Underwater communication is necessary for a variety of applications, including transmission of diver speech, communication between manned and/or unmanned underwater vehicles, and data harvesting for environmental monitoring, to name a few. Examples of communication between underwater vehicles include unmanned or autonomous underwater vehicles (UUV or AUV) for deep water construction, military UUVs such as submarine drones, repair vehicles for deep water oil wells, scientific or resource exploration, etc. Examples of underwater communication between fixed submerged devices are sensor networks deployed on the ocean floor for seismic monitoring and tsunami prediction, pollution monitoring, tactical surveillance, analysis of resource deposits, oceanographic studies, etc. The underwater communication environment is a challenging one. Radio signals experience drastic attenuation, while optical signals suffer from dispersion. Because of these issues, acoustic (sound) signals are usually used for underwater communication. Unfortunately, acoustics has its own problems, including limited bandwidth, slow propagation, and signal distortion. Some of these limitations can be overcome with advanced modulation and coding, but to do so requires better understanding of the underwater acoustic propagation environment, which is significantly different than air- or space-based radio propagation. The underwater environment must be studied and characterized to exploit these advanced modulation and coding techniques. This thesis addresses some of these concerns by proposing a derivation of the envelope level crossing rate of the underwater channel, as well as a simulation model for the channel, both of which agree well with the measured results. A velocity estimator is also proposed, but suffers from a high degree of root mean square error

Thesis (M.S. in Electrical Engineering and M.S. in Applied Physics)--Naval Postgraduate School, June 2005. / Thesis Advisor(s): Joseph Rice, Roberto Cristi. Includes bibliographical references (p. 117-119). Also available online.

A striking feature of swarm robotics is its ability to solve complex tasks through simple local interactions between robots. Those interactions require a good infrastructure in communication and localization. However, in underwater environments, the severe attenuation of radio waves complicates communication and localization of different vehicles. Existing literature on underwater swarms use centralized network topology which require physical vicinity to the central node to ensure reliability. We are interested in building a decentralized underwater swarm with a decentralized network topology that only requires neighbour communication and self-localization. We develop a simple leader-follower interaction rule where the follower estimates the leader's position and acts upon that estimation. The leader shines a 450 nm diffracted blue laser that the follower uses to continuously align its light sensors to the light source. Furthermore, the leader's laser can be modulated for explicit communication purposes. The proposed leader-follower approach produces satisfactory results in surge and sway axes, however, it is not robust against illumination changes in the environment. We then proceed to solve the self-localization problem, by fusing Inertial Measurement Unit (IMU) values with the thrust to estimate a robot’s position. In an Ardusub Simulation in the loop (SITL), the particle filter showed a slightly better performance than the Extended Kalman Filter (EKF) in the surge axis. However, both filters are prone to drifting after a while. We have observed that IMU values need to be filtered properly or another reliable sensor must be used alternatively.

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This thesis is concerned with long-range sound propagation in deep water.  The main area of interest is the stability of acoustic ray paths in wave guides in which there is a transition from single to double duct sound speed profiles, or vice-versa.  Sound propagation is modelled within a ray theoretical framework, which facilitates a dynamical systems approach of understanding long-range propagation phenomena, and the use of its tools of analysis. Alternative reduction techniques to the Poincaré sections are presented, by which the stability of acoustic rays can be graphically determined.  Beyond periodic driving, these techniques prove to be useful in case of the simplest quasiperiodic driving of the ray equations.  One of the techniques facilitates a special representation of ray trajectories for periodic driving. Namely, the space of sectioned trajectories is partitioned into nonintersecting regular and chaotic regions as with the Poincaré sections, when quasiperiodic and chaotic trajectories are represented by curve segments and area filling points, respectively.  In case of the simplest quasiperiodic driving – speaking about the same technique – regular trajectories are represented by curves similar to Lissajous curves, which are opened or closed depending on whether the two driving frequencies involved make relative primes or not. It is confirmed for a perturbed canonical profile that the background sound speed structure controls ray stability. It is also demonstrated for a particular double duct profile, when the singularity of the nonlinearity parameter for the homoclinic trajectory associated with this profile refers to the strong instability of corresponding perturbed trajectories.

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The purpose of this thesis was to design, build, and test a device capable of simulating the acoustic pulse expected from the interaction between an Ultra-High Energy (UHE) neutrino and seawater. When a neutrino interacts with seawater, the reaction creates a long, narrow shower of sub-atomic particles. The energy from this reaction causes nearly instantaneous heating of the seawater on an acoustic timescale. The acoustic pulse created by the resulting thermal expansion of the water is predicted to be bipolar in shape. This work was undertaken to support a Stanford experiment, the Study of Acoustic Ultra-high energy Neutrino Detection (SAUND), that uses existing hydrophone arrays to detect UHE neutrinos from the acoustic pulse generated by their rare interactions with seawater. The device fabricated for this thesis uses the discharge current from a 4 microFarads capacitor charged to 2.5kV to heat the seawater between two copper plates. The anode and cathode plates of this "zapper" design were 6 cm in diameter and 20 cm apart. The acoustic pulse generated by the zapper was measured both in a small test tank at NPS and at the Acoustic Test Facility located at NUWC Keyport. Bipolar pulses observed at NPS on two separate test dates had average pulse lengths of 110 microseconds +/- 10 microseconds and 160 +/- 20 microseconds and average amplitudes at 1m of 1.9 +/- 0.3Pa and 4.7 +/- 0.6Pa. The average pulse length recorded at Keyport was 49 +/- 6 microseconds and the average amplitude at 1m was 6.4 +/- 0.9Pa. The pulse lengths recorded at NPS were reasonably consistent with theory, however all pressure amplitudes were about 100 times lower than predicted. The cause of the amplitude discrepancy is not completely understood at this time.

Approved for public release; distribution is unlimited. / The USS Winston S. Churchill (DDG-81) shock trial was conducted in May and June of 2001 off the coast of Naval Station Mayport, Florida. Because the USS Winston S. Churchill best represented the new line of Flight II-A Arleigh Burkes, it was chosen to undergo ship shock trials for its class. These trials are necessary in order to evaluate the vulnerability and survivability of the hull and the mission essential equipment in a "combat shock environment". However, shock trials are very expensive, require extensive planning and coordination, and represent a potential hazard to the marine environment and its mammals. Computer modeling and simulation are showing themselves to be a plausible alternative in investigating the dynamic response of a ship under these shock trials conditions. This thesis investigates the use of computer ship and fluid modeling, coupled with underwater explosion simulation and compares it to actual shock trial data from the USS Winston S. Churchill. Of particular concern in this study is the amount of fluid that must be modeled to accurately capture the structural response of a full ship finite element model. Four fluid meshes were constructed and used to study the ship's response to an underwater explosion. Each simulation data was analyzed to determine which mesh best represented the actual ship shock trial results. / Lieutenant, United States Navy