Showing papers by "Naval Postgraduate School published in 1994"

A technique for orthogonal frequency division multiplexing frequency offset correction

Abstract: This paper discusses the effects of frequency offset on the performance of orthogonal frequency division multiplexing (OFDM) digital communications. The main problem with frequency offset is that it introduces interference among the multiplicity of carriers in the OFDM signal. It is shown, and confirmed by simulation, that to maintain signal-to-interference ratios of 20 dB or greater for the OFDM carriers, offset is limited to 4% or less of the intercarrier spacing. Next, the paper describes a technique to estimate frequency offset using a repeated data symbol. A maximum likelihood estimation (MLE) algorithm is derived and its performance computed and compared with simulation results. Since the intercarrier interference energy and signal energy both contribute coherently to the estimate, the algorithm generates extremely accurate estimates even when the offset is far too great to demodulate the data values. Also, the estimation error depends only on total symbol energy so it is insensitive to channel spreading and frequency selective fading. A strategy is described for initial acquisition in the event of uncertainty in the initial offset that exceeds 1/2 the carrier spacing, the limit of the MLE algorithm. >

Testing the iron hypothesis in ecosystems of the equatorial Pacific Ocean

Abstract: The idea that iron might limit phytoplankton growth in large regions of the ocean has been tested by enriching an area of 64 km2 in the open equatorial Pacific Ocean with iron This resulted in a doubling of plant biomass, a threefold increase in chlorophyll and a fourfold increase in plant production Similar increases were found in a chlorophyll-rich plume down-stream of the Galapagos Islands, which was naturally enriched in iron These findings indicate that iron limitation can control rates of phytoplankton productivity and biomass in the ocean

Npsnet: A network software architecture for largescale virtual environments

Abstract: This paper explores the issues involved in designing and developing network software architectures for large-scale virtual environments. We present our ideas in the context of NPSNET-IV, the first 3-D virtual environment that incorporates both the IEEE 1278 distributed interactive simulation DIS application protocol and the IP multicast network protocol for multiplayer simulation over the Internet.

MBone provides audio and video across the Internet

Abstract: Researchers have produced the Multicast Backbone (MBone), which provides audio and video connectivity from outer space to under water/spl minus/and virtually everyplace in between. MBone is a virtual network that has been in existence since early 1992. It originated from an effort to multicast audio and video from meetings of the Internet Engineering Task Force. Today. hundreds of researchers use MBone to develop protocols and applications for group communication. Multicast provides one-to-many and many-to-many network delivery services for applications such as videoconferencing and audio where several hosts need to communicate simultaneously. This article describes the network concepts underlying MBone, the importance of bandwidth considerations, various application tools, MBone events, interesting MBone uses, and provides guidance on how to connect your Internet site to the MBone. >

Infragravity-Frequency (0.005–0.05 Hz) Motions on the Shelf. Part I: Forced Waves

Abstract: In Part I, the energy levels of ocean surface waves at infragravity frequencies (nominally 0.005–0.05 Hz) locally forced by swell in 13-m water depth were shown to be predicted accurately by second-order nonlinear wave theory. However, forced infragravity waves were consistently much less energetic than free infragravity waves. Here, in Part II, observations in depths between 8 and 204 m, on Atlantic and Pacific shelves, are used to investigate the sources and variability of free infragravity wave energy. Both free and forced infragravity energy levels generally increase with increasing swell energy and decreasing water depth, but their dependencies are markedly different. Although free waves usually dominate the infragravity frequency band, forced waves contribute a significant fraction of the total infragravity energy with high energy swell and/or in very shallow water. The observed h−1 variation of free infragravity energy with increasing water depth h is stronger than the h−1/2 dependence pre...

Use of Multiquadric Interpolation for Meteorological Objective Analysis

Abstract: The method of multiquadric interpolation is described and compared to the Barnes and Cressman methods of meteorological objective analysis. The method of multiquadric interpolation uses hyperboloid radial basis functions to fit scattered data to a uniform grid. Results for an analytical function indicate that the method is more accurate than the Barnes or Cressman methods. Application to actual meteorological data indicates that multiquadric interpolation produces excellent analyses that retain small-scale features resolved by the observations in any subregion of the analysis.

Film-Cooling From Holes With Compound Angle Orientations: Part 1—Results Downstream of Two Staggered Rows of Holes With 3d Spanwise Spacing

Abstract: Experimental results are presented that describe the development and structure of flow downstream of two staggered rows of film-cooling holes with compound angle orientations. With this configuration, holes are spaced 3d apart in the spanwise direction, inclined at 35 deg with respect to the test surface when projected into the streamwise/normal plane, and inclined at 30 deg with respect to the test surface when projected into the spanwise/normal plane. Results are presented for an injectant to free-stream density ratio near 1.0, and injection blowing ratios from 0.5 to 1.50

AE Source Orientation by Plate Wave Analysis

Abstract: Lead breaks (Hsu-Neilsen source) were used to generate simulated acoustic emission signals in an aluminum plate at angles of 0, 30, 60, and 90 degrees with respect to the plane of the plate. This was accomplished by breaking the lead on slots cut into the plate at the respective angles. The out-of-plane and in-plane displacement components of the resulting signals were detected by broad band transducers and digitized. Analysis of the waveforms showed them to consist of the extensional and flexural plate modes. The amplitude of both components of the two modes was dependent on the source orientation angle. This suggests that plate wave analysis may be used to determine the source orientation of acoustic emission sources.

Optimizing Ship Berthing

Abstract: : Ship berthing plans reserve a location for inbound U.S. Navy surface vessels prior to their port entrance, or reassign ships once in port to allow them to complete, in a timely manner, reprovisioning, repair, maintenance, training, and certification tests prior to redeploying for future operational commitments. Each ship requires different services when in port, such as shore power, crane, ordnance, and fuel. Unfortunately, not all services are offered at all piers, and berth shifting is disruptive and expensive: A port operations scheduler strives to reduce unnecessary berth shifts. We present an optimization model for berth planning and demonstrate it for Norfolk Naval Station, which exhibits all the richness of berthing problem:. the Navy faces.

Toward an improved understanding of thruster dynamics for underwater vehicles

Abstract: This paper proposes a novel approach to modeling the four quadrant dynamic response of thrusters as used for the motion control of ROV and AUV underwater vehicles. The significance is that these vehicles are small in size and respond quickly to commands. Precision in motion control will require further understanding of thruster performance than is currently available. The model includes a four quadrant mapping of the propeller blades lift and drag forces and is coupled with motor and fluid system dynamics. A series of experiments is described for both long and short period triangular, as well as square wave inputs. The model is compared favorably with experimental data for a variety of differing conditions and predicts that force overshoots are observed under conditions of rapid command changes. Use of the model will improve the control of dynamic thrust on these vehicles.

The sizes of the jet drops produced by air bubbles bursting on sea- and fresh-water surfaces

Abstract: The size distributions of the top 6 jet drops produced by air bubbles bursting on sea- and fresh-water surfaces are presented for bubbles with radii from 350 to 1500 μ m. There are no significant differences in the results between sea- and fresh-water. Except for the top drop, the size distributions are often bimodal. For the top jet drop, the dependence of the average droplet radius, R d , on bubble radius, R b , is given by the fit R d = 0.0337 R b 1.208 ( μ m) which compares well with the results for the top drop reported by Blanchard in 1989. Excluding the droplets in the small modes, the droplet size dependence on bubble size for the lower droplets (those subsequent to the top drop) is, within statistical certainty, the same for all the droplets from the second through the fifth. The data for these droplets is fit by R d = 0.0165 R b 1.325 . The probability that a bubble with a given radius produces at least n drops is given for n up to 7, although a small number of bubble bursts were observed to have produced at least 15 jet droplets. DOI: 10.1034/j.1600-0889.1994.t01-2-00007.x

Water mass linkages between the Middle and South Atlantic bights

Abstract: Time and frequency domain analyses are used to relate coastal meteorological data with 7 years of daily surface temperature and salinity collected at three coastal light stations; offshore of the mouth of Chesapeake Bay, Virginia, on Diamond Shoals, at Cape Hatteras, North Carolina and on Frying Pan Shoals, off Cape Fear, North Carolina. Salinity fluctuations at Diamond Shoals are highly correlated with alongshore wind stress, implying wind driven advection of the front between Virginia Coastal Water (VCW) and Carolina Coastal Water (CCW) across Diamond Shoals. The data collected at Diamond Shoals indicate that more than half the time there is significant encroachment of Mid Atlantic Bight water into the South Atlantic Bight around Cape Hatteras, contrary to the notion that VCW is entirely entrained into the Gulf Stream. In fact, VCW can appear as far south as Frying Pan Shoals, thereby extending across the entire North Carolina Capes inner to mid shelf. Temperature and salinity time series also indicate that water masses overlying Diamond Shoals respond quickly to cross-shelf winds. Cross-shelf wind stress is significantly correlated with surface water temperature at Diamond Shoals, for periods between 2 and 12 days. Changes in temperature can be brought about by wind-driven cross-shelf circulation and by wind-induced upwelling. Seasurface temperature satellite (AVHRR) imagery taken during the SEEP II confirm these concepts.

Software merge: semantics of combining changes to programs

Abstract: We present a language-independent semantic model of the process of combining changes to programs. This model extends the domains used in denotational semantics (complete partial orders) to Boolean algebras, and represents incompatible modifications as well as compatible extensions. The model is used to define the intended semantics of change-merging operations on programs and to establish some general properties of software merging. We determine conditions under which changes to subprograms of a software system can be merged independently and illustrate cases where this is not possible.

Sums of like powers of multivariate linear forms

Abstract: (1994). Sums of Like Powers of Multivariate Linear Forms. Mathematics Magazine: Vol. 67, No. 1, pp. 59-61.

Role of Al 2 O 3 particulate reinforcements on precipitation in 2014 Al-matrix composites

Abstract: Precipitation in commercial aluminum alloy 2014, without and with alumina particulate reinforcements, was studied using microhardness, electrical resistivity, differential scanning calorimetry (DSC), and transmission electron microscopy. The precipitation sequence in 2014 Al was confirmed to be αss→α + GPZ →α + λ’→a + λ’ + gH→α + λ (AlCuMgSi) + θ (CuAl2). Reinforcement addition decreased the time to peak hardness, but also reduced the peak matrix microhardness. This was traced to a decrease in the amount of λ’ formed in the composites. Further, it was observed that while Guinier-Preston (GP) zone and θ’ formations are accelerated in the composites, λ’ precipitation is decelerated. The acceleration is attributable primarily to enhanced nucleation resulting from an increase in the matrix dislocation density due to coefficient of thermal expansion (CTE) mismatch between the matrix and the reinforcements, whereas the deceleration is associated with a decrease of low-temperature solute diffusivity due to absorption of vacancies at dislocations and interfaces. It was also observed that the degree of overall acceleration in hardening and the reduction in peak matrix microhardness with reinforcement addition decreased with decreasing aging temperatures. The causal relationships of these observations with the associated mechanisms are discussed.

Automated partial pose measurement system for manipulator calibration experiments

Abstract: The work reported here is concerned with methods used to provide partial pose data for robot calibration tests. Rather than focussing on traditional, precision measurement techniques, the paper discusses calibration using end point motion constraints of various kinds. The preferred method consists of using a wire potentiometer attached to the robot tool by means of two specially designed fixtures. Suitable modeling of the measurement system as well as the internal kinematics of the manipulator allows simulation of the calibration process. Experimental results obtained in the calibration of an industrial manipulator is presented, and means to make the complete calibration process automatic is discussed. >

The relationship between the microwave radar cross section and both wind speed and stress: Model function studies using Frontal Air-Sea Interaction Experiment data

Abstract: The Frontal Air-Sea Interaction Experiment (FASINEX) provided a unique data set with coincident airborne scatterometer measurements of the ocean surface radar cross section (RCS)(at Ku band) and near-surface wind and wind stress. These data have been analyzed to study new model functions which relate wind speed and surface friction velocity (square root of the kinematic wind stress) to the radar cross section and to better understand the processes in the boundary layer that have a strong influence on the radar backscatter. Studies of data from FASINEX indicate that the RCS has a different relation to the friction velocity than to the wind speed. The difference between the RCS models using these two variables depends on the polarization and the incidence angle. The radar data have been acquired from the Jet Propulsion Laboratory airborne scatterometer. These data span 10 different flight days. Stress measurements were inferred from shipboard instruments and from aircraft flying at low altitudes, closely following the scatterometer. Wide ranges of radar incidence angles and environmental conditions needed to fully develop algorithms are available from this experiment.

Indirect control of corporations

Abstract: We consider situations in which shares ofn firms are held by the samen firms as well as by individual investors. The probelm is to determine the actual control of the firms by the individual investors.

Automating output size and reuse metrics in a repository-based computer-aided software engineering (CASE) environment

Abstract: Measurement of software development productivity is needed in order to control software costs, but it is discouragingly labor-intensive and expensive. Computer-aided software engineering (CASE) technologies/spl minus/especially repository-based, integrated CASE/spl minus/have the potential to support the automation of this measurement. We discuss the conceptual basis for the development of automated analyzers for function point and software reuse measurement for object-based CASE. Both analyzers take advantage of the existence of a representation of the application system that is stored within an object repository, and that contains the necessary information about the application system. We also discuss metrics for software reuse measurement, including reuse leverage, reuse value, and reuse classification that are motivated by managerial requirements and the efforts, within industry and the IEEE, to standardize measurement. The functionality and the analytical capabilities of state-of-the-art automated software metrics analyzers are illustrated in the context of an investment banking industry application that is similar to systems deployed at the New York City-based investment bank where these tools were developed and tested. >

Adaptive Quaternion Feedback Regulation for Eigenaxis Rotations

Abstract: A controller to rotate a rigid body between two successive orientations is designed Particular features are the fact that it is based on the quaternion approach, known to provide singularity-free attitude description, and it is adaptive in the sense that it does not need specific knowledge of the inertia matrix Global stability of the overall controller is proved analytically and tested in computer simulations In this paper, we design a nonlinear adaptive controller that per- forms vehicle orientation in the presence of large uncertainties of the inertia matrix The concept of adaptive control for linear time- invariant systems dates back to the 1950s although reliable designs of proven stability did not appear before the late 1970s14'15 Both di- rect and indirect adaptive control schemes are presented The direct adaptive controller has the advantage of simplicity in the imple- mentation, since it is based on a simple gradient-type adaptation law However, the indirect approach we present, although of more complex implementation, is based on a recursive least-squares iden- tification algorithm that has the advantage of converging faster In general, adaptive controllers of nonlinear systems are based on a linearized model of the plant, and stability can be shown only locally In a few cases the particular structure of a nonlinear system lends itself to a globally stable adaptive control algorithm, such as the case of robotic manipulators16 The adaptive controller presented below falls in this category, for which we can show global stability of the overall system composed of the plant and the controller with recursive estimates of the inertia matrix Eigenaxis rotation is presented in Sec II Three control tech- niques are given next: a sliding-mode technique in Sec Ill, direct adaptive control in Sec IV followed by an indirect approach in Sec V, whereas examples and conclusions are given in Sec VI and VII, respectively

A Surface Integral Approach to the Motion Planning of Nonholonomic Systems

Abstract: Nonholonomic mechanical systems are governed by constraints of motion that are nonintegrable differential expressions. Unlike holonomic constraints, these constraints do not reduce the number of dimensions of the configuration space of a system. Therefore a nonholonomic system can access a configuration space of dimension higher than the number of the degrees of freedom of the system. In this paper, we develop an algorithm for planning admissible trajectories for nonholonomic systems that will take the system from onepoint in its configuration space to another. In our algorithm the independent variables are first converged to their desired values. Subsequently, closed trajectories of the independent variables are used to converge the dependent variables

Stability curves for a thermoacoustic prime mover

Abstract: The purpose of this paper is to investigate the stability curves of the fundamental and second modes in a helium filled prime mover. The predicted and measured stability limits are in reasonable agreement for both modes at most mean pressures. There is, however, evidence that the stability of one mode is affected by the presence of the other. It is also observed that one mode can suppress the other. Measurements are also reported on a prime mover modified to selectively inhibit the fundamental mode. Results indicate that the reduced fundamental amplitude allows the stability curve for the second mode to extend into the regions where the fundamental mode previously dominated. This produces a region where both modes are simultaneously excited. Analysis of the waveforms show that the resulting oscillations are quasiperiodic.

Two dimensional wheeled vehicle kinematics

Abstract: This paper develops a fundamental theory of two dimensional wheeled vehicle kinematics. The author first analyzes rigid body robot 2D motions so that a mixture of translational and rotational motions is considered to be a purely rotational motion with its center. Next the author defines the class of tangential motions. Motions in this class are relatively easy to execute by vehicles normally available. A curve with discontinuous curvature is not appropriate for tangential motions for any vehicle architecture. The most important finding in this paper is that any general motion is embodied by an SS type vehicle which does not use any special tires or special complex mechanisms. >

Quantitative study of unsteady compressible flow on an oscillating airfoil

Abstract: Detailed interferometric measurements of the flow near the leading edge of an oscillating airfoil offer the first detailed experimental quantification of the locally compressible flow field that surrounds an oscillating airfoil at moderate subsonic Mach numbers. Interferograms obtained by a specially adapted real-time point-diffraction interferometry technique have revealed significant characteristics of this complex, and very rapidly varying, locally supersonic flow. Instantaneous pressure distributions determined from these interferograms document the effect of unsteadiness on the leading-edge flow environment.

Vortex Element Methods for Flow Simulation

Abstract: Publisher Summary This chapter focuses on vortex element methods for flow simulation. The vortex element methods allow recreating the physically relevant dynamics of two- and three-dimensional incompressible flows through the use of the Lagrangian or the Lagrangian-Eulerian description of the evolution of discretized vorticity fields. The most important accuracy condition in vortex methods is that the cores of neighboring vortex elements must, at all times, overlap. All remeshing efforts are, by and large, directed to fulfill this condition. It should be kept in mind, however, that the stretching of a segment, intensification of vorticity, constancy of the vortex core, distortion of the near-field velocity distribution due to the increase of the particles or nodes, and artificial dissipation may not all be commensurate with the need to maintain a nondivergent vorticity field. In fact, the accuracy of the calculations may be sacrificed by remeshing, particularly in regions of generation and high concentration of vorticity, due to the warping of the velocity and gradient spaces. This chapter describes evolution and applications of vortex element methods. Concepts related to contra flowing streams, quasi-two-dimensional cross flows, and general three-dimensional flows are also discussed in detail.

Evidence of a turbidity current in Monterey Submarine Canyon associated with the 1989 Loma Prieta earthquake

Abstract: Evidence of a turbidity current sweeping through the Monterey Submarine Canyon following the October 1989 Loma Prieta earthquake was documented by the movement of bottom-deployed acoustic transponders used to navigate free-falling oceanographic instrumentation. Measuring sites located along the Canyon at distances of 55, 130 and 190 km from the Canyon head off Moss Landing, CA, all showed evidence of tectonically induced sediment transport. At the site 55 km from the Canyon head, one transponder, located in the axis of the Canyon, was carried 1.9 km down the axis and was deposited among a field of rocks. The other three transponders had been deployed on the sides of the canyon and showed evidence of sediment slumping toward the Canyon axis. Circumstantial evidence from the site 130 km down the canyon suggests that sediment deposition occurred outside the channel axis. Sediment slumping or erosional cutting moved one transponder deeper at the site 190 km from the canyon head.

Motor and plume particle size measurements in solid propellant micromotors

Abstract: Particle size distributions were measured in the chamber, nozzle, and plume of a subscale solid propellant rocket motor. A significant reduction in the mean size of the aluminum/aluminum oxide particles occurred within the motor chamber. The mass fraction of small particles (<2 ft) at the nozzle entrance was less than 10%. Also, most particles were smaller than 50 ft, although a few as large as 85 ft were present. In the converging and throat portions of the nozzle it appeared that particle breakup dominated over collision coalescence. Collision coalescence was observed to occur more dominantly in the supersonic nozzle flow. At the nozzle exit the particle mean size was usually less than 2.5 ft. The particle size distributions were bimodal or trimodal, with the larger particles concentrated near the plume centerline. Because of the short nozzle residence times it is not known whether or not these results are also applicable to full-scale motors. The mean size of the small A12O3 particles in the plume edges was less than 0.5 ft in diam, with an index of refraction of 1.64 ± 0.04 (apparently y-Al2O3), independent of propellant composition, motor operating conditions and nozzle geometry.

Insertion of an articulated human into a networked virtual environment

Abstract: Most distributed interactive simulation (DIS) technology demonstrated in recent years has focused on vehicle interaction. The dismounted infantryman-the individual soldier-has been largely ignored or represented by static models. In six weeks of development, the Naval Postgraduate School, SARCOS Inc., and University of Pennsylvania, under Army Research Laboratory sponsorship, demonstrated the insertion of a fully articulated human figure into a DIS environment. This paper describes the system architecture. >

Eigenmode analysis of dielectric loaded top-hat monopole antennas

Abstract: Eigenmode expansions are used for computing the currents on a dielectric loaded top-hat monopole radiating above an infinite conducting ground plane. The approach is facilitated by adding a parallel ground plane above the antenna, thus allowing use of cylindrical harmonic field expansions in each of three regions. Expansion coefficients are found by enforcing boundary and continuity conditions at conducting surfaces and regional interfaces. Convergence and accuracy are assessed using comparisons with three other methodologies, namely an integral equation solution, a quasi-static analysis, and multi-octave experimental measurements. >