ALBERT

Description: Sonar images are formed by transmitting acoustical pulses and measuring the reflected sound power from the scene surfaces. The recorded signal by a sonar device encodes information about the shape and material properties of these surfaces. In this paper, we present a detailed derivation of an image model for a new class of high-resolution lens-based 2-D forward-looking sonar systems, when the diffuse reflectance of scene/target can be characterized by the Lambertian model. A simplified single patch model is generalized to account for the finite pulse width of the transmitted beam and the simultaneous arrival of scattered signals from multiple patches at the same range in a given azimuthal direction. Validating the model using intensity measurements of isolated cylindrical targets, we then demonstrate application for multipath reflections from bottom surfaces with cylindrical and spherical targets.

Description: In India, several experimental coastal aquaculture cages have been installed on sites along the Indian coast. Many of them have been commercially successful, while some have faced technical difficulties. The construction of the Indian coastal aquaculture cage is based on ad hoc designs adopted from other countries. The performance of the coastal aquaculture cage shows that there is a need for development of a cage design with a location-specific mooring system to withstand local wave conditions for a longer period in different coastal zones of the country. This requires that designs of the cages suiting Indian conditions be made based on sound engineering principles. To design the cage, the current- and wave-induced tensions in the mooring chain and net twine and the cage motions need to be estimated. In this study, a model cage was fabricated and tested in a towing tank under different waves and towing conditions. The tension on the mooring chain was measured during model experiments along with the towing speed and wave parameter. A prototype coastal aquaculture cage with single point mooring near the coast of Visakhapatnam in South East India was identified for measurement of forces and motions. On the prototype cage, measurements of cage motions, tensions on mooring chain, and net twine and cage orientation were carried out. Characteristics of the cage response, observed in model and prototype experiments, are presented in this paper. The model cage is not a geometrically scaled down version of the prototype cage. The diameter/depth ratios of the model and the prototype are different. The mooring pattern and net twine of the model cage is the same as that of the prototype. The data presented in this paper could be used as a reference for validating the numerical model for simulating the cage forces and motions under wave and current loads.

Description: One of the challenges presented in using autonomous underwater vehicles (AUVs) for remote data collection is accurate time synchronization. In the case of bistatic acoustics, synchronization is required between AUV and source so that the time in which each ping is sent out is exactly known. There are three key obstacles to achieving this. First, the vehicle is submerged and therefore unable to access common time references directly. Second, the required accuracy in data acquisition far exceeds that possible using a computer clock to trigger data collection. Finally, to achieve accuracy in microseconds, the system must be characterized to eliminate delays introduced by filtering and analog-to-digital conversion. This paper describes the implementation and characterization of an accurate and precise timing and data acquisition system used on an AUV to collect acoustic array data. The timing was achieved using a combination of global positioning system (GPS) pulse-per-second (PPS) for synchronization on the surface and the Microsemi (Aliso Viejo, CA, USA) chip scale atomic clock (CSAC) for timing while submerged, with a PPS triggered data acquisition system. Characterization and calibration procedures were developed to ensure that the system met the experiment requirements, which included less than one percent of a wavelength error in phase, and one tenth of a meter accuracy in range. Analog and digital delays in the system were determined, and a method was demonstrated to further improve accuracy by dynamically estimating digital delays. The steps outlined in this paper for achieving precision data acquisition could be applied to many other remote systems that require similar microsecond accuracy.

Description: Underwater images suffer from blurring effects, low contrast, and grayed out colors due to the absorption and scattering effects under the water. Many image enhancement algorithms for improving the visual quality of underwater images have been developed. Unfortunately, no well-accepted objective measure exists that can evaluate the quality of underwater images similar to human perception. Predominant underwater image processing algorithms use either a subjective evaluation, which is time consuming and biased, or a generic image quality measure, which fails to consider the properties of underwater images. To address this problem, a new nonreference underwater image quality measure (UIQM) is presented in this paper. The UIQM comprises three underwater image attribute measures: the underwater image colorfulness measure (UICM), the underwater image sharpness measure (UISM), and the underwater image contrast measure (UIConM). Each attribute is selected for evaluating one aspect of the underwater image degradation, and each presented attribute measure is inspired by the properties of human visual systems (HVSs). The experimental results demonstrate that the measures effectively evaluate the underwater image quality in accordance with the human perceptions. These measures are also used on the AirAsia 8501 wreckage images to show their importance in practical applications.

Description: In this paper, an automated change detection technique is presented that compares new and historical seafloor images created with sidescan synthetic aperture sonar (SAS) for changes occurring over time. The method consists of a four-stage process: a coarse navigational alignment that relates and approximates pixel locations of reference and repeat–pass data sets; fine-scale coregistration using the scale-invariant feature transform (SIFT) algorithm to match features between overlapping data sets; local coregistration that improves phase coherence; and finally, change detection utilizing a canonical correlation analysis (CCA) algorithm to detect changes. The method was tested using data collected with a high-frequency SAS in a sandy shallow-water environment. Successful results of this multistage change detection method are presented here, and the robustness of the techniques that exploit phase and amplitude levels of the backscattered signals is discussed. It is shown that the coherent nature of the SAS data can be exploited and utilized in this environment over time scales ranging from hours through several days. Robustness of the coregistration methods and analysis of scene coherence over time is characterized by analysis of repeat pass as well as synthetically modified data sets.

Description: This paper addresses the problem of optimal acoustic sensor placement for underwater target localization in 3-D using range measurements only. By adopting an estimation theoretical framework, the optimal geometric sensor formation that will yield the best achievable performance in terms of target positioning accuracy is computed by maximizing the determinant of an appropriately defined Fisher information matrix (FIM). For mathematical tractability, it is assumed that the measurements of the ranges between the target and a set of acoustic sensors are corrupted with white Gaussian noise. For the sake of completeness, an explicit analytical result for a generic $n$ -sensor network is first obtained for the case when there is no uncertainty in the prior knowledge about the target position. The result is then extended to the practical case where the target is known to lie inside a region of uncertainty. The optimal sensor configuration thus obtained lends itself to an interesting and useful geometrical interpretation. In addition, the “spreading” of the configuration is shown to depend on the number of range measurements, target depth, and the probability distribution function that characterizes the prior knowledge about the target position. Results are also obtained for the problem of optimal sensor placement with constraints, namely, by considering that the sensors can be either located at the sea surface or distributed between the surface and the seabed. The connection between 2-D and 3-D scenarios is clarified. Simulation examples illustrate the key results derived.

Description: In this paper, experimental broadband propagation measurements in a typical sea port scenario at 5.8 GHz are presented. Measurements were performed in Cádiz Bay, Spain, by sounding the channel with a periodic pulsed signal, from which power delay profiles (PDPs) were estimated for several locations. The study focuses on propagation scenarios based on low-height antenna deployments. Experimental PDPs are found with a spiky shape that can be explained from the nature of the environment. This result is significantly different from the one expected in other regular environments (urban, indoor, etc.) where continuous shaped profiles are common. Furthermore, time-dispersive parameters, such as mean delay, delay spread, and coherence bandwidth, are extracted. The estimated values for mean delay and delay spread remain small, which indicates that the channel can be regarded as flat for typical low-bandwidth signals employed over the sea. In fact, the median value for coherence bandwidth is found to be 8 MHz, hence these channels are suitable to support relatively high data rates by using simple communication systems.

Description: In this paper, we study the problem of detection of underwater minefields amidst dense clutter as that of statistical inference under a spatial point-process model. Specifically, we model the locations (mine and clutter) as samples of a Thomas point process with parent locations representing mines and children representing clutter. Accordingly, the parents are distributed according to a homogeneous Poisson process and, given the parent locations, the children are distributed as independent Poisson processes with intensity functions that are Gaussian densities centered at the parents. This provides a likelihood function for parent locations given the observed clutter (children). Under this model, we develop a framework for penalized maximum-likelihood (ML) estimation of model parameters and parent locations. The optimization is performed using a combination of analytical and Monte Carlo methods; the Monte Carlo part relies on a birth–death–move procedure for adding/removing points in the parent set. This framework is illustrated using both simulated and real data sets, the latter obtained courtesy of Naval Surface Warfare Center Panama City Division (NSWC-PCD), Panama City, FL, USA. The results, evaluated both qualitatively and quantitatively, underscore success in estimating parent locations and other parameters, at a reasonable computation cost.

Description: The accurate modeling of underwater acoustical reflection from a wind-roughened ocean surface is a challenging problem. Some complicating factors are the presence of near-surface bubbles and the potential for shadowing of acoustical energy by parts of the surface itself. One essential factor, which is the subject of this paper, is the specular reflection of coherent plane waves at an ocean-like rough surface. We tested the accuracy of one rough surface reflection model, the small-slope approximation (SSA) approach as used by Williams (J. Acoust. Soc. Amer., vol. 116, no. 4, pp. 1975–1984, Oct. 2004), for scenarios for which scattering was entirely in the vertical plane. The SSA model was used to compute values of the coherent plane wave reflection loss per bounce for wind speeds between 5 and 12.5 m/s, frequencies between 1.5 and 9 kHz, and grazing angles between about 1 $^{circ}$ and 10 $^{circ}$ . These values were compared to those obtained from a Monte Carlo approach based on the parabolic equation (PE) method, where realistic ocean surfaces were generated based on the Pierson–Moskowitz spectrum for ocean surface heights. The SSA model compared favorably with the more rigorous PE method for most of the range of parameters considered. An approximation to the SSA model was derived for application to grazing angles less than particular values, and this approximation was shown to compare well with results from PE modeling.

Description: A general numerical model has been developed to predict the probability density function (pdf) of the magnitudes of complex pulse-compressed broadband echoes (broadband echo pdf) due to arbitrary aggregations of scatterers that are detected with a single-beam echosounder. The model is based on physics principles and rigorously accounts for the broadband frequency-dependent characteristics of the system, signal, scatterers, and the beampattern modulation effects of the sonar/radar transceiver. A key aspect to modeling the statistics of broadband echoes is accounting for the scenarios where the pulse-compressed echoes may only partially overlap. The echo statistics under those conditions will be significantly different than those associated with a narrowband system with the same center frequency but whose echoes will completely overlap given the same density of scatterers. As a result, pulse-compressed broadband echoes will generally deviate more from the Rayleigh distribution (i.e., be more “heavy-tailed”) than narrowband echoes—a feature that is critical to analyzing real-world data. As in the case of narrowband signals, the shape of the broadband echo pdf is shown to vary from strongly non-Rayleigh to Rayleigh as the number of dominant scatterers in the beam increases. The model is applied to sonar in the ocean in which the numerical density of fish is inferred using broadband echoes (30–70 kHz). The results are compared with those from conventional echo energy methods. As with narrowband systems, statistics of broadband echoes can be used to estimate the numerical density of scatterers without the need for absolute calibration of the system.