22,500 research outputs found
Propagation modeling for land mobile satellite systems
A simplified empirical model for predicting primary fade statistics for a vegetatively shadowed mobile satellite signal is presented, and predictions based on the model are presented using propagation parameter values from experimental data. Results from the empirical model are used to drive a propagation simulator to produce the secondary fade statistics of average fade duration
LMSS modeling status report
The need to develop accurate models for secondary statistics of fading land mobile satellite signals has motivated a study of fading signal autocorrelations and multipath spectrum. Results of autocorrelations and power spectral densities from measured data are presented and comparisons to multipath spectrum models are made
LMSS propagation modeling at Virginia Tech
Recent efforts in the modeling of land mobile satellite systems are reported. These include descriptions of a simple model for prediction of fading statistics, a propagation simulator, and results from studies using the simulator. Predictions are compared to available measured data
Improvements and modifications to the NASA microwave signature acquisition system
A user oriented description of the modified and upgraded Microwave Signature Acquisition System is provided. The present configuration of the sensor system and its operating characteristics are documented and a step-by-step operating procedure provides instruction for mounting the antenna truss assembly, readying the system for data acquisition, and for controlling the system during the data collection sequence. The resulting data products are also identified
Photoelectrochemical water splitting: silicon photocathodes for hydrogen evolution
The development of low cost, scalable, renewable energy technologies is one of today's most pressing scientific challenges. We report on progress towards the development of a photoelectrochemical water-splitting system that will use sunlight and water as the inputs to produce renewable hydrogen with oxygen as a by-product. This system is based on the design principle of incorporating two separate, photosensitive inorganic semiconductor/liquid junctions to collectively generate the 1.7-1.9 V at open circuit needed to support both the oxidation of H_2O (or OH^-) and the reduction of H^+ (or H_2O). Si microwire arrays are a promising photocathode material because the high aspect-ratio electrode architecture allows for the use of low cost, earth-abundant materials without sacrificing energy-conversion efficiency, due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surfacearea design of the rod-based semiconductor array inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. Arrays of Si microwires grown using the Vapor Liquid Solid (VLS) mechanism have been shown to have desirable electronic light absorption properties. We have demonstrated that these arrays can be coated with earth-abundant metallic catalysts and used for photoelectrochemical production of hydrogen. This development is a step towards the demonstration of a complete artificial photosynthetic system, composed of only inexpensive, earth-abundant materials, that is simultaneously efficient, durable, and scalable
Linear electric field frequency shift (important for next generation electric dipole moment searches) induced in confined gases by a magnetic field gradient
The search for particle electric dipole moments (edm) represents a most
promising way to search for physics beyond the standard model. A number of
groups are planning a new generation of experiments using stored gases of
various kinds. In order to achieve the target sensitivities it will be
necessary to deal with the systematic error resulting from the interaction of
the well-known field with
magnetic field gradients (often referred to as the geometric phase effect
(Commins, ED; Am. J. Phys. \QTR{bf}{59}, 1077 (1991), Pendlebury, JM
\QTR{em}{et al;} Phys. Rev. \QTR{bf}{A70}, 032102 (2004)). This interaction
produces a frequency shift linear in the electric field, mimicking an edm. In
this work we introduce an analytic form for the velocity auto-correlation
function which determines the velocity-position correlation function which in
turn determines the behavior of the frequency shift (Lamoreaux, SK and Golub,
R; Phys. Rev \QTR{bf}{A71}, 032104 (2005)) and show how it depends on the
operating conditions of the experiment. We also discuss some additional issues.Comment: 21 pages, 5 figure
Late stage kinetics for various wicking and spreading problems
The kinetics of spreading of a liquid drop in a wedge or V-shaped groove, in
a network of such grooves, and on a hydrophilic strip, is re-examined. The
length of a droplet of volume Omega spreading in a wedge after a time t is
predicted to scale as Omega^(1/5) * t^(2/5), and the height profile is
predicted to be a parabola in the distance along the wedge. If the droplet is
spreading radially in a sparse network of V-shaped grooves on a surface, the
radius is predicted to scale as Omega^(1/6) * t^(1/3), provided the liquid is
completely contained within the grooves. A number of other results are also
obtained.Comment: 5 pages, 2 figures, RevTeX
First Principles Study of the Electronic and Vibrational Properties of LiNbO2
In the layered transition metal oxide LiNbO the Nb () ion is
trigonal-prismatically coordinated with O ions, with the resulting crystal
field leading to a single band system for low energy properties. A
tight-binding representation shows that intraplanar second neighbor hopping
meV dominates the first neighbor interaction ( meV). The
first and third neighbor couplings are strongly modified by oxygen
displacements of the symmetric Raman-active vibrational mode, and
electron-phonon coupling to this motion may provide the coupling mechanism for
superconductivity in Li-deficient samples (where K). We calculate all
zone-center phonon modes, identify infrared (IR) and Raman active modes, and
report LO-TO splitting of the IR modes. The Born effective charges for the
metal ions are found to have considerable anisotropy reflecting the degree to
which the ions participate in interlayer coupling and covalent bonding. Insight
into the microscopic origin of the valence band density, composed of Nb
states with some mixing of O states, is obtained from examining
Wannier functions for these bands.Comment: 12 pages, 7 figures; Updated with reviewer comments; Updated
reference
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