Naval Postgraduate School

The fair‐weather (wind speeds < ${< }$10 m/s), surface (z= z= 4 m), nearshore wind field modification is examined with multiple cross‐shore arrays spanning from the coastline to 40 km offshore, deployed within four‐month‐long field experiments, measuring winds and air‐water temperatures. The over‐water to coastline winds ratio, RU ${R}_{U}$, was previously explored for offshore winds with limited observations and minimally for onshore and alongshore winds. Array observations provide a complete picture of the nearshore wind field for all wind orientations. The over‐water wind and temperature are linearly related to coastline values near the coastline, with r2 ${r}^{2}$ decreasing with distance from shore, XW ${X}^{W}$, with a decorrelation scale of 10 km. Mean RU ${R}_{U}$ as a function of XW ${X}^{W}$ differs per wind orientation, consistent with prior work. An RˆU ${\widehat{R}}_{U}$ model is developed from exponential Gaussian Process Regression (GPR), which accurately predicts the wind field with 20% data set holdback to elucidate the cross‐shore patterns and variable co‐dependence. The modeled Partial Dependence Plots provide RU ${R}_{U}$ dependency as a function of XW ${X}^{W}$ on coastline winds, and temperature differences without preconceptions. A consistent nearshore wind slowing occurs that varies in amplitude and distance, and changes with variable co‐dependence for wind orientation. The onshore wind slowing is counterintuitive, though consistent with sophisticated numerical models. Wind gustiness exhibits XW ${X}^{W}$ dependence, with linear normalization akin to the open ocean but larger. The RU ${R}_{U}$ observations and GPR highlight nearshore winds' cross‐shore extent and complexity, which are important for atmospheric and oceanic studies.

Optimal feedback control of nonlinear system with free terminal time present many challenges including nonsmooth in the value function and control laws, and existence of multiple local or even global optimal trajectories. To mitigate these issues, the authors introduce an actor-critic method along with some enhancements. The authors demonstrate the algorithm’s effectiveness on a prototypical example featuring each of the main pathological issues present in problems of this type as well as a higher dimensional example to show that the solution method presented can scale.

Underwater position, navigation, and timing messages are transmitted to moored, single acoustic receivers over basin-scale distances. At a 75 Hz center frequency, the lengthy coherence time allows for successive and long-duration symbol transmissions. Analysis of 2700 M-sequence transmissions from Kauai to receiver H11S2 near Wake Island over a 1.5 yr duration and a nominal 3500 km distance yields a mean channel capacity of 0.028 bits/(s Hz). A low signal-to-noise ratio (SNR) telemetry implementation, based on the same data, achieves a raw bitrate of 0.1 bits/s (without preamble and error correction) corresponding to a gross spectral efficiency of 0.0026 bits/(s Hz). By decoding 10 800 transmitted symbols, the empirical probability of symbol error as a function of SNR is determined for three groups of symbols.

There are a great variety of existing codes and algorithms for computing real Givens rotations and it can be difficult to discern what approach is best in terms of speed and accuracy. We consider specific versions of the two most widely used algorithms for computing real Givens rotations and experimentally evaluate their performance both in terms of speed and accuracy. The specific algorithms that we investigate are the result of extensive testing and are very compact and elegant.

Inner shelf circulation studies have focused mainly on alongshore uniform sandy coasts and coral reefs in subtidal and tidal bands, with far less attention given to rocky shores. This study examines depth‐averaged circulation at China Rock, a rocky shore on the Monterey Peninsula, CA, with 15 ADCPs deployed for about a month. The bathymetry varies strongly on multiple lengthscales. Large‐scale bathymetric features include an embayment and two headlands, whereas smaller‐scale features consist of a large variety of rocks extending from the inter‐tidal zone to offshore. Circulation variability encompasses subtidal, diurnal, and semidiurnal frequency bands. Velocity principal‐axes ellipses decay onshore in all frequency bands indicating strong bottom friction, and have orientation variability attributable to nearby large‐scale bathymetric features. Alongshore subtidal currents are reasonably well described by a wind stress and bottom friction balance, with skill similar to previous studies, but with larger linear drag coefficients, particularly in shallower waters. Cross‐shore subtidal currents near the embayment are directed offshore as a bathymetrically controlled rip current strengthened by feeder currents from the headlands, with magnitude related to the incident waves. In the diurnal and semidiurnal bands, alongshore currents are attenuated onshore and the tidal phase (relative to an offshore location) decreases onshore both due to enhanced bottom friction. The attenuation is greater than on a comparable sandy shelf or coral reef, with larger phase shifts more resembling the coral reef observations. The increased linear drag friction can be related to directly measured bottom roughness.

Nearshore wave dissipation by bottom friction can significantly attenuate surface waves when seabed roughness is large. Wave dissipation is parameterized with a friction factor f e , depending upon the wave orbital excursion at the seabed A b , and the seabed roughness k N . Parameterizations have been developed assuming small roughness k N relative to A b , but whether they yield accurate f e for rough seabeds, such as rocky shores, is unclear. Observations from a month-long experiment measured wave transformation on a rough rocky shore, with a large standard deviation of bottom depth σ h of 0.5–1.5 m. The explicit f e dependence on variable rocky seabed σ h has yet to be demonstrated. Sea-swell energy flux consistently decays shoreward of 8 m water depth, which is well offshore of the surfzone given the time-mean incident significant wave height of 1 m. The observed cross-shore flux convergence yields f e estimates across the instrument array. Quality control criteria are implemented to reduce noise in estimated f e . Hourly f e vary from 1 to 10, increase with smaller A b / σ h , and binned-means indicate a power-law scaling. When using a spatially averaged standard deviation σ h ref , the scatter around binned-means increases, demonstrating that f e is related to σ h . Intercomparison with previous experiments is challenging due to different methodologies and definitions of f e . Nevertheless, observations from multiple experiments are broadly consistent with a power-law in terms of A b / σ h . Given high-resolution bathymetry, our empirical f e scaling can be used to parameterize wave dissipation over rough seabeds of coral reefs and rocky shores.

Can large language models be trusted? Not likely. Artificial neuron networks offer a much better option.

This study investigates the microstructural and mechanical properties of cold-sprayed AA7075 coatings reinforced with varying compositions of micro-boron carbide (µB4C) and boron nitride nanoplatelets (BNNPs). Functionally graded coatings (FGCs) with different ceramic content (10-30 vol.% µB4C) were produced and analyzed for surface roughness, microstructure, hardness, and wear resistance. The inclusion of BNNPs improved particle distribution and reduced clustering in dual-reinforced compositions. Nanoindentation and wear tests show that coatings with higher µB4C content exhibited increased hardness and reduced wear rates, with a maximum hardness of 7.2 GPa for Al-30%µB4C. The wear resistance improved up to 46.5% compared to unreinforced AA7075, with µB4C particles enhancing load transfer and minimizing frictional forces.

A commonly invoked concept in radar and communications theory is that of a hypothetical three‐dimensional (3D) omnidirectional isotropic transmission antenna for which the output radiative power depends only on the spherical radial distance from the subject antenna to a given observation point and is independent of the spherical angular coordinates. In the present investigation, a similar transmitter‐receiver antenna system is developed for which the collected power of a linearly polarised receiver antenna depends only on the spherical radial distance from a specially designed transmission antenna to this receiver antenna and is independent of the spherical angular coordinates. This system design capitalises on the radiative properties of a particular spherical transmission antenna that is characterised by azimuthal rotation of the radiative fields and power pattern. This property of 3D isotropic power reception applies exactly in the near field, far field and all intermediate ranges from the spherical transmitter to the linearly polarised receiver. Likewise, this 3D isotropic receive power property is applicable for all radio frequency (RF) wavelengths, both larger and smaller than the radius of the spherical transmission antenna. This proposed antenna system concept could offer utility in multiple applications, including communications beaconing and radar surveillance.

We present cross-entropy search for multiple-aircraft facing inter- and intra-aircraft constraints, a problem that cannot be solved by traditional Dynamic Programming algorithms such as A*.

Unmanned underwater vehicles (UUVs) present an opportunity to reduce risk for manned platforms in support of strategic objectives; however, in an era of shrinking budgets, how the Navy can quickly integrate these platforms remains an open question. This research explores using model-based conceptual design (MBCD) with Monterey Phoenix (MP) to expose hidden requirements and emergent behaviors in complex systems. To establish the context of this work, this article first uses open-source information on UUVs and missions to build a narrative vignette and system architecture. MP modeling then identifies paths to mission success and failure along with refining new concepts to boost the likelihood of mission success. This process demonstrates the capability to verify requirements and validate new concepts before expensive development. Coupled with user experience, this approach results in a more complete understanding of the system, including unconsidered outcomes. The methods described in this research are immediately applicable to UUVs and related unmanned systems. In addition, future work can generalize and apply these methods to a broad range of operational and process problems to capture, understand, and control the behavior of any system or process of interest.

For purposes of scientific investigation and operational monitoring of lakes, we have been testing the application of Coastal Acoustic Tomography ("CAT") to a deep subtropical lake, Lake Biwa in Japan. This article reports work on measuring nonreciprocity of transmission between transducer pairs, with a final target of assimilating into a 3D hydrodynamic model of the lake.

Purpose of Review This review discusses the major factors that challenge circadian rhythms in the military operational environment and describes recent efforts to mitigate the effects of circadian misalignment. Recent Findings The biological clocks of military service members are challenged by multiple factors: shiftwork, unpredictable work schedules, excessively long work hours, and their age-dependent sleep needs. We describe recent efforts to mitigate the effects of circadian misalignment by classifying them into seven groups: fixed (circadian) watchstanding schedules, improvements to sleep-related habitability, light management, biomathematical models, real-time monitoring of military members’ sleep/wake patterns, training, and policy/regulations. Summary The issue of circadian desynchrony is part of the bigger problem of fatigue in the operational military environment. Even though none of the described mitigation efforts, on their own, is sufficient to remedy circadian disruption, when combined in a systematic approach, these efforts can benefit the military by contributing to reducing fatigue.

A lumped parameter approach to the problem of acoustic wave scattering by a perforated cylinder has been presented. The proposed framework enables analytical evaluation of the scattering amplitudes of all harmonics and derivation of the dispersion relations for the guided wave propagating inside the cylinder. The lumped parameter boundary condition enables straightforward estimation of the effect of different perforation patterns on the scattering characteristics and internal resonances of the perforated cylinder. The derived equations were treated analytically and validated numerically. It was demonstrated how the proposed theory can be applied for estimation of the fundamental frequency of a two-dimensional Helmholtz resonator with the complex configurations of openings. The predictions are in good agreement with the previously published results.

Tor users derive anonymity in part from the size of the Tor user base, but Tor struggles to attract and support more users due to performance limitations. Previous works have proposed modifications to Tor’s path selection algorithm to enhance both performance and security, but many proposals have unintended consequences due to incorporating information related to client location. We instead propose selecting paths using a global view of the network, independent of client location, and we propose doing so with a machine learning classifier to predict the performance of a given path before building a circuit. We show through a variety of simulated and live experimental settings, across different time periods, that this approach can significantly improve performance compared to Tor’s default path selection algorithm and two previously proposed approaches. In addition to evaluating the security of our approach with traditional metrics, we propose a novel anonymity metric that captures information leakage resulting from location-aware path selection, and we show that our path selection approach leaks no more information than the default path selection algorithm.

Despite decades of emission control measures aimed at improving air quality, Los Angeles (LA) continues to experience severe ozone pollution during the summertime. We incorporate cooking volatile organic compound (VOC) emissions in a chemical transport model and evaluate it against observations in order to improve the model representation of the present-day ozone chemical regime in LA. Using this updated model, we investigate the impact of adopting zero-emission vehicles (ZEVs) on ozone pollution with increased confidence. We show that mitigating on-road gasoline emissions through ZEV adoption would benefit both air quality and climate by substantially reducing anthropogenic nitrogen oxides (NOx) and carbon dioxide (CO2) emissions in LA by 28 and 41% during the summertime, respectively. This would result in a moderate reduction of O3 pollution, decreasing the average number of population-weighted O3 exceedance days in August from 9 to 6 days, and would shift the majority of LA, except for the coastline, into a NOx-limited regime. Our results also show that adopting ZEVs for on-road diesel and off-road vehicles would further reduce the number of O3 exceedance days in August to an average of 1 day.

This paper presents a general method to solve space logistics trajectory optimization problems in the case of low-thrust propulsion. The problem of multiple orbital transfers is considered for a servicing satellite that must visit a set of client satellites only once. A near-optimal distance metric based on the Q-law feedback controller is adopted in order to quantify the cost of the transfer of the servicing satellite between each pair of client-satellite orbits. This distance metric is evaluated for a discrete set of departure orbits, arrival orbits, initial servicer masses, i.e., mass before the transfer, and departure times. A four-dimensional array representing the overall cost is then constructed, and its elements are interpolated in order to efficiently solve the tour-optimization problem using genetic algorithms, particle swarm optimization, and simulated annealing. Both minimum-time and minimum-fuel problems are considered. The proposed method accounts for the following time-dependent factors: secular J 2 perturbations, eclipse power constraints, and fuel mass depletion. The proposed method is tested on a case study involving 20 client satellites with low eccentricity and low inclination, between medium Earth orbit and geosynchronous Earth orbit, and with randomly distributed right ascension of the ascending node and argument of perigee. The efficiency and accuracy of the proposed approach are assessed by comparing the results to solutions found by a direct optimization method.

Saltwater intrusion (SWI) into coastal freshwater systems is a growing concern in the face of climate change‐driven sea level rise and hydrologic variability. Saltwater contamination of surface freshwater in the coastal California Pajaro Valley exemplifies this concern, where surface water cannot be diverted for agriculture if it is too saline. Closures at the mouth of the Pajaro River Lagoon, a bar‐built estuary in the Pajaro Valley, are associated with SWI. Closures and SWI are driven by a combination of offshore climate, coastal hydrodynamics, estuarine dynamics, inland hydrology, and infrastructure and management. Here, we describe the Pajaro Valley coastal water system and identify the oceanic and inland hydrologic drivers of SWI using available observational data between 2012 and 2020. We use time series and exploratory statistical analyses of coastal total water levels (TWLs), slough stage and salinity, river discharge, and contextual knowledge from local water managers. We observe that wet season lagoon closure and SWI events follow high oceanic TWLs coupled with low stage and discharge in the inland freshwater network, revealing how both wave and inland flow conditions govern lagoon closures and coincident SWI. This study yields novel empirical findings and a methodology for connecting coastal oceanography, estuarine coupled hydro‐ and morpho‐dynamics, inland hydrology, and water management practices relevant to climate change adaptation in human‐modified coastal water systems.

In this article we consider the compound question of whether, and if so, how IT security can be measured and tested.