Description: Underwater pulse position modulation (PPM) transmission uses acoustic pulses since they allow communication, remote control of underwater vehicles, and positioning estimation; the last one when more than one detector is present by using only one technology, that is, the same transmission module. On the other hand, PPM pulses are difficult to detect since they carry information on signal delays, assuring that reliable information delivery both in shallow- and deep-sea channels may be considered a timely challenge due to strong multipath and other interference sources/causes giving rise to signal fluctuation and high noise level. This paper proposes a transceiver architecture characterized by a signal analyzer and an adaptive detection based on the channel features. The signal analysis is based on the Wigner–Ville transform (WVT) that converts a 1-D signal into an image. Edge detection procedures performed on the obtained image so as to discover the interference features and understand the main characteristics of multipath with their adverse effects on detection of pulse position information have been taken into account. Based on this, maximal ratio combining (MRC) and equal gain combining (EGC) for RAKE reception and truncated channel equalization have been considered. Numerical results showing the ability of the proposed transceiver to detect pulses in the presence of multipath and interference generated by external acoustic sources have been carried out also by taking into account the estimation error effects and by comparing the actual performance with ideal cases. Finally, field tests corroborate the reliability of the proposed approach.

Description: This paper addresses the modeling and experimental identification of six different six degree-of-freedom (6-DOF) coupled nonlinear second-order plant models. We report a comparative experimental evaluation of six different candidate plant models whose unknown plant parameters are estimated from data obtained in free-motion vehicle trials. The parameter estimation methodologies of ordinary least squares (OLS), total least squares (TLS), and underdetermined TLS were employed to identify experimentally the unknown plant model parameters. We evaluate the performance of each of the six different 6-DOF coupled nonlinear finite-dimensional plant models of an underwater vehicle estimated by OLS and the plant model identified by TLS by comparing the mean absolute error between the experimentally observed vehicle velocities and the velocities obtained by a numerical simulation of the identified plant models. We also report a cross validation which evaluates the performance of a plant model to accurately reproduce observed plant velocities for experimental trials differing from the trials from which the plant model parameters were estimated. We conclude that: 1) plant models identified by TLS generally perform better (i.e., more accurately reproduce observed experimental behavior) than models identified by OLS; and 2) plant models including fully parametrized coupled quadratic drag terms perform best overall in cross validation. This study has the following contributions: it is the first reported experimental 6-DOF fully coupled plant model identification and cross validation of a low-speed, fully actuated, and neutrally buoyant underwater vehicles; it is the first experimental 6-DOF plant model identification for this same class of underwater vehicles during free-flight experiments; and it is the first reported use of TLS to perform 6-DOF plant model identification of an underwater vehicle.

Description: In this paper, the authors study the problem of robust adaptive path-following control for underactuated ships with model uncertainties and nonzero-mean time-varying disturbance. A concise adaptive neural network (NN)-based control scheme is proposed using backstepping, feedforward approximations, dynamic surface control, and minimal learning parameter techniques. In addition, to tackle the strong couplings among state variables (including the underactuated state variable) and underactuated characteristics, much effort is put into guaranteeing semiglobal uniform ultimate boundedness of the ship motion control system. The outstanding advantage of this scheme is that the control law has a concise form and is easy to implement in practice due to a smaller computational burden, with only two online parameters being tuned to tackle the uncertainties. The simulation results demonstrate the effectiveness of the proposed algorithm, especially including the experiment in the simulated marine environment.

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: 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: Lists the reviewers who contributed to IEEE Journal of Oceanic Engineering.

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.