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At least 415 records · Page 23

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NASA NTRS1985

Active microwave remote sensing of an anisotropic random medium layer

A two-layer anisotropic random medium model has been developed to study the active remote sensing of the earth. The dyadic Green's function for a two-layer anisotropic medium is developed and used in conjunction with the first-order Born approximation to calculate the backscattering coefficients. It is shown that strong cross-polarization occurs in the single scattering process and is indispensable in the interpretation of radar measurements of sea ice at different frequencies, polarizations, and viewing angles. The effects of anisotropy on the angular responses of backscattering coefficients are also illustrated.

Lee, J. K.↗

NASA NTRS1986

Antenna pattern correction for the Nimbus-7 SMMR

This paper describes the philosophy and method used to develop the antenna pattern correction (APC) algorithm that was used on the data from the Scanning Multichannel Microwave Radiometer (SMMR) on Nimbus-7. There are limitations on what can be accomplished with such a procedure; these limitations are explored with the aid of Fourier analysis, even though the algorithm used on the SMMR data does not perform any Fourier transforms. The resulting analysis showed that, for the SMMR instrument, no useful improvement could be made in the data in terms of reduction of side lobes, but the quality of the sea surface temperature retrievals could be improved considerably by matching the antenna beamwidths at the different frequencies.

Milman, A. S.↗

NASA NTRS1987

Visual Speech-Training Aid for the Deaf

Teaching deaf to speak aided by electronic system provides striking colored, pictorial representation of sound; energy at different frequencies as function of time. Other modalities, such as nasality, intra-oral pressure, and lip-muscle contraction, pictorialized simultaneously. Use of standard components, including personal microcomputer, helps reduce cost below prior voice-training systems. Speech-training system, microphone output separated by filters into narrow frequency bands, changed into digital signals, formatted by computer, and displayed on television screen. Output from other sensors displayed simultaneously or screen split to allow sound produced by student to be compared with that of teacher.

Miller, Robert J.↗

NASA NTRS1987

Loran-C approach guidance project current status

There are four areas of work in the Loran-C flight test project. Current results provide performance data on the effects of Signal Noise Ratio (SNR) on the dynamic performance of the receiver filters for Loran-C data, and data on Loran-C grid deformation at a microscale of 100 meters. The Loran-C receiver provides a line of position (LOP) Master and Slave transmitter at an angle 0 to magnetic north. No transformation to latitude-longitude reference frame is required since this is the major source of Loran-C navigation errors. A local coordinate frame is established centered at touchdown point on the runway with directions along and across the runway. A Loran-C data collection system was set up. The Loran-C data are sent directly to an Apple II computer with a 12 inch monitor. The effect of SNR on Loran-C precision is shown for two receiver filters of different frequency response. A set of ground level static readings of touchdown was taken around Hanscom Field and transferred to an accurate detailed layout drawing; this showed local distortions of the average touchdown values.

Elias, A. L.↗

NASA NTRS1986

Interactions of coupled acoustic and vortical instability

In the past, the acoustic combustion instability was studied independently of the hydrodynamic instability induced by vortex motions. This paper is intended to combine the two different sources of energy everywhere within the spatial domain and determine the effect of one upon the other. This can be achieved by calculating the mean flow velocities and vorticities and their fluctuating parts of velocities and vortices, as well as the fluctuating pressure. The Orr-Sommerfeld equation is utilized to determine the wavenumbers and unsteady stream functions from which vortically coupled acoustic instability growth constants are calculated. This process demonstrates that there are two different frequencies, acoustic and hydrodynamic, various combinations of which contribute to either damping or amplification. It is found that stability boundaries for coupled acoustic and vortical oscillations are somewhat similar to the classical hydrodynamic stability boundaries, but they occur in the form of multiple islands.

Chung, T. J.↗

NASA NTRS1987

Uncertainty relations for light waves and the concept of photons

A Lorentz-covariant localization for light waves is presented. The unitary representation for the electromagnetic four-potential is constructed for a monochromatic light wave. A model for covariant superposition is constructed for light waves with different frequencies. It is therefore possible to construct a wave function for light waves carrying a covariant probability interpretation. It is shown that the time-energy uncertainty relation (Delta-t)(Delta-omega) = about 1 for light waves is a Lorentz-invariant relation. The connection between photons and localized light waves is examined critically.

Han, D.↗

NASA NTRS1988

Acoustical Measurement Of Mine-Shaft Length

Acoustical system proposed to measure depth of a "blind" shaft. Acoustic wave guided by shaft and provides estimate of shaft length, from which volume estimated. Acoustic-generator system determines resonant-frequency difference to measure shaft length.

Heyman, Joseph S.↗

NASA NTRS1988

Signal Generator Compensates For Phase Shift In Cable

Stabilized reference signal delivered to remote unit. Signals at two different frequencies transmitted back and forth along cable and mixed to measure propagation delay in cable. Delay-measuring circuitry part of feedback loop constraining overall delay at stable value. Intended for use in system generating precise time and frequency signals, signal-generating subsystem compensates for phase shift in cable.

Sigman, E. H.↗

NASA NTRS1988

Estimating Electron Content Of The Ionosphere

Method for estimation total electron content of ionosphere. Based on measurements of signals transmitted from global positioning satellites (GPS's). Ionospheric delays obtained by measuring differential arrival times of signals at two different frequencies. Since GPS observations cover certain regions of sky, location of space probe to be calibrated might not overlap GPS field of view. Certain assumptions made about behavior of ionosphere so total electron content estimated in any direction from receiver.

Lanyi, G. E.↗

NASA NTRS1988

Nonlinear Analysis Of Rotor Dynamics

Study explores analytical consequences of nonlinear Jeffcott equations of rotor dynamics. Section 1: Summary of previous studies. Section 2: Jeffcott Equations. Section 3: Proves two theorems that provide inequalities on coefficients of differential equations and magnitude of forcing function in absence of side force. Section 4: Numerical investigation of multiple-forcing-function problem by introducing both side force and mass imbalance. Section 5: Examples of numberical solutions of complex generalized Jeffcott equation with two forcing functions of different frequencies f1 and f2. Section 6: Boundedness and stability of solutions.Section 7: Concludes report reviewing analytical results and significance.

Day, William B.↗

NASA NTRS1987

On the onset of three-dimensionality and time-dependence in the Goertler vortex problem

The instability of large amplitude Goertler vortices in a growing boundary layer is discussed in the fully nonlinear regime. It is shown that a three-dimensional breakdown to a flow with wavy vortex boundaries similar to that which occurs in the Taylor vortex problem takes place. However, the instability is confined to the thin shear layers which have been shown to trap the region of vortex activity. The disturbance eigenfunctions decay exponentially away from the center of these layers so that the upper and lower shear layers can support independent modes of instability. The structure of the instability, in particular its location and speed of downstream propagation, is found to be entirely consistent with recent experimental results. Furthermore, it is shown that the upper and lower layers support wavy vortex instabilities with quite different frequencies. This result is again consistent with the available experimental observations.

Hall, Philip↗

NASA NTRS1988

Experimental investigation of propfan aeroelastic response in off-axis flow with mistuning

Measured vibratory strain amplitudes from off-axis flow are compared for the blades of two, 8-bladed propfan model rotors with mistuning. One rotor had inherent mistuning. The other was intentionally mistuned by replacing every other blade of the first rotor with a blade of same geometry but different frequencies and mode shapes. The data shows that the intentional mistuning had a beneficial effect on the aeroelastic response of the propfan motors for a wide range of off-axis flow angles, blade pitch angles, and rotational speeds. Statistical trends of blade strain amplitudes are compared for both the rotors in terms of the ratio of the maximum to the mean and the coefficient of variation.

Mehmed, Oral↗

NASA NTRS1987

Whistler mode emissions in the Uranian radiation belts

Voyager 2 detected intense whistler mode emissions and fluxes of energetic electrons during the outbound pass through the region of auroral L shells. The observed energetic (E greater than 22 keV) electron distribution, a model warm (E less than 27.5 keV) electron distribution, and the cold plasma density profile deduced by Kurth et al. (1987) are used to calculate the ray path-integrated spatial amplification of whistlers which arrive at Voyager 2 from the magnetic equator. By matching the calculated amplification and the relative gains at different frequencies deduced from the observed whistler power spectrum, the pitch angle anisotropy parameters of the electron distributions are determined to within a fairly narrow range of values. The estimated bounce average pitch angle diffusion coefficient indicates that electrons are on strong diffusion over a wide range in energies. The electron precipitation energy flux is sufficient to produce the observed auroral light emissions.

Coroniti, F. V.↗

NASA NTRS1987

Estimates of the solar internal angular velocity obtained with the Mt. Wilson 60-foot solar tower

Estimates are obtained of the solar internal angular velocity from measurements of the frequency splittings of p-mode oscillations. A 16-day time series of full-disk Dopplergrams obtained during July and August 1984 at the 60-foot tower telescope of the Mt. Wilson Observatory is analyzed. Power spectra were computed for all of the zonal, tesseral, and sectoral p-modes from l = 0 to 89 and for all of the sectoral p-modes from l = 90 to 200. A mean power spectrum was calculated for each degree up to 89. The frequency differences of all of the different nonzonal modes were calculated for these mean power spectra.

Rhodes, Edward J., Jr.↗

NASA NTRS1988

High-dynamic GPS tracking

The results of comparing four different frequency estimation schemes in the presence of high dynamics and low carrier-to-noise ratios are given. The comparison is based on measured data from a hardware demonstration. The tested algorithms include a digital phase-locked loop, a cross-product automatic frequency tracking loop, and extended Kalman filter, and finally, a fast Fourier transformation-aided cross-product frequency tracking loop. The tracking algorithms are compared on their frequency error performance and their ability to maintain lock during severe maneuvers at various carrier-to-noise ratios. The measured results are shown to agree with simulation results carried out and reported previously.

Hinedi, S.↗

NASA NTRS1989

An analytical and experimental investigation of edge delamination in laminates subjected to tension, bending, and torsion

An integrated two-dimensional finite element was developed to calculate interlaminar stresses and strain energy release rates for the study of delamination in composite laminates subjected to uniaxial tension, bending, and torsion loads. Addressed are the formulation, implementation, and verification of the model. Parametric studies were conducted on the effect of Poisson's ratio mismatch between plies and the stacking sequence on interlaminar stress, and on the effect of delamination opening height and delamination length, due to bending, on strain energy release rate for various laminates. A comparison of strain energy release rates in all-graphite and graphite/glass hybrid laminates is included. The preliminary results of laminates subjected to torsion are also included. Fatigue tension tests were conducted on Mode 1 and mixed mode edge-delamination coupons to establish the relationship between fatigue load vs. onset of delamination cycle. The effect on the fatigue delamination onset of different frequencies (1 and 5 Hz) was investigated for glass, graphite,and their hybrid laminates. Although a 20 percent increase in the static onset-of-delamination strength and a 10 percent increase in ultimate strength resulted from hybridizing the all-graphite laminate with a 90 deg glass ply, the fatigue onset is lower in the hybrid laminate than in the all-graphite laminate.

Chan, Wen S.↗

NASA NTRS1988

Experimental investigation of propfan aeroelastic response in off-axis flow with mistuning

Mehmed, Oral↗

NASA NTRS1989

False lock in quadriphase receivers with unequal I- and Q-channel data rates

Results are presented concerning the noise-free false-lock problem in quadriphase receivers with unequal I-channel and Q-channel data rates. It is shown that false lock may occur when the incoming carrier frequency and the loop VCO (voltage-controlled oscillator) frequency differ by an amount that is a linear combination of a quarter of I-channel and Q-channel data rates. General expressions for the false-lock performance of quadriphase receivers are given for random as well as periodic data patterns. Specific closed-form expressions for the false-lock margin are are derived when the arm filters of the quadriphase receivers are single-pole Butterworth filters, and the format of the random data is NRZ (nonreturn-to-zero). Numerical evaluations of false-lock margin as a function of the product of 3-dB arm-filter bandwidth and symbol duration are carried out for random data using various arm filters and for different periodic I-chanel and Q-channel data patterns.

Cideciyan, Roy D.↗