NUWC Keyport Division

The future naval combatant ships will feature a fully integrated power system. These systems will allow unprecedented control of shipboard power to propel the ship, sense the battlespace, and engage the enemy. One crucial enabling technology is robust power conversion modules like the hybrid dc to ac inverter. This paper is a further exploration of the hybrid inverter scheme consisting of a six-step voltage-source inverter and a hysteresis-controlled current-regulated inverter. In this implementation, the six-step controller was designed to be independent of the hysteresis controller. The hysteresis controller is fed a reference signal extracted from the total output current going to the load. The signal is filtered and modified by a closed-loop system such that the total output current approaches a perfect sine wave; the quality of which is limited only by the bandwidth of the hysteresis converter. The modified closed-loop controller was compared to previous efforts and found to improve current total harmonic distortion from 3.2% to 1.8%. This paper proves that existing power electronic technology can be used to produce high-fidelity waveforms for high-power Naval propulsion drives which have a power range of 50 MW to 100 MW.

Deep-water acoustic tracking of marine mammals typically requires correlating hydrophone signals across both short and large-aperture hydrophone arrays. Here, we demonstrate how a single drifting, depth-controlled platform can obtain two and three-dimensional positional fixes of marine mammal sounds using an acoustic vector sensor and short-aperture arrays, both mounted on an autonomous drifting platform. In February and June 2023 two autonomous opto-acoustic drifters were deployed 50km off San Diego, CA at 250 m depth in 1030 m deep water. Both drifters were equipped with a CTD and acoustic recording system comprised of a 1.75 m aperture vertical hydrophone array, a tetrahedral hydrophone array, and either a 2-D Geospectrum M-35 or 3-D Wilcoxon VS-209 acoustic vector sensor. Throughout the two to three-day deployments, all acoustic sensors detected numerous marine mammal calls, including humpback whales and a pod of common dolphins. Estimates of azimuth and elevation were obtained from the simultaneously sampled acoustic vector sensor and hydrophone arrays, while multi-path and cross-platform processing were employed to extract range information. The results suggest that sparse deployments of drifting vector sensor platforms may be able to map biological distributions over spatial scales that would otherwise require larger numbers of hydrophone-only platforms. [Work supported by ONR TFO.]

Conducting unobtrusive observations of physical, acoustic, and biological properties of the mesopelagic zone remains challenging, despite the proliferation of mobile autonomous platforms. Presented here is the Optical Passive Acoustic Adaptive Drifter System (OPAADS), a 2-m tall Lagrangian drifter platform that incorporates a CTD, stereo video cameras, vertical and tetrahedral hydrophone arrays, and acoustic vector sensors. The OPAADS uses a commercial buoyancy engine to dynamically adjust its target isobar via either a preprogrammed dive sequence or real-time commands relayed by an ultra-short baseline underwater tracking and communication system. The platform can descend to depths to 1 km for 3- to 5-day missions. Since 2022 we have conducted 29 deployments of three units off the New England Seamounts, Southern California, and Kona-Kohala Hawaii, all in water depths greater than 700 m. Many deployments find bioacoustic activity a dominant component of the ambient sound, from clusters of sperm whale clicks off seamounts, to fish chorusing off Hawaii, to distinctive 0.5–2 and 3–5 kHz diffuse noise bands off San Diego and Hawaii often associated with dolphin echolocation but whose origin remains uncertain. We also discuss the directionality and localization of these sources. [Work sponsored by ONR TFO.]

Between 2022 and 2024, multiple deployments of deep-water, drifting, opto-acoustic platforms were conducted near the New England Seamounts (∼4000 m depth) and in the Southern California Bight (∼1000 m depth). The platforms were equipped with conductivity, temperature, and depth (CTD) sensors, vertical and tetrahedral hydrophone arrays, and a three-axis acoustic vector sensor. For one deployment in Southern California, an 80-m thermistor string was suspended from the platform, providing high-resolution temperature profiles complementing the acoustic measurements. The drifters continuously recorded mid-frequency (500–20 000 Hz) ambient sound for 1–4 days at depths ranging from 100 to 800 m, entering a “hibernation” mode to minimize self-noise. The directionality and spatial coherence between hydrophones were computed and compared to analytical models and numerical simulations of surface-generated noise. Data between the hydrophone arrays and the acoustic vector sensor were also compared. Temporal and spatial variations in the acoustic environment, particularly 15–20 min oscillations in the surface noise directionality, were observed at both sites. The seamounts data displayed more dynamic changes in surface-generated noise directionality but less bio-acoustic activity. These results highlight the utility of these platforms for characterizing complex acoustic environments and cross-checking beamforming and vector sensor measurements. [Work sponsored by ONR TFO.]

Periodic fluctuations in the vertical directionality of surface-generated noise have been observed during multiple deep-water drifting platform deployments in the New England Seamount, Southern California Bight, and Hawaiian Ridge regions. Ambient sound in the 500–20 000 Hz band was recorded using co-located hydrophone arrays and an acoustic vector sensor. Consistent 15- to 20-min oscillations in the apparent vertical angle and intensity of surface noise were detected at depths between 100 and 800 m, suggesting that upper-ocean processes modulate surface-noise generation and/or its propagation. To investigate these potential physical drivers, we analyze environmental data, including from a high-resolution thermistor string and upward-looking echosounder, collected during earlier deployments in a subset of the study regions, along with new experiments conducted in the Southern California Bight. Acoustic observations are compared with numerical simulations of surface noise generation and propagation in stratified, range-dependent environments. The results provide new insight into how temporal variability in near-surface structure, such as bubble plume distribution or internal wave sound speed fluctuations, can influence the angular dependence of ambient noise. [Work sponsored by ONR TFO.]

Conducting unobtrusive observations of the physical, acoustic, and biological properties of the mesopelagic zone remains challenging, despite the proliferation of mobile ROV, AUV, and glider platforms. Here, we discuss the Optical Passive Acoustic Adaptive Drifter System (OPAADS), a 2 m tall Lagrangian drifter platform that incorporates a CTD, stereo video cameras, vertical and tetrahedral hydrophone arrays, and acoustic vector sensors. The platform has also suspended a 91-m vertical thermistor array and acoustic doppler current profiler. The OPAADS uses a commercial buoyancy engine to dynamically adjust its target isobar via either a preprogrammed dive sequence or real-time commands relayed by an ultra-short baseline underwater tracking and communication system. Three platforms have been built and deployed over ten times since 2022, off both southern California and off the New England Seamounts, between depths of 100 and 700 m for durations between 24 and 72 h. We discuss the sensors, operations, and observations collected by OPAADS, including data useful for (1) characterizing bioacoustic chorusing, wind-driven surface noise directionality, and turbulence dynamics; (2) conducting species identification in the deep scattering layer; (3) performing marine mammal tracking; and (4) collecting pilot data studying potential approaches for long-range underwater navigation. [Work sponsored by ONR TFO.]