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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

MTF Analysis of LANDSAT-4 Thematic Mapper

The spatial radiance distribution of a ground target must be known to a resolution at least four to five times greater than that of the system under test when measuring a satellite sensor's modulation transfer function. Calibration of the target requires either the use of man-made special purpose targets with known properties, e.g., a small reflective mirror or a dark-light linear pattern such as line or edge, or use of relatively high resolution underflight imagery to calibrate an arbitrary ground scene. Both approaches are to be used, in addition a technique that utilizes an analytical model of the scene spatial frequency power spectrum is being investigated as an alternative to calibration of the scene.

Schowengerdt, R.↗
NASA NTRS1983

High latitude electromagnetic plasma wave emissions

The principal types of electromagnetic plasma wave emission produced in the high latitude auroral regions are reviewed. Three types of radiation are described: auroral kilometric radiation, auroral hiss, and Z mode radiation. Auroral kilometric radiation is a very intense radio emission generated in the free space R-X mode by electrons associated with the formation of discrete auroral arcs in the local evening. Theories suggest that this radiation is an electron cyclotron resonance instability driven by an enhanced loss cone in the auroral acceleration region at altitudes of about 1 to 2 R sub E. Auroral hiss is a somewhat weaker whistler mode emission generated by low energy (100 eV to 10 keV) auroral electrons. The auroral hiss usually has a V shaped frequency time spectrum caused by a freqency dependent beaming of the whistler mode into a conical beam directed upward or downward along the magnetic field.

Gurnett, D. A.↗
NASA NTRS1986

Theory and simulations of broadband electrostatic noise in the geomagnetic tail

The excitation mechanism for broadband electrostatic noise (BEN) and the effects of BEN particles in the geomagnetic tail are examined using the linear analysis theory and particle simulations. The linear theory for electrostatic instabilities is discussed. The plasma sheet particle population is simulated using counter-streaming cold ion beams, and warm ions and electrons. The ion-ion instability, ion-acoustic mode, and the electrostatic ion cyclotron harmonic waves are studied. The velocity distributions, electric field intensity, and electron plasma waves for the plasma sheet boundary layer are evaluated. The frequency wave spectrum and particle distributions are computed and analyzed. The conditions for the two simulations, which differ only in beam ion drift speed, are described; it is observed that in the first simulation the dominate modes propagate parallel to the magnetic field and in the second simulation the propagation modes are oblique. The simulation data reveal that when beam temperature is smaller than plasma sheet temperature ion-acoustic and ion-ion instabilities grow to large amplitudes heating both electrons and ions. The data are compared to ISEE-1 observations and good correlation is obtained.

Ashour-Abdalla, M.↗
NASA NTRS1988

Radio flares from AE Aquarii - A low-power analog to Cygnus X-37

The magnetic cataclysmic variable AE Aquarii has been observed at 1.5, 4.9, 15, and 22.5 GHz. The source shows temporal variations in the flux density, often with a flarelike morphology. The flux variations are at times extreme, with values ranging from 1-12 mJy at 15 GHz; the degree of variability increases with frequency. The spectrum shows a variety of short-term behavior, at times increasing as nu, at other times decreasing with nu, and at still others flat. The long-term average of the spectrum increases as nu exp 0.3-0.4. No significant degree of circular polarization was observed. The source properties can be explained as a superposition of flare events in which electrons are accelerated to relativistic energies which then emit synchrotron radiation in discrete, expanding sources. The origin of the flare events may be connected to the disruption of the accretion disk some ditance from the white dwarf primary. The radio emission of AE Aqr shows similarities to the low-level flares from Cygnus X-3.

Bastian, T. S.↗
NASA NTRS1989

PACOSS program status and results

Future large space systems (LSS), both civilian and military, will have performance objectives which require stringent pointing accuracies, relatively fast retargeting times, short settling times, accurate dynamic shape requirements, or combinations thereof. Many of these structures will be large but lightweight and will exhibit a dense, low-frequency modal spectrum with significant content within the control bandwidth. Although it is possible in principle to achieve structural vibration control with purely active means, experience with complex structures has shown that the realities of plant model inaccuracies and real sensor and actuator dynamics frequently combine to produce disappointing results. It was shown that a combination of passive and active control will result in a simpler system which can be expected to be more reliable and less expensive than a corresponding system utilizing active control exclusively. The goals of the Passive and Active Control of Space Structures (PACOSS) program consist of a thorough investigation of the relative roles of passive active vibration control, and the development of validated means of vibration control. The program approach, representative system article, dynamic test article, and test status and results are outlined.

Richards, K. E., Jr.↗
NASA NTRS1989

Statistical characteristics of MST radar echoes and its interpretation

Two concepts of fundamental importance are reviewed: the autocorrelation function and the frequency power spectrum. In addition, some turbulence concepts, the relationship between radar signals and atmospheric medium statistics, partial reflection, and the characteristics of noise and clutter interference are discussed.

Woodman, Ronald F.↗
NASA NTRS1992

Effective equations and the inverse cascade theory for Kolmogorov flows

We study the two dimensional Kolmogorov flows in the limit as the forcing frequency goes to infinity. Direct numerical simulation indicates that the low frequency energy spectrum evolves to a universal kappa (exp -4) decay law. We derive effective equations governing the behavior of the large scale flow quantities. We then present numerical evidence that with smooth initial data, the solution to the effective equation develops a kappa (exp -4) type singularity at a finite time. This gives a convenient explanation for the kappa (exp -4) decay law exhibited by the original Kolmogorov flows.

Weinan, E.↗
NASA NTRS1991

Effect of wave-particle interaction on the outflow of ions at high latitudes

The objective of the research is to use the low-frequency wave spectrum measured by the Plasma Wave Instrument on the DE-1 spacecraft to include the wave-particle interaction (WPI) in the different polar wind models that are available at Utah State University. A Monte Carlo technique was used to simulate the ion diffusion in the velocity space due to scattering by the waves. This enabled us to study the effect of WPI on the magnitude as well as the composition of the outflow of the ionospheric ions. In particular, in the first year the 1-D semi-kinetic code developed by Barakat and Schunk (1983) was modified in order to include the effect of the WPI.

Barakat, Abdallah R.↗
NASA NTRS1991

Spectral performance of monolithic holmium and thulium lasers

Fabry-Perot resonators have been used to demonstrate single-mode lasing of holmium and neodymium YAG. The previous demonstration in the holmium laser required TE cooling the crystal to -15 C in order to achieve threshold. The present study extends that result, demonstrating +25 C operation in a 1-mm thick plano/plano resonator. The experimental configuration of lasing both the holmium and thulium lasers used a 500-mW diode laser which was collimated, circularized, and focused into a beam radius of 60 microns. The single-frequency lasing spectrum of the holmium laser is shown. By adjusting the mirror reflectivity, the ability to control the laser's wavelength is demonstrated. This laser operated with 11 mW of optical power, a 57-percent slope efficiency, and 120-mW threshold vs absorbed diode power laser for the 60-micron beam radius. The thulium laser operated very efficiently at room temperature, but on seven longitudinal modes. The Tm:TAG laser exhibits typical characteristics of spatial hole burning not seen in the Ho:Tm:YAG for flat/flat resonators.

Storm, Mark E.↗
NASA NTRS1993

Effective equations and the inverse cascade theory for Kolmogorov flows

We study the two-dimensional Kolmogorov flows in the limit as the forcing frequency goes to infinity. Direct numerical simulation indicates that the low frequency energy spectrum evolves to a universal kappa (exp -4) decay law. We derive effective equations governing the behavior of the large scale flow quantities. We then present numerical evidence that with smooth initial data, the solution to the effective equation develops a kappa (exp -4) type singularity at a finite time. This gives a convenient explanation for the kappa (exp -4) decay law exhibited by the original Kolmogorov flows.

Weinan, E.↗
NASA NTRS1993

Performance results of a digital test signal generator

Performance results of a digital test signal-generator hardware-demonstration unit are reported. Capabilities available include baseband and intermediate frequency (IF) spectrum generation, for which test results are provided. Repeatability in the setting of a given signal-to-noise ratio (SNR) when a baseband or an IF spectrum is being generated ranges from 0.01 dB at high SNR's or high data rates to 0.3 dB at low data rates or low SNR's. Baseband symbol SNR and carrier SNR (Pc/No) accuracies of 0.1 dB were verified with the built-in statistics circuitry. At low SNR's that accuracy remains to be fully verified. These results were confirmed with measurements from a demodulator synchronizer assembly for the baseband spectrum generation, and with a digital receiver (Pioneer 10 receiver) for the IF spectrum generation.

Gutierrez-Luaces, B. O.↗
NASA NTRS1993

Polarimetric thermal emission from rough surfaces

Recent theoretical works have suggested the potential of passive polarimetry in the remote sensing of geophysical media. It was shown that the third Stokes parameter U of the thermal emission may become larger for azimuthally asymmetric fields of observation. In order to investigate the potential applicability of passive polarimetry to the remote sensing of ocean surface, measurements of the polarimetric thermal emission from a sinusoidal water surface and a numerical study of the polarimetric thermal emission from randomly rough ocean surfaces were performed. Measurements of sinusoidal water surface thermal emission were performed using a sinusoidal water surface which was created by placing a thin sheet of fiberglass with a sinusoidal profile in two dimensions extended infinitely in the third dimension onto a water surface. The theory of thermal emission from a 'two-layer' periodic surface is derived and the exact solution is performed using both the extended boundary condition method (EBC) and the method of moments (MOM). The theoretical predictions are found to be in good agreement with the experimental results once the effects of the radiometer antenna pattern are included and the contribution of background noise to the measurements is modeled. The experimental results show that the U parameter indicates the direction of periodicity of the water surface and can approach values of up to 30 K for the surface observed. Next, a numerical study of polarimetric thermal emission from randomly rough surfaces was performed. A Monte Carlo technique utilizing an exact method for calculating thermal emission was chosen for the study to avoid any of the limitations of the commonly used approximate methods in rough surface scattering. In this Monte Carlo technique, a set of finite rough surface profiles in two dimensions with desired statistics was generated and extended periodically. The polarimetric thermal emission from each surface of the set was then calculated using both the EBC and the MOM and the results were averaged. The surface statistics chosen were intended to model a wind perturbed ocean surface in the X to Ku band microwave region. The results indicate that the U parameter is sensitive to the azimuthal angle between the surface periodicity and the looking angle and to the rms height of the surface, and that the U parameter is fairly insensitive to variations in polar angle, permittivity, surface power law spectrum, and surface spectrum high frequency cutoff. These properties give further strength to the idea of using the U parameter to detect wind direction over the ocean.

Johnson, J. T.↗
NASA NTRS1994

Analysis of DE-1 PWI electric field data

The measurement of low frequency electric field oscillations may be accomplished with the Plasma Wave Instrument (PWI) on DE 1. Oscillations at a frequency around 1 Hz are below the range of the conventional plasma wave receivers, but they can be detected by using a special processing of the quasi-static electric field data. With this processing it is also possible to determine if the electric field oscillations are predominately parallel or perpendicular to the ambient magnetic field. The quasi-static electric field in the DE 1 spin/orbit plane is measured with a long-wire 'double probe'. This antenna is perpendicular to the satellite spin axis, which in turn is approximately perpendicular to the geomagnetic field in the polar magnetosphere. The electric field data are digitally sampled at a frequency of 16 Hz. The measured electric field signal, which has had phase reversals introduced by the rotating antenna, is multiplied by the sine of the rotation angle between the antenna and the magnetic field. This is called the 'perpendicular' signal. The measured time series is also multiplied with the cosine of the angle to produce a separate 'parallel' signal. These two separate time series are then processed to determine the frequency power spectrum.

Weimer, Daniel↗
NASA NTRS1998

Damping Mechanisms for Microgravity Vibration Isolation (MSFC Center Director's Discretionary Fund Final Report, Project No. 94-07)

As a research facility for microgravity science, the International Space Station (ISS) will be used for numerous investigations such as protein crystal growth, combustion, and fluid mechanics experiments which require a quiescent acceleration environment across a broad spectrum of frequencies. These experiments are most sensitive to low-frequency accelerations and can tolerate much higher accelerations at higher frequency. However, the anticipated acceleration environment on ISS significantly exceeds the required acceleration level. The ubiquity and difficulty in characterization of the disturbance sources precludes source isolation, requiring vibration isolation to attenuate the anticipated disturbances to an acceptable level. This memorandum reports the results of research in active control methods for microgravity vibration isolation.

Whorton, M. S.↗
NASA NTRS1999

A TREETOPS Simulation of the STABLE Microgravity Vibration Isolation System

As a research facility for microgravity science, the International Space Station (ISS) will be used for numerous experiments which require a quiescent acceleration environment across a broad spectrum of frequencies. For many micro-gravity science experiments, the ambient acceleration environment on ISS will significantly exceed desirable levels. The ubiquity of acceleration disturbance sources and the difficulty in characterization of these sources precludes source isolation, requiring, vibration isolation to attenuate the disturbances to an acceptable level at the experiment. To provide a more quiescent acceleration environment, a vibration isolation system named STABLE (Suppression of Transient Accelerations By LEvitation) was developed. STABLE was the first successful flight test of an active isolation device for micro-gravity science payloads and was flown on STS-73/USML-2 in October 1995. This report documents the development of the high fidelity, nonlinear, multibody simulation developed using TREETOPS which was used to design the control laws and define the expected performance of the STABLE isolation system.

Nurre, G. S.↗
NASA NTRS2001

Damage Assessment of Creep Tested and Thermally Aged Metallic Alloys Using Acousto-Ultrasonics

In recent years emphasis has been placed on the early detection of material changes experienced in turbine powerplant components. During the scheduled overhaul of a turbine, the current techniques of examination of various hot section components aim to find flaws such as cracks, wear, and erosion, as well as excessive deformations. Thus far, these localized damage modes have been detected with satisfactory results. However, the techniques used to find these flaws provide no information on life until the flaws are actually detected. Major improvements in damage assessment, safety, as well as more accurate life prediction could be achieved if nondestructive evaluation (NDE) techniques could be utilized to sense material changes that occur prior to the localized defects mentioned. Because of elevated temperatures and excessive stresses, turbine components may experience creep behavior. As a result, it is desirable to monitor and access the current condition of such components. Research at the NASA Glenn Research Center involves developing and utilizing an NDE technique that discloses distributed material changes that occur prior to the localized damage detected by the current methods of inspection. In a recent study, creep processes in a nickel-base alloy were the life-limiting condition of interest, and the NDE technique was acousto-ultrasonics (AU). AU is an NDE technique that utilizes two ultrasonic transducers to interrogate the condition of a test specimen. The sending transducer introduces an ultrasonic pulse at a point on the surface of the specimen while a receiving transducer detects the signal after it has passed through the material. The goal of the method is to correlate certain parameters of the detected waveform to characteristics of the material between the two transducers. Here, the waveform parameter of interest is the attenuation due to internal damping for which information is being garnered from the frequency domain. The parameters utilized to indirectly quantify the attenuation are the ultrasonic decay rate as well as various moments of the frequency power spectrum. A new, user-friendly, graphical interface AU system was developed at NASA Glenn. This system is an all-inclusive, multifunction system that controls the sending and receiving ultrasonic transducers as well as all posttest signal analysis. The system's postprocessing software calculates the multiple parameters used to study the material of interest.

Gyekenyesi, Andrew L.↗
NASA NTRS2010

Communication

NASA s communication work for the UAS Command and Control area will build upon work currently being conducted under NASA Recovery Act funds. Communication portions of UAS NextGen ConOps, Stateof- the-Art assessment, and Gap Analysis. Preliminary simulations for UAS CNPC link scalability assessment. Surrogate UAS aircraft upgrades. This work will also leverage FY10 in-guide funding for communication link model development. UAS are currently managed through exceptions and are operating using DoD frequencies for line-of-sight (LOS) and satellite-based communications links, low-power LOS links in amateur bands, or unlicensed Instrument/Scientific/Medical (ISM) frequencies. None of these frequency bands are designated for Safety and Regularity of Flight. No radio-frequency (RF) spectrum has been allocated by the International Telecommunications Union (ITU) specifically for UAS command and control links, for either LOS or Beyond LOS (BLOS) communication.

Griner, James↗