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At least 1,009 records · Page 56
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NASA NTRS1987

Pioneer 10 observations of the Beta Cephei stars Gamma Pegasi and Delta Ceti

The results of analyzing broad-band Pioneer 10 photometric observations of the low-amplitude pulsating Beta Cephei stars Gamma Pegasi and Delta Ceti are reported. Periods and light curve amplitudes of 3.649 + or - 0.020 hr, 0.05 + or - 0.02 mag for Gamma Peg and 3.869 + or - 0.020 hr, 0.13 + or - 0.02 mag for Delta Ceti are obtained; a power spectrum analysis of the data reveals no other periods. No evidence is found for a phase shift between the light curve maxima in the UV and visible regions. The observed amplitudes combined with published visual and near-UV data suggest a flux and temperature variability of about 200 solar luminosities and 250 K for Gamma Peg and about 600 solar luminosities and 450 K for Delta Cet. These results are compared with others obtained with satellite and ground-based instrumentation.

Peters, Geraldine J.↗
NASA NTRS1987

A unique measurement technique to study laminar-separation bubble characteristics on an airfoil

A 'nonintrusive', multielement heat-transfer sensor was designed to study laminar-separation bubble characteristics on a NASA LRN (1)-1010 low-Reynolds number airfoil. The sensor consists of 30 individual nickel films, vacuum-deposited on a thin substrate (0.05 mm) that was bonded to the airfoil model with the sensor array placed streamwise on the airfoil upper surface. Experiments were conducted on a 15-cm chord model in the 50,000-300,000 chord Reynolds number range. Time history as well as spectral analysis of signals from surface film gauges were simultaneously obtained to determine the location of laminar separation and the subsequent behavior of the separated shear layer. In addition to the successful determination of laminar separation, a new phenomenon involving a large phase shift in dynamic shear stresses across the separation and reattachment points was observed.

Stack, J. P.↗
NASA NTRS1987

Beaming of Uranian kilometric radiation and implications for the planetary magnetic field

Radio waves from Uranus at 481.2 kHz detected by Voyager 2 show a strong beaming effect. When the data are analyzed according to the magnetic latitude of the observer in an offset tilted dipole field model, a sharply defined emission beam extending from 25 to 55 deg latitude is evident consistent with a source over the south magnetic pole. The lack of prominent emission from the north magnetic pole can be attributed to the large spatial offset of the magnetic dipole resulting in much lower source frequencies. For proper alignment of the beam, an advance of the dipole axis longitude by 23 deg from the standard model is required. This phase shift is interpreted as the result of higher order magnetic moments producing a skew of the field lines at low altitudes in the polar source region. This conclusion further substantiates models of the interior of Uranus which predict a complex intrinsic field with significant nondipolar contributions to the surface magnetic field.

Barbosa, D. D.↗
NASA NTRS1987

Solitary waves in the resonant phenomenon between a surface gravity wave packet and an internal gravity wave

A two-layer inviscid incompressible fluid system of intermediate depth is considered. A multiple-scales perturbation technique is applied to the basic equations and boundary conditions for a two-layer fluid system to derive a system of weakly nonlinear partial integrodifferential equations governing the resonant interaction between a surface gravity wave packet and an internal gravity wave at an intermediate depth, providing a bridge between the existing shallow and deep fluid theories. The convolution integral term in these equations accounts for the dispersion in the lower-layer fluid. An iterative fast Fourier transform scheme is developed to find solitary wave solutions to this system of equations. The overtaking collision of two pairs of solitary waves, simulated using a spectral method, is found to be inelastic. It is found that the amplitude of the solitary waves changes slightly after the collision. The phase shifts these solitary waves undergo was calculated numerically.

Sepulveda, Nicasio↗
NASA NTRS1988

Dual Pilot-Tone Calibration Technique

Signal-to-noise ratio increased. Proposed phase-shift-keying scheme for radio transmission of digital signals include two pilot tones, one near each of frequency band. Carrier frequency synthesized from two pilot-tone frequencies in dual-pilot-calibrated receiver. Effective composite carrier signal derived from pilot tones used to demodulate coherently data portion of received signal.

Simon, Marvin K.↗
NASA NTRS1988

Radio-Frequency Strain Monitor

Radio-frequency (RF) strain monitor developed to measure lengths of objects. RF waveguide or cable bonded to structure monitored. Propagation of RF signal along waveguide results in phase shift proportional to length of path traveled. Impedance mismatches placed in RF cable at nodes of structure. Records mismatches and detects overall length of line and lengths of intervals between nodes. Used to detect changes in elements of large structure with single cable. Monitor has potential for many applications, including monitoring stability of such large structures as aircraft, bridges, and buildings in Earthquake zones.

Heyman, Joseph S.↗
NASA NTRS1988

Microwave Deflection Sensor

Doppler-radar instrument measures small deflections or vibrations of reflecting surface. Acting as interferometric micrometer, instrument includes combination of analog and digital circuits measuring change in phase of radar return due to movement of reflecting surface along signal-propagation path. Includes homodyne Doppler-radar transceiver and digital signal-processing circuitry to measure change in phase shift as target deflects.

Shores, Paul↗
NASA NTRS1987

Bandwidth efficient block codes for M-ary PSK modulation

A class of bandwidth efficient block codes for M-ary PSK modulation is presented. A soft-decision decoding for this class of codes is devised. Some specific short codes for Quad Phase Shift Key (QPSK), 8-PSK and 16-PSK modulations are constructed. These codes have good minimum squared Euclidean distances and provide 2 to 5.8 dB coding gains over uncoded QPSK modulation without (or with little) bandwidth expansion. The complete weight distributions of these specific codes are determined. Based on these weight distributions, their error probabilities are evaluated. Some of these codes have simple trellis structures and hence can be decoded by Viterbi decoding algorithm with relatively simple implementation. Moreover, the codes are very suitable for use as inner codes for various cascaded coding schemes with Reed-Solomon codes as outer codes.

Lin, Shu↗
NASA NTRS1987

Active control of flexural vibrations in beams

The feasibility of using piezoelectric actuators to control the flexural oscillations of large structures in space is investigated. Flexural oscillations are excited by impulsive loads. The vibratory response can degrade the pointing accuracy of cameras and antennae, and can cause high stresses at structural node points. Piezoelectric actuators have the advantage of exerting localized bending moments. In this way, vibration is controlled without exciting rigid body modes. The actuators are used in collocated sensor/driver pairs to form a feedback control system. The sensor produces a voltage that is proportional to the dynamic stress at the sensor location, and the driver produces a force that is proportional to the voltage applied to it. The analog control system amplifies and phase shifts the sensor signal to produce the voltage signal that is applied to the driver. The feedback control is demonstrated to increase the first mode damping in a cantilever beam by up to 100 percent, depending on the amplifier gain. The damping efficiency of the control system when the piezoelectrics are not optimally positioned at points of high stress in the beam is evaluated.

Gerhold, Carl H.↗
NASA NTRS1988

Evaluation of the communications impact of a low power arcjet thruster

The interaction of a 1 kW arcjet thruster plume with a communications signal is evaluated. A two-parameter, source flow equation has been used to represent the far flow field distribution of the arcjet plume in a realistic spacecraft configuration. Modelling the plume as a plasma slab, the interaction of the plume with a 4 GHz communications signal is then evaluated in terms of signal attenuation and phase shift between transmitting and receiving antennas. Except for propagation paths which pass very near the arcjet source, the impacts to transmission appear to be negligible. The dominant signal loss mechanism is refraction of the beam rather than absorption losses due to collisions. However, significant reflection of the signal at the sharp vacuum-plasma boundary may also occur for propagation paths which pass near the source.

Carney, Lynnette M.↗
NASA NTRS1988

Omni-directional L-band antenna for mobile communications

The principle and design of an L-band omni-directional mobile communication antenna are discussed. The antenna is a circular wave guide aperture with hybrid circuits attached to higher order mode excitation. It produces polarized and symmetric two split beams in elevation. The circular waveguide is fed by eight probes with a 90 degree phase shift between their inputs. Radiation pattern characteristics are controlled by adjusting the aperture diameter and mode excitation. This antenna satisfies gain requirements as well as withstanding the harsh environment.

Kim, C. S.↗
NASA NTRS1988

Trellis-coded MPSK modulation techniques for MSAT-X(sup)1

Various trellis-coded modulated phase shift keying (MPSK) modulation techniques for transmitting 4.8 kbps over a 5 kHz RF channel are considered. The tradeoffs between coherent versus differentially coherent types of demodulation, and interleaving are discussed as well as optimum trellis-codes designed for fading channels. Simulation results are presented.

Divsalar, Dariush↗
NASA NTRS1988

An 8-DPSK TCM modem for MSAT-X

This paper describes the real-time digital implementation of an 8-differentiated phase-shift keying (DPSK) trellis-coded modulation (TCM) modem for operation on an L-band, 5 kHz wide, land mobile satellite (LMS) channel. The modem architecture as well as some of the signal processing techniques employed in the modem to combat the LMS channel impairments are described, and the modem performance over the fading channel is presented.

Jedrey, Thomas C.↗
NASA NTRS1988

Two dimensional convolute integers for machine vision and image recognition

Machine vision and image recognition require sophisticated image processing prior to the application of Artificial Intelligence. Two Dimensional Convolute Integer Technology is an innovative mathematical approach for addressing machine vision and image recognition. This new technology generates a family of digital operators for addressing optical images and related two dimensional data sets. The operators are regression generated, integer valued, zero phase shifting, convoluting, frequency sensitive, two dimensional low pass, high pass and band pass filters that are mathematically equivalent to surface fitted partial derivatives. These operators are applied non-recursively either as classical convolutions (replacement point values), interstitial point generators (bandwidth broadening or resolution enhancement), or as missing value calculators (compensation for dead array element values). These operators show frequency sensitive feature selection scale invariant properties. Such tasks as boundary/edge enhancement and noise or small size pixel disturbance removal can readily be accomplished. For feature selection tight band pass operators are essential. Results from test cases are given.

Edwards, Thomas R.↗
NASA NTRS1987

Optical generation of a circular harmonic filter for rotation and translation invariant optical pattern recognition

A new method to generate a circular harmonic filter with both rotation and translation invariance is described, which is based on a phase-shifted double-exposure (PSDE) technique. An expression for the peak correlation intensity at the origin for the correlation plane as a function of target orientation for the PSDE filter is derived. Experimental results confirming the theoretical predictions are provided.

Cheng, Yeou-Yen↗
NASA NTRS1987

On a theory of the evolution of surface cold fronts

The governing vorticity and divergence equations in the surface layer are derived and the roles of the different terms and feedback mechanisms are investigated in semigeostrophic and nongeostrophic cold-frontal systems. A planetary boundary layer model is used to perform sensitivity tests to determine that in a cold front the ageostrophic feedback mechanism as defined by Orlanski and Ross tends to act as a positive feedback mechanism, enhancing vorticity and convergence growth. Therefore, it cannot explain the phase shift between convergence and vorticity as simulated by Orlanski and Ross. An alternative plausible, though tentative, explanation in terms of a gravity wave is offered. It is shown that when the geostrophic deformation increases, nonlinear terms in the divergence equation may become important and further destabilize the system.

Levy, Gad↗
NASA NTRS1987

High-efficiency helical traveling-wave tube with dynamic velocity taper and advanced multistage depressed collector

A NASA-sponsored research and development contract has been established with the Watkins-Johnson Company to fabricate high-efficiency 20-watt helical traveling wave tubes (TWTs) operating at 8.4 to 8.43 GHz. The TWTs employ dynamic velocity tapers (DVTs) and advanced multistage depressed collectors (MDCs) having electrodes with low secondary electron emission characteristics. The TWT designs include two different DVTs; one for maximum efficiency and the other for minimum distortion and phase shift. The MDC designs include electrodes of untreated and ion-textured graphite as well as copper which has been treated for secondary electron emission suppression. Objectives of the program include achieving at least 55 percent overall efficiency. Tests with the first TWTs (with undepressed collectors) indicate good agreement between predicted and measured RF efficiencies with as high as 30 percent improvement in RF efficiency over conventional helix designs.

Curren, Arthur N.↗
NASA NTRS1987

Multiple trellis coded modulation (MTCM) performance on a fading mobile satellite channel

The author recently introduced the notion of multiple trellis coding, in which more than one channel symbol per trellis branch is transmitted. He showed that on the ideal additive white Gaussian noise (AWGN) channel, the combination of multiple trellis coding with M-ary modulation yields a performance gain with symmetric signal sets comparable to and in some cases better than that previously achieved only with signal constellation asymmetry. The combination of conventional trellis coding with multiple phase-shift-keyed (MPSK) signaling has recently been shown by the author to be a well-suited modulation/coding scheme for transmission over the fading mobile satellite channel. In particular, a rate 2/3 coded 8-PSK scheme operating at 4800 b/s is currently under development for use in NASA's Mobile Satellite Experiment (MSAT-X). The author applies the multiple trellis-coded modulation technique in the same fading mobile satellite environment, extending the analysis results previously found for its performance over the AWGN channel to the MSAT-X channel.

Simon, Marvin K.↗