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Active sonar systems have been developed using larger arrays and broadband 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 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 false alarm. A model is proposed that links the number of scatterers in a resolution cell to the resulting PDF of the matched filter envelope. In particular, it is shown that when the amplitude of the echo from a single scatterer is exponentially distributed, the envelope PDF follows the K-distribution with the shape parameter related to the number of scatterers within the resolution cell. The probability of detection for an additive nonfluctuating target in K-distributed reverberation is then considered and seen to be well approximated by a location shifted gamma distribution. The SNR required to obtain a probability of detection and false alarm performance specification is then used to determine that, as long as the target is not over-resolved, decreasing the size of the resolution cell always results in an improvement in performance. The amount of improvement is then quantified by the signal excess, which was seen to less than one decibel per doubling of bandwidth when the reverberation is extremely non-Rayleigh as opposed to the expected 3 dB gain when the reverberation is Rayleigh distributed.

Multibeam sonar requires calibration for use in absolute backscattering measurements of water-column targets. In an initial study, a least-squares method has been developed to locate a standard calibration target in the sonar transmit plane from the individual transducer element receive signals. By enabling the target position to be specified in azimuth, the overall transmit and receive sensitivity may be measured simultaneously for a number of beams, considerably rationalizing the amount of work required for calibration by standard target. The localization technique has been applied to data derived from test measurements with the Simrad SM2000 Multibeam Echo Sounder, operating at 200 kHz, with 128 receive beams of nominal beamwidth 1.5/spl times/20 deg. The approximate root-mean-square phase mismatch is about 0.1 rad, and the resultant error of the predicted azimuth angle is less than 0.1 deg.

Phase-coded pingers, which came into practical application in the 1970s on fixed undersea tracking ranges, were one of the early uses of a complex waveform in undersea tracking applications. Prior to this, gated continuous-wave (CW) pings utilizing peak detection algorithms were used on undersea tracking ranges and for other navigation and tracking applications. Today's tracking and navigation applications require much higher accuracy, higher probability of detection, portability (which includes low power), and high system availability. An advanced ocean transponder (AOT) and deckset system has been developed which meets these challenges. Utilizing advanced complex digital waveform signaling techniques and high-performance commercial off-the-shelf (COTS) components, the new AOT/deckset system meets today's tracking and navigation requirements. The complex waveforms used by this system provide enhanced processing gain, narrow correlation peaks, and robust communication in highly reverberant environments. The need for portable systems has also added the challenge of extensive in-water processing requirements. New COTS, low-powered field-programmable gate-arrays, digital signal processors, and micro-controllers provide the necessary processing bandwidth to adequately detect, classify, and process the complex waveforms. These functions are provided as part of the portable in-water AOT. The low power characteristics and operational scenarios incorporated provide for long battery life. This paper describes the commercially available AOT/Deckset system, which utilizes these advanced complex waveform techniques. It includes a discussion of the signaling scheme, the processing architecture, and the detection algorithm, as well as results from recent field tests.

Conventional towed array beamformers process different frequency bands using separate hydrophone nests. While appropriate for narrowband signals, for broadband signals this approach unnecessarily throws away signal information and splits up the signal over several displays. This results in performance degradation when processing broadband signals such as impulsive echoes. AETC has developed a full-array, full-bandwidth beamformer that represents a significant improvement to the conventional towed array beamformer. We present performance predictions and results for the SQR-19 array using data recorded at sea.

In shallow tropical and subtropical waters, the ambient is often dominated by biological noise from snapping shrimp. This noise can severely limit various acoustic devices that operate between 1 kHz and several hundred kHz. There are other applications that rely on this noise as an incoherent signal for imaging. In either case, there is interest in a better characterization of the fundamental properties of the source of this noise. In particular, it would be of interest to experimentally determine its natural spatial-temporal distribution. There is an opportunity to measure these characteristics with existing horizontal arrays designed and deployed for other purposes. Arrays that are designed for low frequency operation but that record data up to a frequency of 1 kHz or more are well suited for this purpose. Because they are long, they can potentially provide very high-resolution images of sound sources close to the array. We explore the potential of using broadband fully focused beamforming with such arrays to localize nearby shrimp snaps in space and time. Depth of field and cross-range resolution for typical arrays are calculated. For some arrays we expect to be able to achieve a spatial resolution of about 2 m/spl times/10 m over the area within several hundred meters of the array. We believe this is adequate to resolve the locations of individual nearby shrimp. These expectations are compared to data taken in the Timor Sea with a multi-frequency towed array in a water depth of about 100 m. Data were recorded up to a frequency of 1200 Hz. The data clearly contain impulsive noise, highly suggestive of snapping shrimp, which dominate the ambient level above 600 Hz. We find that fully focused beamforming can clearly identify individual snaps and localize them in two dimensions. Achieved resolution is about a factor of two less than theoretical in both the cross-range and range dimensions. A preliminary count yielded a density of about 70 snaps/km/sup 2//sec in one particular region.

A new approach for processing active sonar data for the estimation of ocean bottom (sediment) acoustic scattering strength is presented. This approach uses an algorithm that compares statistical parameters derived from the output of sonar beamformed time series to the modeled eigenstructure of the ocean environment. Events (echoes) are recognized in the time series of the reverberation. Time of arrival, duration, measures of energy and higher-order moments, and an initial estimate of the georeferenced location of the echo recomputed. These derived and computed data are compared to the modeled eigenstructure of the ocean environment. The eigenstructure is computed by discretization of the ocean environment in depth and range intervals determined by sonar design characteristics. The resulting eigenstructure is sorted by time and then compared to events in the observed time series. Typically, a given arrival time observed in the time series will have multiple matches in time in the eigenstructure file. A system of tests and rules are applied to the matching eigenray set to resolve ambiguities. If the rulebased ambiguity resolution algorithm falls to identify a particular eigenray as the probable source of dominant energy, then the event (echo) is flagged and not used in the scattering strength calculation. Unique features of this automated algorithm are (1) the ability to match observed events in the time series with a specific propagation path in the ocean, (2) the ability to identify the launch angle at the sonar, (3) the ability to identify the grazing angle at the ocean bottom, (4) the ability to resolve eigenray (multipath) arrival ambiguities in shallow water, and (5) a potential to make high-angle measurements on beam pattern side lobes. Results are presented in shallow water, where the ability to resolve particular eigenrays in a multipath shallow-water environment has been previously thought to be difficult or impossible. The results also demonstrate the ability to generate georeferenced scattering strength as a function of grazing angle, utilizing this technology.

Buried target detection and classification is of significant interest to the underwater mine countermeasures (MCM) community. In order to penetrate the seafloor, low frequency sources are required. For adequate temporal (cross-range) resolution, a wideband source is necessary. In this low frequency regime, the different scattering mechanisms behave as a strong function of frequency. Buried elastic targets, such as mines, exhibit strong modal behavior, generating responses delayed in time and occupying only a fraction of the full source frequency band. There is also much more sub-bottom interaction by the lower frequencies, and as such the statistics of the reverberation signal vary with frequency. This may impact the platform motion estimation capability in synthetic aperture sonar (SAS) applications. This paper details an algorithm for exploiting the temporal and frequency diversity of the signals for SAS target detection and classification. An example is provided using an at-sea bistatic SAS application with an AUV-borne sonar receiver.

The distribution of sediment types in the south section of Kaneohe Bay was surveyed using Datasonics SIS 1502 sidescan sonar interfaced to GeoDAS (Geographical Data Acquisition System). The GIS (Geographic Information Systems) software, ERmapper, was used to classify the digital sidescan image based on color differences using an unsupervised classification technique. Sediment samples were taken from the study area and analyzed for grain size distribution to verify the use of ERmapper as a classification tool. The imagery clearly revealed different sediment properties. These differences were also distinguished by the unsupervised classification process. The authors were not able to process a sufficient number of sediment samples to allow for a statistical comparison of sediment grain size distribution and the sidescan image. However, the sediment size distribution observed in the samples they analyzed showed a clear relationship to the sidescan image. This analysis therefore indicates that the use of sidescan sonar coupled with GIS software, has the potential to be used to remotely characterize sediment types within the coastal zone.

Target signature estimation is important in passive sonar signal processing. Higher order spectral methods such as bispectral estimation are often used as a complementary tool to conventional analysis. However, these methods require larger data sets to achieve consistent estimates. The sensitivity for non-stationarities are also more pronounced for bispectral methods. Here, we use a stepwise outlier rejection algorithm as both a stationarity test and a data conditioner for experimental sonar data. We demonstrate the importance of testing for significant quadratic phase coupling and using outlier rejection as a first step in bicoherence analysis of sonar data.

This paper describes developments in the Sonar Active Boundary Loss Estimation (SABLE) project. The objective of the project is to produce high-resolution, georeferenced signal and feature attribute databases from combatant sonar systems and to derive geoacoustic parameters from the databases in support of Fleet requirements. The SABLE program is motivated by a variety of factors, including the need to operate in anisotropic (littoral and shallow-water) environments, decreasing resources, and advances in ADP hardware that now make approaches such as SABLE possible. We present an overview of a prototype full implementation of the SABLE process from the ship's sensor to the production of a gridded, georeferenced database. The overview includes the integration of environmental information into the database as well as sorted/matched eigenray information derived from a customized eigenray model. Unique features of the full database include the ability to sort/display feature attributes as a function of various constraints such as geometric conditions (sonar depression/elevation angle, azimuth of observation, bottom grazing angle, etc.), sonar acoustic parameters (pulse type, frequency, bandwidth, etc.), or any other feature attribute in the database. Examples of various data visualization and analysis tools are also presented.

Synthetic aperture sonar (SAS) is a promising technology for rapidly detecting and classifying underwater mines. Previous SAS systems have been mounted on swimming vehicles that typically undergo smoothly varying motion. EDO Electro-Ceramics built an SAS for us and Foster-Miller, Inc. mounted it on one of their underwater crawling vehicles. This SAS-Crawler operates at 160 kHz, has a theoretical resolution of 2.54 cm (1 in) in range and 1.27 cm (0.5 in) in cross-range (azimuth) out to 50 m, and is intended to operate in the very shallow water and surf zones. The motion of a bottom crawling SAS is expected to be jerky as it follows the rough terrain. This presents new problems in motion estimation and correction. Furthermore, the acoustic environment, e.g., coherence properties, of the very shallow water and surf zones is less stable than the deeper waters. The slow speed of the vehicle in conjunction with a relatively high ping rate, however, yields many overlapping, or redundant, phase centers, which should help us overcome the motion estimation and coherence problems.

An inversion method using a chirp signal and two near field hydrophones is applied to the geo-acoustical parameter estimation of the shallow water sub-bottom structure. An experiment was carried out in the East Sea using a simple experimental setup. The data are processed such that a good resolution and robustness is achieved via the matched filtering and the K-L transform. The inversion is formulated as an optimization problem which minimizes the difference between the measured and the simulated signals. The genetic algorithm is applied to the global search problem where the search space can be rotated such that parameter coupling can be exploited for a more efficient search. Numerical simulations are performed both in the original coordinates and in the rotated coordinates. Finally, the inversion method is applied to the East Sea experimental data.

Geoacoustic inversion is a topic of much interest in the underwater acoustics community these days. Shallow water geoacoustic properties include water depth, sediment layer thicknesses, sediment sound-speed profiles, density profiles, and attenuations. It is not only difficult to explicitly "measure" some of these parameters directly, it is also impossible to survey any region extensively. Thus, inversion methods which are non-invasive and which can rapidly and efficiently estimate properties over a volume are very desirable. Over the past decade the effort to develop such schemes has grown, particularly those which are MFP (matched field processing) based, i.e., which compare measured acoustic data on an array with model predictions for such data and which are sensitive to and based on environmental inputs. This presentation will briefly survey some workshops and more definitively discuss progress in a tomographic method presently under development.

Calibration of multibeam sonar in confined environments may require treatment of surface reverberation. In the case of tanks, the method of coherent echo subtraction may be employed to remove static surface reverberation, allowing the tanks to be used for standard-target calibration. This is illustrated for the Simrad SM2000 Multibeam Echo Sounder operating at 200 kHz. The signal-to-noise ratio is typically improved by about 25 dB.

A new sea mine computer-aided-detection/computer-aided-classification (CAD/CAC) processing string has been developed. The CAD/CAC processing string consists of preprocessing, adaptive clutter filtering (ACF), normalization, detection, feature extraction, classification and fusion processing blocks. The range-dimension ACF is an adaptive linear FIR filter, which is matched both to average highlight and shadow information, while also simultaneously suppressing background clutter. For each detected object, features are extracted and processed through an orthogonalization transformation, enabling an efficient application of the optimal log-likelihood-ratio-test (LLRT) classification rule, in the orthogonal feature space domain. The classified objects of 3 distinct processing strings, developed by 3 different researchers, are fused, using the classification confidence values as features and logic-based, M-out-of-N, or novel LLRT-based fusion rules. The utility of the overall processing strings and their fusion was demonstrated with new very shallow water high-resolution sonar imagery data. The processing string detection and classification parameters were tuned and the string classification performance was optimized, by appropriately selecting a subset of the original feature set. It was shown that LLRT-based fusion algorithms outperform the logic-based or the M-out-of-N ones. The LLRT-based fusion of the CAD/CAC processing strings resulted in a four-fold false alarm rate reduction, compared to the best single CAD/CAC processing string results, while maintaining a constant correct mine classification probability.

Results are presented of a geoacoustic inversion methodology applied to a variety of measured acoustic data. These data include impulsive time series data, transmission loss versus range, and broadband data from a moving surface ship. Common features of the inversions address key issues associated with the general problem of geoacoustic inversion in shallow water environments.

A method is formalised for the analysis of signals containing one or more linear chirps. This is done with a view to aiding in the evaluation of data produced through sub-bottom profiling using Hamming shaped linear chirp pulses in the frequency range of about 1 to 12 kHz. A one dimensional convolution model of the physical process is also developed, and used to demonstrate the range of possibilities open for consideration through use of the outlined technique.

To reduce the errors in detecting a target caused by the perturbed array shape, a simple pre-processing for a perturbed linear towed array is proposed. The proposed method linearizes a nonlinear phase of steering vector with position information measured by two reference sensors. It is based on the concept that a hydrophone's position may be decided by two adjacent hydrophones. Through the numerical experiments, the performance of the proposed method for the direction-of-arrival estimation is evaluated for various signal-to-noise ratio situations. The results show that the proposed method estimated the characteristic of sources with more accuracy.

Today, the US Navy's submarine fleet is equipped with a multitude of advanced, highly capable acoustic sensors and systems that are utilized in many aspects of tactical, navigational, and operational command and control (C2). These modern sonar systems collect, process, store, disseminate, and display sensor data. It is the real-time operational understanding of this collective sensor data that we term sensorspace awareness. Rapid advances in processing techniques, computing power, and sensor technology present a new challenge - sonar operators require extensive training and experience in order to deal with an enormous amount of acoustic data using limited 2D display and interaction tools - nearly unchanged in the past 25 years. Fraunhofer CRCG and the Naval Undersea Warfare Center present a highly interactive Large Scale Visualization Environment (LSVE) for performing the essential tasks of detection and classification of sonar contacts. Our prototype application employs a semi-immersive 3D display system, multiple and mixed modalities of interaction and feedback, and state-of-the-art volumetric visualization and abstraction techniques. The described use-case includes simulated passive sonar data from an advanced towed array system, chosen for both the amount and high degree of dimensionality typical of the data produced. In our scenario, an operator uses the LSVE to rapidly detect a contact in a low signal to noise environment and performs the task of classification. This paper describes the application scenario, approach and implementation of the LSVE and concludes with experiences, lessons learned, and future directions from both the interactive large-scale visualization as well as an application point of view.