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At least 775 records · Page 43

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NASA NTRS2015

Systems and Methods for Automated Vessel Navigation Using Sea State Prediction

Systems and methods for sea state prediction and autonomous navigation in accordance with embodiments of the invention are disclosed. One embodiment of the invention includes a method of predicting a future sea state including generating a sequence of at least two 3D images of a sea surface using at least two image sensors, detecting peaks and troughs in the 3D images using a processor, identifying at least one wavefront in each 3D image based upon the detected peaks and troughs using the processor, characterizing at least one propagating wave based upon the propagation of wavefronts detected in the sequence of 3D images using the processor, and predicting a future sea state using at least one propagating wave characterizing the propagation of wavefronts in the sequence of 3D images using the processor. Another embodiment includes a method of autonomous vessel navigation based upon a predicted sea state and target location.

Huntsberger, Terrance L.↗

NASA NTRS2017

Systems and Methods for Automated Vessel Navigation Using Sea State Prediction

Huntsberger, Terrance L.↗

NASA NTRS1991

An evaluation of HEMT potential for millimeter-wave signal sources using interpolation and harmonic balance techniques

A large-signal analysis method based on an harmonic balance technique and a 2-D cubic spline interpolation function has been developed and applied to the prediction of InP-based HEMT oscillator performance for frequencies extending up to the submillimeter-wave range. The large-signal analysis method uses a limited number of DC and small-signal S-parameter data and allows the accurate characterization of HEMT large-signal behavior. The method has been validated experimentally using load-pull measurement. Oscillation frequency, power performance, and load requirements are discussed, with an operation capability of 300 GHz predicted using state-of-the-art devices (fmax is approximately equal to 450 GHz).

Kwon, Youngwoo↗

NASA NTRS1991

A study of baroclinic instability in a cylindrical annulus with the temperature gradient imposed on the lower surface

Results are presented from experimental and numerical-modeling studies carried out on a rotating thermally driven fluid system in a cylindrical annulus with the horizontal gradient imposed upon the lower horizontal surface. The rotation rate and the temperature difference were varied to construct a regime diagram in thermal Rossby-number/Taylor-number space. It is shown that the curve separating the axisymmetric flow from the wave flow is 'knee-shaped', similar to the conventional side-heated and side-cooled baroclinic annulus. The experimentally observed transition curve agrees well with numerical calculations and the agreement between the predicted and measured wavenumbers is good both near the transition and within the wave regime. Away from the transition curve and well within the wave regime, there is an evolution from larger to smaller wavenumbers as the flow equilibrates, which is well captured by the numerical model for these cases.

Miller, Timothy L.↗

NASA NTRS2000

Thermal and Driven Stochastic Growth of Langmuir Waves in the Solar Wind and Earth's Foreshock

Statistical distributions of Langmuir wave fields in the solar wind and the edge of Earth's foreshock are analyzed and compared with predictions for stochastic growth theory (SGT). SGT quantitatively explains the solar wind, edge, and deep foreshock data as pure thermal waves, driven thermal waves subject to net linear growth and stochastic effects, and as waves in a pure SGT state, respectively, plus radiation near the plasma frequency f(sub p). These changes are interpreted in terms of spatial variations in the beam instability's growth rate and evolution toward a pure SGT state. SGT analyses of field distributions are shown to provide a viable alternative to thermal noise spectroscopy for wave instruments with coarse frequency resolution, and to separate f(sub p) radiation from Langmuir waves.

Cairns, Iver H.↗

NASA NTRS1977

Reynolds number effects on shock-wave turbulent boundary-layer interactions - A comparison of numerical and experimental results

An experiment is described that tests and guides computations of a shock-wave turbulent boundary-layer interaction flow over a 20-deg compression corner at Mach 2.85. Numerical solutions of the time-averaged Navier-Stokes equations for the entire flow field, employing various turbulence models, are compared with the data. Each model is critically evaluated by comparisons with the details of the experimental data. Experimental results for the extent of upstream pressure influence and separation location are compared with numerical predictions for a wide range of Reynolds numbers and shock-wave strengths.

Horstman, C. C.↗

NASA NTRS1997

Physics of Rotation Powered Pulsars and Their Nebulae

This letter is my progress report for the Astrophysics Theory grant. The first goal of the research supported by this grant is to produce a time dependent theory of the unsteady relativistic collisionless shock wave terminating the relativistic wind from a pulsar, and compare the predicted surface brightness fluctuations to Hubble Space Telescope observations of the wisps in the Crab Nebula. The second goal is to model the production of electron-positron pairs over the polar caps of rotation powered pulsars, and use the results to predict the heating of the surface due to particle trapping and bombardment of the atmosphere at the base of the polar field lines. We have succeeded in creating a one dimensional hybrid code, in which the electron-positron pairs incident on the shock structure are modeled as a relativistic, adiabatic, ideal MHD fluid, while the heavy ions are treated as particles using a particle-in-cell algorithm. The electromagnetic fields are evaluated from the currents and charge densities in the pairs and the ions, while the particles and the fluid accelerate in response to the computed self-consistent electromagnetic fields. The results are promising, in that the underlying ion cyclotron instability generates finite amplitude, propagating magnetosonic waves in the pairs, whose wavelengths and frequencies, when translated into physical units, are comparable to the observed running waves of brightness observed by HST near the Crab pulsar. The code is undergoing a number of tests, to assure us that this preliminary correspondence is not an artifact. In the coming year, the observational appearance of the models will be computed and compared to the HST observations of the Crab now in hand, and used to predict the HST results which will be obtained the year after next. WE also developed a one dimensional cascade theory for pair creation over pulsars' polar caps. A linear integral equation describing the synchrotron cascade has been derived and solved by iterative techniques, in the case when a high energy electron moving parallel to a star centered dipole magnetic field initiates the cascade through curvature gamma ray emission.

Arons, Jonathan↗

NASA NTRS1996

An acoustic emission and acousto-ultrasonic analysis of impact damaged composite pressure vessels

The use of acoustic emission to characterize impact damage in composite structures is being performed on composite bottles wrapped with graphite epoxy and kevlar bottles. Further development of the acoustic emission methodology will include neural net analysis and/or other multivariate techniques to enhance the capability of the technique to identify dominant failure mechanisms during fracture. The acousto-ultrasonics technique will also continue to be investigated to determine its ability to predict regions prone to failure prior to the burst tests. Characterization of the stress wave factor before, and after impact damage will be useful for inspection purposes in manufacturing processes. The combination of the two methods will also allow for simple nondestructive tests capable of predicting the performance of a composite structure prior to its being placed in service and during service.

Workman, Gary L.↗

NASA NTRS1989

Observations on electromagnetic bias in radar altimeter sea surface measurements

Because the relative radar cross section of the sea surface increases below mean sea level and decreases above it, the range measurements of satellite radar altimeters are biased toward the wave troughs. Published and unpublished direct measurements of this electromagnetic (EM) bias are examined as well as the predictions of theoretical developments. The EM bias is predominantly a function of the radar frequency used, averaging 1.2 percent of the wave height of Ka band and 3.3 percent of the wave height at X band. The airborne measurements present a consistent picture of the variation of the relative radar cross section as a function of deviation from mean sea level. A technique to measure EM bias at the Ku and C band operating frequencies of the Topex satellite altimeter is described.

Walsh, Edward J.↗

NASA NTRS1987

An experimental investigation of compressible three-dimensional boundary layer flow in annular diffusers

An experimental study is described in which detailed wall pressure measurements have been obtained for compressible three-dimensional unseparated boundary layer flow in annular diffusers with and without normal shock waves. Detailed mean flow-field data were also obtained for the diffuser flow without a shock wave. Two diffuser flows with shock waves were investigated. In one case, the normal shock existed over the complete annulus whereas in the second case, the shock existed over a part of the annulus. The data obtained can be used to validate computational codes for predicting such flow fields. The details of the flow field without the shock wave show flow reversal in the circumferential direction on both inner and outer surfaces. However, there is a lag in the flow reversal between the inner nad the outer surfaces. This is an interesting feature of this flow and should be a good test for the computational codes.

Om, Deepak↗

NASA NTRS1994

Focused interplanetary transport of approximately 1 MeV solar energetic protons through self-generated Alfven waves

We present a model of the focused transport of approximately 1 MeV solar energetic protons through interplanetary Alfven waves that the protons themselves amplify or damp. It is based on the quasi-linear theory but with a phenomenological pitch angle diffusion coefficient in the 'resonance gap.' For initial Alfven wave distributions that give mean free paths greater than approximately 0.5 AU for approximately 1 MeV protons in the inner heliosphere, the model predicts greater than roughly an order of magnitude amplification (damping) in the outward (inward) propagating resonant Alfven waves at less than or approximately equal to o.3 AU heliocentric distance. As the strength of proton source is increased, the peak differential proton intensity at approximately 1 MeV at 1 AU increases to a maximum of approximately 250 particles (/(sq cm)(s)(sr)(MeV)) and then decreases slowly. It may be attenuated by a factor of 5 or more relative to the case without wave evolution, provided that the proton source is sufficiently intense that the resulting peak differential intensity of approximately 1 MeV protons at 1 AU exceeds approximately 200 particles (/(sq cm)(s)(sr)(MeV)). Therefore, in large solar proton events, (1) one may have to take into account self-amplified waves in studying solar particle propagation, (2) the number of accelerated protons escaping from a flare or interplanetary shock may have been underestimated in past studies by a significant factor, and (3) accelerated protons escaping from a traveling interplanetary shock at r less than or approximately equal to 0.3 AU should amplify the ambient hydromagnetic waves siginificantly to make the shock an efficient accelerator, even if initially the mean free path is greater than or approximately equal to 1 AU.

Ng, C. K.↗

NASA NTRS1986

An extreme clear air turbulence incidence associated with a strong downslope windstorm

The meteorological conditions present during an occurrence of clear-air turbulence (CAT) associated with the presence of a mountain wave are documented. The incident caused severe vertical accelerations of two passenger aircraft travelling at 33,000 ft altitude. Satellite, pilot report and surface and upper air meteorological data were examined to characterize the situation. The CAT was apparently produced by an unusually strong westerly flow over the Rocky Mountains, causing a lee wave over the foothill regions where the turbulence was experienced. A downslope windstorm formed at the same time as wave activity on three scales: 60 nm, 17 nm and 1 nm in the troposphere. The data suggest that surface observations of severe windstorms in the mountain foothill regions could be used to predict the presence of CAT conditions, especially if combined with available data on lee waves.

Lester, P. F.↗

NASA NTRS1995

Hybrid simulations of the saturated Farley-Buneman instability in the ionosphere

Numerical simulations of the Farley-Buneman instability in 2-1/2 dimensions using particle ions and fluid electrons show the growth, saturation and nonlinear behavior of two-stream waves. This hybrid technique models the saturated state of the instability for a much longer period of time than the pure particle codes that preceded it. While focusing principally on modeling the topside E region equatorial electrojet, many of these results apply to the auroral two-stream instability as well. The following features are seen in all our hybrid simulations: (1) wave growth at an angle offset from the electron drift direction where the angle depends on the strength of the driving electric field, (2) nonlinear coupling to waves traveling perpendicular to the propagation direction of the principal two-stream waves, (3) a saturated wave phase velocity at or above the sound speed but well below the velocity predicted by linear theory and (4) phase velocities which remain almost constant as a simulated radar sweeps from a horizontal direction to nearly vertical. The nonlinear electron motion dominates the behavior of these waves. Further, these simulations indicate that ion kinetic effects are not essential for the saturation of the instability and that electron temperature effects have a minor impact on the final saturated state.

Oppenheim, Meers↗

NASA NTRS1980

Block 4 receiver tracking loop performance in the presence of a CW RFI

A model that allows one to predict the tracking performance of the Block 4 receiver in the presence of a continuous wave radio frequency interference is discussed. Experimental and analytical results are provided for a typical Deep Space Network operational mode. Simulation and experimental results show good agreement with theoretical prediction for the static phase error and out-of-lock values. Predicted phase jitter is consistently lower than the experimental and simulated results by a factor of one-half for small interference to signal ratio (ISR) when the offset frequency is small. For large ISR, good agreement is observed. The analytical model assumes a noiseless condition, which is valid only when the loop is operated at strong signal levels. Experimental data indicate, however, that even at the minimum operating signal level of 10-dB carrier margin, reasonably good prediction can still be obtained. A curve of protection criteria that extends the current recommendation is also presented.

Sue, M. K.↗

NASA NTRS1992

Multiple frequency interference in photorefractive media

The paper describes the use of a numerical simulation to predict the dynamic behavior of a photorefractive crystal exposed to interfering light waves at two different frequencies. Unlike static recording media, photorefractive materials allow for the simultaneous diffraction from and generation of refractive index gratings. The grating properties are evaluated in terms of their effect on the performance of a dynamic distributed sensor which uses the crystal as a holographic recording medium. Experimental results are presented which support the behavior predicted by simulation.

Cox, David E.↗

NASA NTRS1973

Source altitude for experiments to simulate space-to-earth laser propagation.

The bias in scintillation measurements caused by the proximity of a spherical-wave source to the turbulence region of the atmosphere is predicted, and the laser-source altitude required for meaningful experiments simulating space-to-earth laser propagation is estimated. It is concluded that the source should be located at two or more times the maximum altitude of the tropopause to ensure that all measurements are not biased by more than 25%. Thus the vehicle used for experiments of this type should be capable of reaching a minimum altitude of 32 km.

Minott, P. O.↗

NASA NTRS1974

Recent results from satellite beacon measurements

A review is made of the techniques for measuring ionospheric electron content, the most important parameter in the study of transionospheric propagation. Data collected since 1964 have yielded a synoptic description of the behavior of the electron content in mid-latitudes. Empirical relationships between the level of solar activity and the electron content have been developed permitting the prognostication of the electron content values. Marked discrepancies between prognostication and observation which occur during ionospheric storms are being studied to identify their causes. Electron content bite-outs during solar eclipses fall off with distance from totality more rapidly than simple theory predicts suggesting the action of eclipse induced neutral winds. Gravity waves propagating in the thermosphere leave a signature in the electron content records. Studies of such records have allowed the identification of the position of the gravity wave source, and its radiation pattern.

Da Rosa, A. V.↗

NASA NTRS1978

Shock wave ignition of magnesium powders

The paper is concerned with magnesium powder ignition caused by the interaction of powder with incident shock wave-induced flows. Experimentally observed ignition delays are correlated with predicted times for particle melting and the onset of thermal erosion, and a good agreement is found between experiment and theory. Ignition delays were measured as functions of particle screening, cloud concentration, incident shock strength, initial pressure, and oxygen concentration (diluted with nitrogen). An expression for the Nusselt number in the Reynolds number range 0-10,000 and the Mach number range 0-6 is proposed. The expression, when applied to predict particle behavior, provides results in agreement with experimental data for M/Re less than 0.5.

Fox, T. W.↗