Description: A novel and fully automatic rigid fish cage system has recently been developed for deployment in the waters of Korea. The cage structure has 12 sides, incorporating a steel framework with a diameter and depth of 5.92 and 2.91 m, respectively. Attached to the steel framework is a housing for motor valves controlling variable ballast tanks, eight housings for two air compressors, a main control system, four batteries, a reserve air tank, four high air pressure tanks, 12 variable ballast tanks, and a seawater pump housing. The net of the fish cage is tightened across the frame to minimize volume reduction due to currents. The cage is outfitted with a control station located above the valve housing. With the control system, the buoyancy can be adjusted by utilizing compressed air stored in the four high air pressure tanks. The mechanical components of the ballast systems are operated by automated software that incorporates control and monitoring algorithms. The software initiates control of the ballasting components so the fish cage system can submerge if a preselected sea state occurs. The automatic control station incorporates a wind gauge, wireless communication printed circuit boards (PCBs), and a transmitting antenna. During operation, it monitors the wind speed, so the cage can be submerged before extreme sea states and then surfaced after the weather has passed and the conditions are considered safe. The control station also regulates the flow of air and seawater to and from the variable ballast tanks in response to the surface environmental conditions. Control can also be done remotely by a facility operator. In the development process, in situ tests were conducted to assess the performance of the submersion mechanism and the reliability of the automatic control system with a 1/4 size fish cage of similar construction. During the tests, the vertical position and inclination of the fish cage in the water column were measured. During the tests, the clo- e/open states of the motor valves that control the 12 variable ballast tanks were also assessed during descent and ascent operations. The successful performance of the 1/4 size fish cage during the tests showed promise that such a system could possibly be used on a much larger scale to avoid the rigors of the environment in support of commercial level offshore aquaculture.

Description: In this paper, an exact solution is derived, evaluated, and numerically validated for describing the statistics of echoes from mixed assemblages of scatterers. Here, a “mixed assemblage” involves the geometry in which there is more than one type of scatterer spatially interspersed and uniformly distributed within the analysis window, which is much larger than the resolution cell of the system (i.e., there are many independent samples per window). The scatterers are generally not resolvable and the signals are narrowband. The scattering geometry is in the backscattering direction in the direct-path case in which there are no interfering echoes from neighboring boundaries. The probability density functions (pdfs) of echo envelopes in such cases can be highly non-Rayleigh and possess heavy tails, and the shape of the pdf curves contains information for characterizing and discriminating the composition of mixed assemblages. The general formulation is based on characteristic functions (CFs; hereafter referred to as the CF-based mixed assemblage pdf) and incorporates effects due to the scatterers being randomly located in the beam. Comparisons are made between the performance of the CF-based mixed assemblage pdf and the commonly used mixture model for simulated cases involving two different types of scatterers, arranged either interspersed or segregated in the analysis window. Both models can be made to fit the shape of the echo pdf of simulated data in some conditions. However, mismatch between model assumptions and the actual physical scattering processes can lead to order of magnitude errors in the inferred numerical density and backscattering amplitude of each type of scatterers.

Description: Acoustical seabed classification is a technology for mapping seabed sediments. Processed multibeam sonar data yield the variation of the seabed scattering strength with incidence angle, and this paper examines the effect of this on classification. A simple Gaussian statistical model is developed for the observed scattering strength, whereby an observation is represented by a piecewise constant function of incidence angle. Provided some data for which the sediment types are known (training data), the statistics for each type can be robustly estimated. Subsequently, a standard Bayesian theory is applied to classify new observations. The model was used to compute limits on classification accuracy in terms of the intrinsic scattering strength statistics of the seabed, and to predict whether a logarithmic or linear scale for the data is preferable. Systematic experiments on a North Sea data set with four sediment classes tested how the classification accuracy depends on the piecewise function approximation, incidence angle range, amount of training data, and spatial averaging (combining consecutive pings into one observation). The classifier based on Gaussian statistics performed at least as well as sophisticated algorithms with no assumptions about the data statistics. The best accuracy (95%) was attained for logarithmic data. The amount of training data needed to achieve this was about 500 pings per class; spatial averaging could be limited to 10–20 pings. Comparable across-track spatial resolution was possible by dividing the full swath into separate independent sectors, but only at reduced accuracy (87% or less). However, comparable accuracy may be possible by taking into account the spatial relationships of observations.

Description: Accurate environmental information is required for obtaining confident sonar performance predictions. This environmental information is, however, often unreliable or unavailable. To support antisubmarine warfare (ASW) operations, a through-the-sensor approach has been developed in which relevant acoustic seabed properties are derived from reverberation data, and a demonstrator system has been installed on a Royal Norwegian Navy frigate. It determines relevant acoustic seabed parameters from the reverberation data near real time. This demonstrator system has been validated in several sea trials conducted off the coast of Bergen, Norway. The acoustic seabed parameters derived in these trials have a good correspondence with the available prior information. Furthermore, the results show that acoustic seabed parameters derived from reverberation data in previous trials can be used to improve reverberation prediction for subsequent trials, even when environmental conditions, i.e., sound-speed profiles, are different. Because the demonstrator makes information on acoustic seabed properties directly available for in situ sonar performance prediction, it can be used as a tactical decision aid.

Description: The motivation of this work is to investigate the source number detectability by an oversampled or undersampled linear acoustic vector sensor (AVS) array. Sufficient conditions that ensure the extension of existing source number estimators and beamforming techniques from the acoustic pressure sensor (APS) array to the AVS array are established based on the linear independence of steering vectors of the AVS array, an issue closely related to the number of sources whose parameters [e.g., the direction of arrival (DOA)] can be uniquely determinable. The upper bound on the detectable number of sources with distinct DOAs under oversampled and undersampled array configurations is derived, where the difference of definitions of the distinct DOAs between the APS array and the AVS array is clarified. The number constraint on detectable sources with distinct spatial aliasing DOAs using the undersampled AVS array is also investigated. Under these conditions, advantages of eigenvalue-based source number estimators using AVS arrays are demonstrated, and the minimum description length (MDL) estimator is extended to the AVS array as an illustration to verify the established theories.

Description: In this paper, a narrowband method based on the acoustical Doppler effect is proposed for estimating the full set of five flight parameters, along with the propeller blade rate, of a turboprop aircraft as it transits over a wide aperture hydrophone array in a straight line at constant speed and constant altitude. The five flight parameters describe fully the linear trajectory of the aircraft's transit. The basic principle of the proposed method is to measure the temporal variation of the instantaneous frequency (IF) of the acoustic signal received by each sensor of the array and then to minimize the sum of the squared deviations of the IF estimates from their predicted values over a sufficiently long period of time for all sensors. The minimization is performed numerically subject to a set of constraints on the source parameters to be estimated. The IF model for the received signal at each sensor of the array and the initial parameter estimates required for the numerical minimization are derived. The effectiveness of the proposed method is demonstrated using both simulated and real data.

Description: In this paper, sound scattering at a water/sediment periodic interface is modeled by an integral equation method to investigate the range of validity of the Rayleigh hypothesis. Due to the periodicity of surface, coupled integral equations are simplified by introducing the quasi-periodic Green's function, which can be evaluated with a relatively small number of terms of the spectral series. By comparing the scattering amplitudes obtained from the integral equation method and the method based on the Rayleigh hypothesis, the range of validity of the Rayleigh hypothesis is determined. For a given wavenumber $kappa$ of the periodic surface, the maximum surface amplitude $h_{max}$ beyond which the Rayleigh hypothesis is invalid decreases with frequency of incident waves. For the grazing angle $theta={hbox {90}}^{circ}$ , $h_{max}$ is larger than $h_{c}=0.448/kappa$ (the upper limit of surface amplitude determined by the criterion of the Rayleigh hypothesis) in the frequency range 2.5–40 kHz. However, for a small grazing angle, such as, $theta={hbox {35}}^{circ}$ , $h_{max}$ is observed to be smaller than $h_{c}$ for the large surface period $Lambda$ at frequency 40 kHz.

Description: In this paper, we present a system design for single-input–multiple-output (SIMO) orthogonal frequency-division multiplexing (OFDM) communications over underwater acoustic (UWA) channels. The design targets at improving bandwidth efficiency by reducing the amount of overheads (e.g., guard time intervals, training/pilot symbols) typically associated with SIMO–OFDM UWA communications. The design consists of three key components: 1) preamble-based parameter estimation that exploits full knowledge of the preamble; 2) time-reversal-based symbol detection that enables the use of short guard time intervals for interblock interference avoidance; and 3) decision-directed (DD) channel tracking with error propagation (EP) control that eliminates the need of periodic training for channel tracking. The proposed design is analyzed and tested at a seagoing experiment. The experimental results confirm the merits of our design and demonstrate considerable improvements over a competing alternative.