7,917 research outputs found
Research in LMSS propagation
The Virginia Tech Satellite Communications Group has participated in the Land Mobile Satellite System (LMSS) program through JPL sponsorship since 1985. Involvement has mainly been in modeling and simulation of propagation characteristics and effects. Models developed to predict cummulative fade distributions for fading LMSS signals include LMSSMOD and the Simple Models which approximate LMSSMOD. Models to predict the mean and standard deviation of signal attenuation through roadside vegetation, namely the Average Path Model, were developed. In the area of simulation, efforts have centered around the development of a software simulator that uses data bases derived from experimental data to generate simulated data with arbitrary statistical behavior. This work has progressed to the development of an integrated analysis and simulation package, LIPS. The basic theory and results for the models and simulator have been previously documented in reports and papers. All LMSS activities are summarized and details of this year's efforts are given
Investigation of the effects of short chain processing additives on polymers
The effects of low level concentrations of several short chain processing additives on the properties of the 4,4'-bis(3,4-dicarboxyphenoxy) diphenylsulfide dianhydride (BDSDA)/4,4'-diaminodiphenyl ether (ODA)/1,3'-diaminobenzene (m-phenylene diamine) (MPA) (422) copolyimide were investigated. It was noted that 5 percent MPD/phthalic anhydride (PA) is more effective than 5 percent ODA/PA and BDSDA/aniline (AN) in strengthening the host material. However, the introduction of 10 percent BDSDA/AN produces disproportionately high effects on free volume and free electron density in the host copolyimide
Zig-zag Sort: A Simple Deterministic Data-Oblivious Sorting Algorithm Running in O(n log n) Time
We describe and analyze Zig-zag Sort--a deterministic data-oblivious sorting
algorithm running in O(n log n) time that is arguably simpler than previously
known algorithms with similar properties, which are based on the AKS sorting
network. Because it is data-oblivious and deterministic, Zig-zag Sort can be
implemented as a simple O(n log n)-size sorting network, thereby providing a
solution to an open problem posed by Incerpi and Sedgewick in 1985. In
addition, Zig-zag Sort is a variant of Shellsort, and is, in fact, the first
deterministic Shellsort variant running in O(n log n) time. The existence of
such an algorithm was posed as an open problem by Plaxton et al. in 1992 and
also by Sedgewick in 1996. More relevant for today, however, is the fact that
the existence of a simple data-oblivious deterministic sorting algorithm
running in O(n log n) time simplifies the inner-loop computation in several
proposed oblivious-RAM simulation methods (which utilize AKS sorting networks),
and this, in turn, implies simplified mechanisms for privacy-preserving data
outsourcing in several cloud computing applications. We provide both
constructive and non-constructive implementations of Zig-zag Sort, based on the
existence of a circuit known as an epsilon-halver, such that the constant
factors in our constructive implementations are orders of magnitude smaller
than those for constructive variants of the AKS sorting network, which are also
based on the use of epsilon-halvers.Comment: Appearing in ACM Symp. on Theory of Computing (STOC) 201
Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations
We study lithium depletion in low-mass and solar-like stars as a function of
time, using a new diffusion coefficient describing extra-mixing taking place at
the bottom of a convective envelope. This new form is motivated by
multi-dimensional fully compressible, time implicit hydrodynamic simulations
performed with the MUSIC code. Intermittent convective mixing at the convective
boundary in a star can be modeled using extreme value theory, a statistical
analysis frequently used for finance, meteorology, and environmental science.
In this letter, we implement this statistical diffusion coefficient in a
one-dimensional stellar evolution code, using parameters calibrated from
multi-dimensional hydrodynamic simulations of a young low-mass star. We propose
a new scenario that can explain observations of the surface abundance of
lithium in the Sun and in clusters covering a wide range of ages, from
50 Myr to 4 Gyr. Because it relies on our physical model of convective
penetration, this scenario has a limited number of assumptions. It can explain
the observed trend between rotation and depletion, based on a single additional
assumption, namely that rotation affects the mixing efficiency at the
convective boundary. We suggest the existence of a threshold in stellar
rotation rate above which rotation strongly prevents the vertical penetration
of plumes and below which rotation has small effects. In addition to providing
a possible explanation for the long standing problem of lithium depletion in
pre-main sequence and main sequence stars, the strength of our scenario is that
its basic assumptions can be tested by future hydrodynamic simulations.Comment: 7 pages, 3 figures, Accepted for publication in ApJ Letter
The FEM-2 design method
The FEM-2 parallel computer is designed using methods differing from those ordinarily employed in parallel computer design. The major distinguishing aspects are: (1) a top-down rather than bottom-up design process; (2) the design considers the entire system structure in terms of layers of virtual machines; and (3) each layer of virtual machine is defined formally during the design process. The result is a complete hardware/software system design. The basic design method is discussed and the advantages of the method are considered. A status report on the FEM-2 design is included
Anomalous temperature evolution of the internal magnetic field distribution in the charge-ordered triangular antiferromagnet AgNiO2
Zero-field muon-spin relaxation measurements of the frustrated triangular
quantum magnet AgNiO2 are consistent with a model of charge disproportionation
that has been advanced to explain the structural and magnetic properties of
this compound. Below an ordering temperature of T_N=19.9(2) K we observe six
distinct muon precession frequencies, due to the magnetic order, which can be
accounted for with a model describing the probable muon sites. The precession
frequencies show an unusual temperature evolution which is suggestive of the
separate evolution of two opposing magnetic sublattices.Comment: 4 pages, 3 figure
Effect of the source charge on charged-beam interferometry
We investigate quantal perturbations of the interferometric correlations of
charged bosons by the Coulomb field of an instantaneous, charged source. The
source charge increases the apparent source size by weakening the correlation
at non-zero relative momenta. The effect is strongest for pairs with a small
total momentum and is stronger for kaons than for pions of the same momenta.
The experimental data currently available are well described by this effect
without invoking Pratt's exploding source model. A simple expression is
proposed to account for the effect.Comment: 9 pages TEX, 3 Postscript figures available at
http://www.krl.caltech.edu/preprints/MAP.htm
Superconducting Pairing Symmetries in Anisotropic Triangular Quantum Antiferromagnets
Motivated by the recent discovery of a low temperature spin liquid phase in
layered organic compound -(ET)Cu(CN) which becomes a
superconductor under pressure, we examine the phase transition of Mott
insulating and superconducting (SC) states in a Hubbard-Heisenberg model on an
anisotropic triangular lattice. We use a renormalized mean field theory to
study the Gutzwiller projected BCS wavefucntions. The half filled electron
system is a Mott insulator at large on-site repulsion , and is a
superconductor at a moderate . The symmetry of the SC state depends on the
anisotropy, and is gapful with symmetry near the
isotropic limit and is gapless with symmetry at small anisotropy
ratio.Comment: 6 pages, 5 figure
Primjenljivost Comptonovog raspršenja za određivanje impulsne raspodjele elektrona
We present new insights regarding the validity of the impulse approximation (IA) in determining electron momentum density (EMD) from Compton scattering. These insights are obtained utilizing the code we have recently developed for the full calculation of the triple differential cross-section (TDCS) for Compton scattering. We find that, due to the averaging, at lower energies IA is more accurate for the double differential cross-section (DDCS) than for TDCS. We conclude that at such energies an EMD determination from the DDCS is more accurate than its direct determination through the measurement of the TDCS at the same energy. We also discuss the validity of IA for calculations of other less averaged Compton observables.Predstavljamo nove poglede o valjanosti impulsne aproksimacije (IA) za određivanje impulsne raspodjele elektrona (EMD) Comptonovim raspršenjem. Ti se pogledi osnivaju na primjeni nedavno razvijenog programa za računanje trostrukog diferencijalnog udarnog presjeka (TDCS) za Comptonovo raspršenje. Nalazimo da zbog računanja prosjeka, IA je na niskim energijama točnija za dvostruke diferencijalne presjeke (DDCS) nego za TDCS. Zaključujemo kako je na tim energijama određivanje EMD na osnovi DDCS točnije nego mjerenjem TDCS na istoj energiji. Raspravljamo također valjanost IA za određivanje drugih Comptonovih manje usrednjenih veličina
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