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At least 235 records · Page 13

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NASA NTRS1989

Correlation with a spatial light modulator having phase and amplitude cross coupling

In correlation filtering a spatial light modulators is traditionally modeled as affecting only the phase or only the amplitude of light. Usually, however, a single operating parameter affects both phase and amplitude. An integral constraint is developed that is a necessary condition for optimizing a correlation filter having single parameter coupling between phase and amplitude. The phase-only filter is shown to be a special case.

Juday, Richard D.↗

NASA NTRS1990

Effects of amplitude distortions and IF equalization on satellite communication system bit-error rate performance

Satellite communications links are subject to distortions which result in an amplitude versus frequency response which deviates from the ideal flat response. Such distortions result from propagation effects such as multipath fading and scintillation and from transponder and ground terminal hardware imperfections. Laboratory experiments performed at NASA Lewis measured the bit-error-rate (BER) degradation resulting from several types of amplitude response distortions. Additional tests measured the amount of BER improvement obtained by flattening the amplitude response of a distorted laboratory-simulated satellite channel. This paper presents the results of these experiments.

Kerczewski, Robert J.↗

NASA NTRS1991

RW Per - Nodal motion changes its amplitude by 1.4 mag

RW Per was found to have large secular changes in its eclipse amplitude. In blue light, for example, the amplitude was 3.2 mag in the early 1900s, 2.2 mag in the late 1960s, and 1.75 mag in 1990. Throughout this time, the brightness at maximum was constant in all colors. It is shown that the only possible explanation is nodal motion, where the inclination varies with a period of roughly 100,000 yr. The nodal motion is caused by a third star, for which the light curve, the colors, and the O - C curve already provide evidence. Thus, RW Per is only the fourth known star with large changes of eclipse amplitude and is only the second example of nodal motion.

Schaefer, Bradley E.↗

NASA NTRS1991

The finite-amplitude behavior of the Joule mode under astrophysical conditions

Magnetized astrophysical plasmas reveal a great deal of structure when spatially resolved. One possible explanation for this structuring is based on the existence of filamenting instabilities driven by radiation. In this paper, previous linear calculations are extended by considering the weakly nonlinear (finite-amplitude) development of such filamenting instabilities in magnetized unstratified plasmas. It is shown that under most conditions these instabilities (in particular, the Jouse model) are unstable at finite amplitude; in particular, in the temperature and plasma-beta domains characteristic of, for example, much of the solar transition region, these modes - which can be linearly stable under these conditions - become unstable to finite-amplitude perturbations. The relevance of this to the problem of heating the solar low transition region by current dissipation is discussed.

Bodo, G.↗

NASA NTRS1991

Amplitude and phase-shaping effects in beamwaveguides

Results are presented of an investigation on improvements in a geometrical optics (GO) design of a beam-waveguide antenna for operation at multiple frequency bands. Improvements might be possible by changing the design of the lower-frequency input pattern to the beam waveguide. The effects of amplitude and phase shaping the input pattern have been studied with an aperture diffraction model. Accurate vector near-field computations were made rapidly with a spherical wave expansion of the input and scattered fields. Numerical results indicate that for aperture sizes of less than 30 wavelengths, there is insufficient control on defocusing due to amplitude and phase shaping. Design tradeoffs on spillover loss and defocusing are possible by changing the amplitude and phase distribution of the input wavefront for larger size apertures.

Rengarajan, Sembiam R.↗

NASA NTRS1990

Time-dependent treatment of scattering - Integral equation approaches using the time-dependent amplitude density

The time-dependent form of the Lippmann-Schwinger integral equation is used as the basis of several new wave packet propagation schemes. These can be formulated in terms of either the time-dependent wave function or a time-dependent amplitude density. The latter is nonzero only in the region of configuratiaon space for which the potential is nonzero, thereby in principle obviating the necessity of large grids or the use of complex absorbing potentials when resonances cause long collision times (leading, consequently, to long propagation times). Transition amplitudes are obtained in terms of Fourier transforms of the amplitude density from the time to the energy domain. The approach is illustrated by an application to a standard potential scattering model problem where, as in previous studies, the action of the kinetic energy operator is evaluated by fast Fourier transform (FFT) techniques.

Hoffman, David K.↗

NASA NTRS1992

Some effects of oscillation waveform and amplitude on unsteady turbulent shear flows

Some physical features of several unsteady separating turbulent boundary layers are presented for practical Reynolds numbers and reduced frequencies such as for helicopter and turbomachinery flows. The effects of unsteadiness amplitude and waveform are examined for flows along the floor of a converging and diverging wind tunnel test section. At the end of the converging portion, the mean skin friction coefficient normalized on the mean dynamic pressure is independent of the waveform and amplitude within low experimental uncertainties. In the detaching and detached portions of the flow, wall values of the fraction of time that the flow moves downstream of gamma sub pu, which is a separated flow state variable, shows that oscillation waveform and amplitude strongly influence the detached flow behavior. Distributions of gamma sub pu during a cycle indicate hysteresis within the detached flow and the effects of the higher harmonics of pressure gradient and velocity.

Agarwal, Naval K.↗

NASA NTRS1992

Nonlinear development and secondary instability of large-amplitude Goertler vortices in hypersonic boundary layers

The weakly nonlinear development of large-amplitude Goertler vortices in a small neighborhood of the neutral stability position given by the linear theory is considered. It is shown that if the basic state and the wall curvature satisfy a certain condition, the Goertler vortices will grow until they become so large as to drive a mean flow correction as large as the basic state. Attention is then given to the further downstream evolution of these large-amplitude vortices, and it is shown that the vortices have a triple layer structure which consists of a region of vortex activity bounded by two transition layers over which the amplitude of the harmonic part of the vortex decays to zero exponentially. The positions of the two transition layers are found to be governed by a free boundary problem, which is solved for a number of curvature distribution cases. The secondary instability of the two transition layers is investigated with respect to traveling waves which are pi/2 out of phase in the spanwise direction with the steady Goertler vortices.

Fu, Y.↗

NASA NTRS1992

Frequency, phase, and amplitude changes of the hydrogen maser oscillation

The frequency, the phase, and the amplitude changes of the hydrogen maser oscillation, which are induced by the modulation of the cavity resonant frequency, are considered. The results obtained apply specifically to one of the H-maser cavity autotuning methods which is actually implemented, namely the cavity frequency-switching method. The frequency, the phase, and the amplitude changes are analyzed theoretically. The phase and the amplitude variations are measured experimentally. It is shown, in particular, that the phase of oscillation is subjected to abrupt jumps at the times of the cavity frequency switching, whose magnitude is specified. The results given can be used for the design of a phase-locked loop (PLL) aimed at minimizing the transfer of the phase modulation to the slaved VCXO.

Audoin, Claude↗

NASA NTRS1993

Broad-band linear polarization in cool stars. II - Amplitude and wavelength dependence for magnetic and scattering regions

We have developed a model to estimate the amplitude and wavelength dependence of broad-band linear polarization (BLP) from magnetic regions on cool stars. The model includes corrections both for line blends and for the partial cancellation of polarization in the vector sum over the stellar disk. Our results agree with recent calculations of BLP in the red, but show larger amplitudes and a different wavelength dependence in the blue. We find that the detailed wavelength dependence of the polarization is complex and varies with effective temperature and gravity due to changes in line blanketing. The BLP amplitudes depend strongly on field strength, blanketing, and magnetic region filling factor and geometry. We make rough estimates of the maximum BLP for stars of various spectral types and demonstrate a method for deriving a lower limit to the filling factor from the maximum observed BLP. This lower limit is related to the fractional area covered by the spatially asymmetric component of magnetic regions.

Saar, Steven H.↗

NASA NTRS1993

The radial evolution of the amplitudes of 'dissipationless' turbulent solar wind fluctuations

We examine the evolution with heliocentric distance of the amplitude of fluctuations in the interplanetary magnetic and velocity fields assuming a model of homogeneous, steady state turbulence. Both the Alfven ratio and the normalized cross helicity are taken to be constant, compared to other variations, and the turbulence is assumed to be nearly incompressive. The fluctuation amplitudes are found by assuming the heating of the plasma by the fluctuations to be negligible; quasi-steady state turbulence with damping balanced by large-scale energy input will lead to the same conclusions for inertial range fluctuations. While the assumptions of this model contrast sharply with those for purely Alfvenic fluctuations, the radial dependence of the amplitude of the fluctuations for reasonable parameters is very nearly that found from both WKB analysis and recent turbulence modeling. The robustness of this result suggests why some predictions of WKB theory are apparently correct in solar wind conditions where the theory is not expected to be valid.

Verma, M. K.↗

NASA NTRS1958

Experimental Determination of Effects of Frequency and Amplitude on the Lateral Stability Derivatives for a Delta, a Swept, and Unswept Wing Oscillating in Yaw

Three wing models were oscillated in yaw about their vertical axes to determine the effects of systematic variations of frequency and amplitude of oscillation on the in-phase and out-of-phase combination lateral stability derivatives resulting from this motion. The tests were made at low speeds for a 60 degree delta wing, a 45 degree swept wing, and an unswept wing; the swept and unswept wings had aspect ratios of 4. The results indicate that large changes in the magnitude of the stability derivatives due to the variation of frequency occur at high angles of attack, particularly for the delta wing. The greatest variations of the derivatives with frequency take place for the lowest frequencies of oscillation; at the higher frequencies, the effects of frequency are smaller and the derivatives become more linear with angle of attack. Effects of amplitude of oscillation on the stability derivatives for delta wings were evident for certain high angles of attack and for the lowest frequencies of oscillation. As the frequency became high, the amplitude effects tended to disappear.

Fisher, Lewis R↗

NASA NTRS1994

Segmentation Of Multifrequency Complex-Amplitude SAR Data

Several mathematical models and associated algorithms implement method of segmenting multifrequency, highly speckled, high-resolution, complex-amplitude (amplitude and phase) synthetic-aperture-radar (SAR) digitized image into regions, within each of which radar backscattering characteristics are similar or homogeneous from place to place. Typically, each region represents different type of terrain or other surface; e.g., forest, agricultural land, sea ice, or water. Method of segmentation of SAR scene into regions is product of generalization, to multifrequency case, of single-frequency method described in "Algorithms for Segmentation of Complex-Amplitude SAR Data" (NPO-18524).

Rignot, Eric J.↗

NASA NTRS1994

A method of measuring the amplitude-modulated vacuum field near a conducting mirror

Electromagnetic fields of the vacuum mode near a conducting mirror are modified with respect to those in free space, with their amplitudes having a sinusoidal spatial dependence from the mirror. Therefore if we combine this spatially amplitude-modulated vacuum field mode and intense coherent light with a beam splitter, we may detect this fluctuation of the vacuum mode in a homodyne detection scheme. It will give a new method to produce squeezed states of light with a single mirror placed close to an unused port of a beam splitter. We show that the amplitude fluctuation of the combined light can be reduced by a factor of 2 below that of the coherent light. We also discuss the limitations due to the finite line width of the laser and the effective absorption length of the photodiodes.

Youn, Sun-Hyun↗

NASA NTRS1995

Continuous phase and amplitude holographic elements

A method for producing a phase hologram using e-beam lithography provides n-ary levels of phase and amplitude by first producing an amplitude hologram on a transparent substrate by e-beam exposure of a resist over a film of metal by exposing n is less than or equal to m x m spots of an array of spots for each pixel, where the spots are randomly selected in proportion to the amplitude assigned to each pixel, and then after developing and etching the metal film producing a phase hologram by e-beam lithography using a low contrast resist, such as PMMA, and n-ary levels of low doses less than approximately 200 micro-C/sq cm and preferably in the range of 20-200 micro-C/sq cm, and aggressive development using pure acetone for an empirically determined time (about 6 s) controlled to within 1/10 s to produce partial development of each pixel in proportion to the n-ary level of dose assigned to it.

Maker, Paul D.↗

NASA NTRS1995

On the modulational instability of large amplitude waves in supersonic boundary layers

The evolution of large amplitude Tollmien-Schlichting waves in a supersonic boundary layer is investigated. Disturbances which have their wavenumber and frequency slowly varying in time and space are described using a phase equation type of approach. Unlike the incompressible case we find that the initial bifurcation to a finite amplitude Tollmien-Schlichting wave is subcritical for most Mach numbers. In fact the bifurcation is only supercritical for a small range of Mach numbers and even then for only a finite range of wave propagation angles. The modulational instability of large amplitude wavetrains is considered and is shown to be governed by an equation similar to Burgers equation but with the viscous term replaced by a fractional derivative. A numerical investigation of the solution of this equation is described. It is shown that uniform wavetrains are unstable.

Hall, Philip↗

NASA NTRS1992

Optimizing binary phase and amplitude filters for PCE, SNR, and discrimination

Binary phase-only filters (BPOFs) have generated much study because of their implementation on currently available spatial light modulator devices. On polarization-rotating devices such as the magneto-optic spatial light modulator (SLM), it is also possible to encode binary amplitude information into two SLM transmission states, in addition to the binary phase information. This is done by varying the rotation angle of the polarization analyzer following the SLM in the optical train. Through this parameter, a continuum of filters may be designed that span the space of binary phase and amplitude filters (BPAFs) between BPOFs and binary amplitude filters. In this study, we investigate the design of optimal BPAFs for the key correlation characteristics of peak sharpness (through the peak-to-correlation energy (PCE) metric), signal-to-noise ratio (SNR), and discrimination between in-class and out-of-class images. We present simulation results illustrating improvements obtained over conventional BPOFs, and trade-offs between the different performance criteria in terms of the filter design parameter.

Downie, John D.↗

NASA NTRS1994

Localized injection of large-amplitude Pc 1 waves and electron temperature enhancement near the plasmapause observed by DE2 in the upper ionosphere

The relation between electron temperature enhancement and large amplitude Pc 1 wave injections in the upper ionosphere is investigated using the data obtained by the Dynamics Explorer 2 spacecraft. Results can be summarized as follows: (1) The region of the temperature enhancement coincides with that of the wave injection which is latitudinally very narrow (less than 100 km) in comparison with the wavelength along the ambient magnetic field (several hundred kilometers). (2) The duration of the wave injection (or the temperature enhancement) seems to be less than a few hours even under quiet geomagnetic conditions, and/or the injection seems to be very localized, not only latitudinally, but also longitudinally. (3) The appearance and the magnitude of temperature enhancement depend on both the wave amplitude and the satellite altitude. (4) Two of the 22 events that were analyzed show a clear enhancement of low-energy electron flux (5 to 30 eV) at the wave injection, and the flux is field-aligned both downward and upward. The region of the temperature enhancement coincides with that of the downward electron flux. From these results, it is suggested that the temperature enhancement which accompanies large-amplitude waves with Pc 1 pulsation frequencies (0.2 to 5 Hz) is caused by the direct acceleration of thermal electrons at low altitudes by the parallel electric field (0.01 to 0.001 mV/m) of the ion-cyclotron waves (kinetic Alfven waves) having an oblique wave normal.

Iyemori, T.↗