboard work in order to raise confidence level for application to the C/PAS System. Section II contains a summary of the important features of the signal processing techniques which have been studied to date as well as a table showing requirements under which studies were made.

A previous report entitled 'The First Cut Signal Processor for Conformal/Planar Array Sonar System', in which a feasible signal processing sub-system for the C/PAS System was recommended, contains much of the background material for the work reported here. In the course of this work various techniques were studied under the requirements imposed by the C/PAS System concept. It was deemed appropriate to review the technique studies that were previously made and to determine their range of applicability in terms of signal bandwidth and number of beams to be processed. Therefore, the purpose of the work reported here is to: (1) Review the technique studies previously conducted to determine feasibility of these techniques for various combinations of signal bandwidth and number of beams. Specifically, bandwidths in the range of 50 to 200 cps for the wideband signals, 1 to 4 cps for the narrow band signal, and number of beams in the range of 100 to 1000 were assumed. (2) Extend the studies previously conducted to include cost and size estimates for hardware implementations of the First Cut Signal Processing Sub-System, as well as for other techniques studied, as a function of signal bandwidth and number of beams. (3) Identify critical components of the most promising techniques which require further detailed study and/or laboratory breadboard work in order to raise confidence level for application to the C/PAS System. Section II contains a summary of the important features of the signal processing techniques which have been studied to date as well as a table showing requirements under which studies were made.