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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS2010

Phased Array Radiometer Calibration Using a Radiated Noise Source

Electronic beam steering capability of phased array antenna systems offer significant advantages when used in real aperture imaging radiometers. The sensitivity of such systems is limited by the ability to accurately calibrate variations in the antenna circuit characteristics. Passive antenna systems, which require mechanical rotation to scan the beam, have stable characteristics and the noise figure of the antenna can be characterized with knowledge of its physical temperature [1],[2]. Phased array antenna systems provide the ability to electronically steer the beam in any desired direction. Such antennas make use of active components (amplifiers, phase shifters) to provide electronic scanning capability while maintaining a low antenna noise figure. The gain fluctuations in the active components can be significant, resulting in substantial calibration difficulties [3]. In this paper, we introduce two novel calibration techniques that provide an end-to-end calibration of a real-aperture, phased array radiometer system. Empirical data will be shown to illustrate the performance of both methods.

Srinivasan, Karthik↗
NASA NTRS1995

Heating and ionization of stellar chromospheres by nonthermal proton beams: Implications for impulsive phase, redshifted Lyman-alpha radiation in stellar flares

We investigate the physical basis for the timescale of impulsive-phase, redshifted Lyman-alpha emission in stellar flares on the assumption that it is determined by energy losses in a nonthermal proton beam that is penetrating the chromosphere from above. The temporal evolution of ionization and heating in representative model chromospheres subjected to such beams is calculated. The treatment of 'stopping' of beam protons takes into account their interactions with (1) electrons bound in neutral hydrogen, (2) nuclei of neutral hydrogen, (3) free electrons, and (4) ambient thermal protons. We find that, for constant incident beam flux, the system attains an equilibrium with the beam energy input to the chromosphere balanced by radiative losses. In equilibrium, the beam penetration depth is constant, and erosion of the chromosphere ceases. If the redshifted, impulsive-phase stellar flare Lyman-alpha emission is produced by downstreaming hydrogen formed through charge exchange between beam protons and ambient hydrogen, then the emission should end when the beam no longer reaches neutral hydrogen. The durations of representative emission events calculated on this assumption range from 0.1 to 14 s. The stronger the beam, the shorter the timescale over which the redshifted Lyman-alpha emission can be observed.

Brosius, Jeffrey W.↗
NASA NTRS1980

The application of artificial electron beams to magnetospheric research

Scientific and technical progress made in the utilization of electron beams injected from large sounding rockets for the investigation of magnetospheric structure and dynamics and plasma physics is reviewed. Consideration is given to the problem of vehicle neutralization during the generation of electron beams in the ionosphere and the stability and electromagnetic wave emissions of electron beams propagating in space. Optical effects observed during the two Hess artificial aurora experiments, the Minnesota Echo IV and V experiments, the Zarnitsa-1 artificial aurora experiment and the Precede-Excede experiments are discussed, together with atmospheric scattering effects observed during Echo I, II and III, and results of the three experimental programs which have detected and analyzed beams after passage through the distant magnetosphere (the second Hess experiment, ARAKS and the Electron Echo experiments) are presented. Finally, plans for future research are outlined, and a chronology and complete bibliography of the programs conducted to date are included.

Winckler, J. R.↗
NASA NTRS1990

A computational procedure for multibody systems including flexible beam dynamics

A computational procedure suitable for the solution of equations of motions for flexible multibody systems has been developed. The flexible beams are modeled using a fully nonlinear theory which accounts for both finite rotations and large deformations. The present formulation incorporates physical measures of conjugate Cauchy stress and covariant strain increments. As a consequence, the beam model can easily be interfaced with real-time strain measurements and feedback control systems. A distinct feature of the present work is the computational preservation of total energy for undamped systems; this is obtained via an objective strain increment/stress update procedure combined with an energy-conserving time integration algorithm which contains an accurate update of angular orientations. The procedure is demonstrated via several example problems.

Downer, J. D.↗
NASA NTRS1997

Characterization of Cadmium-Zinc Telluride Crystals Grown by 'Contactless' PVT Using Synchrotron White Beam Topography

Crystals of Cd(1-x)Zn(x)Te grown by Physical Vapor Transport (PVT) using self-seeding 'contactless' techniques were characterized using synchrotron radiation (reflection, transmission, and Laue back-reflection X-ray topography). Crystals of low (x = 0.04) and high (up to x approx. = 0.4) ZnTe content were investigated. Twins and defects such as dislocations, precipitates, and slip bands were identified. Extensive inhomogeneous strains present in some samples were found to be generated by interaction (sticking) with the pedestal and by composition gradients in the crystals. Large (up to about 5 mm) oval strain fields were observed around some Te precipitates. Low angle grain boundaries were found only in higher ZnTe content (x greater than or equal to 0.2) samples.

Palosz, W.↗
NASA NTRS1989

Variable-Energy Ion Beams For Modification Of Surfaces

Beam of low-energy negative oxygen ions used to grow layer of silicon dioxide on silicon. Beam unique both in purity, contains no molecular oxygen or other charged species, and in low energy, which is insufficient to damage silicon by physically displacing atoms. Low-energy growth accomplished with help of ion-beam apparatus. Directs electrons into crosswise stream of gas, generating stream of negative ions. Pair of charged plates separates ions from accompanying electrons and diverts ion beam to target - silicon substrate. Diameter of beam at target 0.5 to 0.75 cm. Promises useful device to study oxidation of semiconductors and, in certain applications, to replace conventional oxidation processes.

Chutjian, Ara↗
NASA NTRS1986

Method and apparatus for a Fabry-Perot multiple beam fringe sensor

A method and, in one embodiment of the invention, the resulting apparatus for implementing a unique multiple beam fringe sensor that is adapted to be interfaced with a low cost, compact fiber optic transmission system in order to provide an accurate digital representation of a physical parameter (e.g. temperature) of a remote sample. The sensor is fabricated so as to include a Fabry-Perot gap formed between the ends of two mated optical fibers. By examining the optical characteristics of light that is transmitted through the Fabry-Perot sensor gap, an indication of gap width can be ascertained. Accordingly, a change in Fabry-Perot sensor gap width is related to a change in the particular physical parameter to be measured. In another embodiment of the invention, a second unique multiple beam fringe sensor having a Fabry-Perot gap is disclosed that is also adapted to provide an accurate digital representation of a physical parameter (e.g. temperature) of a remote sample. The sensor may be fabricated in two segments. A fiber containing segment includes each of a driving optical fiber for supplying incident light signals to the Fabry-Perot gap and a sensing optical fiber for receiving output light signals that have been transmitted twice through the Fabry-Perot gap, the optical characteristics of which output signals provide an indication of the parameter to be sensed. A transducer segment includes the Fabry-Perot gap formed therein and means responsive to the physical parameter for changing the width of the Fabry-Perot gap and, accordingly, the optical characteristics of the light signals passing therethrough.

James, Kenneth A.↗
NASA NTRS1987

Effect of bonding on the performance of a piezoactuator-based active control system

The utilization of piezoelectric actuators in controlling the structural vibrations of flexible beams is studied. A Modified Independent Modal Space Control (MIMSC) method is devised to select the optimal location, control gains and excitation voltage of the piezoelectric actuators in a way that would minimize the amplitudes of vibrations of beams to which these actuators are bonded, as well as the input control energy necessary to suppress these vibrations. The presented method accounts for the effects that the piezoelectric actuators and the bonding layers have on changing the elastic and inertial properties of the flexible beams. Numerical examples are presented to illustrate the application of the MIMSC method and to demonstrate the effect of the physical and geometrical properties of the bonding layer on the dynamic performance of the actively controlled beams. The obtained results emphasize the importance of the devised method in designing more realistic active control systems for flexible beams, in particular, and large flexible structures in general.

Baz, A.↗
NASA NTRS1980

Distance corrections for single- and dual-color lasers by ray tracing

The physical arc length difference between the red and blue beam of a dual-color laser generates an error term in determining Delta-n as well as n (the refractive index) and D (the line length). Numerical ray trace examples and theoretical approximations show that the resulting relative error Delta-D/D increases as D-squared. Error reduction by a factor of nearly 10 is possible by using the pressure, temperature, and humidity of both endpoints to calculate n. The present study results from a desire to improve the precision of monitoring the local and regional position of the Lunar Laser Ranging Observatory atop Haleakala, Maui, Hawaii; the monitoring of the earth's crustal deformation and strain is a primary concern of the study.

Berg, E.↗
NASA NTRS1983

A revised version of the transfer matrix method to analyze one-dimensional structures

A new and general method to analyze both free and forced vibration characteristics of one-dimensional structures is discussed in this paper. This scheme links for the first time the classical transfer matrix method with the recently developed integrating matrix technique to integrate systems of differential equations. Two alternative approaches to the problem are presented. The first is based upon the lumped parameter model to account for the inertia properties of the structure. The second releases that constraint allowing a more precise description of the physical system. The free vibration of a straight uniform beam under different support conditions is analyzed to test the accuracy of the two models. Finally some results for the free vibration of a 12th order system representing a curved, rotating beam prove that the present method is conveniently extended to more complicated structural dynamics problems.

Nitzsche, F.↗
NASA NTRS1984

Electron capture by U(91+) and U(92+) and ionization of U(90+) and U(91+)

U(92+)/U(91+) and U(91+)/U(90+) electron-capture and ionization cross sections and equilibrium charge-state distributions are measured experimentally in mylar, Cu and Ta of varying thickness. Relativistic U(68+) ions at 437 or 962 MeV/nucleon are produced by a heavy-ion linear accelerator and synchrotron in tandem and passed through the target material into a magnetic specrometer and position-sensitive proportional counter for evaluation of charge states. The results are presented graphically and discussed. At 962 MeV/nucleon, beams containing 85 percent bare U(92+) nuclei are obtained using 150-mg/sq cm Cu or 85-mg/sq cm Ta; at 437 MeV/nucleon, 50 percent bare U(92+) nuclei are obtained with 90-mg/sq cm Cu. The techniques decribed can be applied to produce beams of bare U nuclei for acceleration to ultrarelativistic speeds or beams of few-electron U for atomic-physics experiments on quantum electrodynamics.

Gould, H.↗
NASA NTRS1986

Electric field measurements during the Condor critical velocity experiment

The instrumentation of the Condor critical velocity Ba experiment (Wescott et al., 1986) for the measurements of the energetic particles and the electric field associated with a Ba explosion is described. The Ba explosion created a complex electric field pulse detected in situ by a single-axis double electric-field probe on a separate spacecraft. The measurements provide evidence of several important links in the critical-velocity chain, and are consistent with two hypotheses. The first hypothesis involves the creation of large polarization electric field due to charge separation; the second hypothesis implies a polarization of the beam by currents flowing across it. The chain of physical processes inferred from the observations is in agreement with most theories for the Alfven process.

Kelley, M. C.↗
NASA NTRS1990

MBE HgCdTe heterostructure detectors

HgCdTe has been the mainstay for medium (3 to 5 micron) and long (10 to 14 micron) wavelength infrared detectors in recent years. Conventional growth and processing techniques are continuing to improve the material. However, the additional ability to tailor composition and placement of doped layers on the tens of angstroms scale using molecular beam epitaxy (MBE) provides the opportunity for new device physics and concepts to be utilized. MBE-based device structures to be discussed here can be grouped into two categories: tailored conventional structures and quantum structures. The tailored conventional structures are improvements on familiar devices, but make use of the ability to create layers of varying composition, and thus band gap, at will. The heterostructure junction can be positioned independently of doping p-n junctions. This allows the small band gap region in which the absorption occurs to be separated from a larger band gap region in which the electric field is large and where unwanted tunneling can occur. Data from hybrid MBE/liquid phase epitaxy (LPE)/bulk structures are given. Quantum structures include the HgTe-CdTe superlattice, in which the band gap and transport can be controlled by alternating thin layers (tens of angstroms thick) of HgTe and CdTe. The superlattice has been shown to exhibit behavior which is non-alloy like, including very high hole mobilities, two-dimensional structure in the absorption coefficient, resonant tunneling, and anisotropic transport.

Schulman, Joel N.↗
NASA NTRS2003

Double Layers in Expanding Plasmas and Their Relevance to the Auroral Plasma Processes

When a dense plasma consisting of a cold and a sufficiently warm electron population expands, a rarefaction shock forms [Bezzerides et al., 1978]. In the expansion of the polar wind in the magnetosphere, it has been previously shown that when a sufficiently warm electron population also exists, in addition to the usual cold ionospheric one, a discontinuity forms in the electrostatic potential distribution along the magnetic field lines [Barakat and Schunk, 1984]. Despite the lack of spatial resolution and the assumption of quasi-neutrality in the polar wind models, such discontinuities have been called double layers (DLs). Recently similar discontinuities have been invoked to partly explain the auroral acceleration of electrons and ions in the upward current region [Ergun et al., 2000]. By means of one-dimensional Vlasov simulations of expanding plasmas, for the first time we make here the connection between (1) the rarefaction shocks, (2) the discontinuities in the potential distributions, and (3) DLs. We show that when plasmas expand from opposite directions into a deep density cavity with a potential drop across it and when the plasma on the high-potential side contains hot and cold electron populations, the temporal evolution of the potential and the plasma distribution generates evolving multiple double layers with an ,extended density cavity between them. One of the DLs is the rarefaction-shock (RFS) and it forms by the reflections of the cold electrons coming from the high-potential side; it supports a part of the potential drop approximately determined by the hot electron temperature. The other DLs evolve from charge separations arising either from reflection of ions coming from the low-potential side or stemming from plasma instabilities; they support the rest of the potential drop. The instabilities forming these additional double layers involve electron-ion (e-i) Buneman or ion-ion (i-i) two-stream interactions. The electron-electron two-stream interactions on the high-potential side of the RFS generate electron-acoustic waves, which evolve into electron phase-space holes. The ion population originating from the low-potential side and trapped by the RFS is energized by the e-i and i-i instabilities and it eventually precipitates into the high-potential plasma along with an electron beam. Applications of these findings to the auroral plasma physics are discussed.

Singh, Nagendra↗
NASA NTRS2012

On Applications of Disruption Tolerant Networking to Optical Networking in Space

The integration of optical communication links into space networks via Disruption Tolerant Networking (DTN) is a largely unexplored area of research. Building on successful foundational work accomplished at JPL, we discuss a multi-hop multi-path network featuring optical links. The experimental test bed is constructed at the NASA Glenn Research Center featuring multiple Ethernet-to-fiber converters coupled with free space optical (FSO) communication channels. The test bed architecture models communication paths from deployed Mars assets to the deep space network (DSN) and finally to the mission operations center (MOC). Reliable versus unreliable communication methods are investigated and discussed; including reliable transport protocols, custody transfer, and fragmentation. Potential commercial applications may include an optical communications infrastructure deployment to support developing nations and remote areas, which are unburdened with supporting an existing heritage means of telecommunications. Narrow laser beam widths and control of polarization states offer inherent physical layer security benefits with optical communications over RF solutions. This paper explores whether or not DTN is appropriate for space-based optical networks, optimal payload sizes, reliability, and a discussion on security.

Hylton, Alan Guy↗
NASA NTRS2020

Thermal Decomposition of the Murchison CM2 Carbonaceous Chondrite: Implications of Space Weathering Processes for Sample Return Missions

Primitive carbonaceous asteroids are the target bodies for the JAXA Hayabusa2 mission to Ryugu and the NASA OSIRIS-REx mission to Bennu. Both asteroids share spectral characteristics of CI/CM type carbonaceous chondrites. Ryugu, in particular, appears to have undergone thermal processing that has modified its spectral properties. The nature and extent of space weathering processes on the surfaces of Bennu and Ryugu are under active investigation using remote sensing data from the missions [4] and through laboratory studies on analog materials. The analog studies are needed in order to understand the mineralogical and chemical changes that occur in space weathered samples that give rise to the observed optical effects measured by remote-sensing and to prepare for the analysis of returned samples. The space weathering effects of micrometeorite impact and solar wind irradiation on primitive carbonaceous chondrites have been simulated by analog studies on the Murchison CM2 chondrite. We performed a coordinated mineralogical, chemical and spectroscopic study to examine in detail the effects of thermal metamorphism on Murchison samples as an analog to processes that may have occurred on Ryugu. The bulk measurements including X-ray diffraction (XRD), Mössbauer spectroscopy, UV-VIS-NIR spectroscopy, thermogravimetric analysis, and evolved gas analysis are reported in a companion paper. Here we report on our preliminary nanoscale mineralogical and chemical analyses of pre- and post-heated Murchison samples using multiple electron beam techniques to understand how the mineralogical, chemical, and physical characteristics of carbonaceous chondrites change with increasing thermal effects.

Lee, S.↗
NASA NTRS2008

Five-Year Wilkinson Microwave Anisotropy Probe (WMAP)Observations: Beam Maps and Window Functions

Cosmology and other scientific results from the WMAP mission require an accurate knowledge of the beam patterns in flight. While the degree of beam knowledge for the WMAP one-year and three-year results was unprecedented for a CMB experiment, we have significantly improved the beam determination as part of the five-year data release. Physical optics fits are done on both the A and the B sides for the first time. The cutoff scale of the fitted distortions on the primary mirror is reduced by a factor of approximately 2 from previous analyses. These changes enable an improvement in the hybridization of Jupiter data with beam models, which is optimized with respect to error in the main beam solid angle. An increase in main-beam solid angle of approximately 1% is found for the V2 and W1-W4 differencing assemblies. Although the five-year results are statistically consistent with previous ones, the errors in the five-year beam transfer functions are reduced by a factor of approximately 2 as compared to the three-year analysis. We present radiometry of the planet Jupiter as a test of the beam consistency and as a calibration standard; for an individual differencing assembly. errors in the measured disk temperature are approximately 0.5%.

Hill, R.S.↗