155,556 research outputs found
Precise near-earth navigation with GPS: A survey of techniques
The tracking accuracy of the low earth orbiters (below about 3000 km altitude) can be brought below 10 cm with a variety of differential techniques that exploit the Global Positioning System (GPS). All of these techniques require a precisely known global network of GPS ground receivers and a receiver aboard the user satellite, and all simultaneously estimate the user and GPS satellite orbits. Three basic approaches are the geometric, dynamic, and nondynamic strategies. The last combines dynamic GPS solutions with a geometric user solution. Two powerful extensions of the nondynamic strategy show considerable promise. The first uses an optimized synthesis of dynamics and geometry in the user solution, while the second uses a novel gravity-adjustment method to exploit data from repeat ground tracks. These techniques will offer sub-decimeter accuracy for dynamically unpredictable satellites down to the lowesst possible altitudes
Defect chemistry and transport properties of BaxCe0.85M0.15O3-d
The site-incorporation mechanism of M3+ dopants into A2+B4+O3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A2+-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B4+-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3-d (x = 0.85 - 1.20; M = Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs
Factors Responsible for the Stability and the Existence of a Clean Energy Gap of a Silicon Nanocluster
We present a critical theoretical study of electronic properties of silicon
nanoclusters, in particular the roles played by symmetry, relaxation, and
hydrogen passivation on the the stability, the gap states and the energy gap of
the system using the order-N [O(N)] non-orthogonal tight-binding molecular
dynamics and the local analysis of electronic structure.Comment: 26 pages including figure
The effect of supernova heating on cluster properties and constraints on galaxy formation models
Models of galaxy formation should be able to predict the properties of
clusters of galaxies, in particular their gas fractions, metallicities, X-ray
luminosity-temperature relation, temperature function and mass-deposition-rate
function. Fitting these properties places important constaints on galaxy
formation on all scales. By following gas processes in detail, our
semi-analytic model (based on that of Nulsen & Fabian 1997) is the only such
model able to predict all of the above cluster properties. We use realistic gas
fractions and gas density profiles, and as required by observations we break
the self-similarity of cluster structure by including supernova heating of
intracluster gas, the amount of which is indicated by the observed
metallicities. We also highlight the importance of the mass-deposition-rate
function as an independent and very sensitive probe of cluster structure.Comment: 5 pages, 4 figures, accepted for publication in MNRAS as a lette
Origin of Tidal Dissipation in Jupiter: II. the Value of Q
The process of tidal dissipation inside Jupiter is not yet understood. Its
tidal quality factor () is inferred to lie between and . We
examine effects of inertial-modes on tidal dissipation in a neutrally bouyant,
core-less, uniformly rotating planet. The rate of dissipation caused by
resonantly excited inertial-modes depends on the following three parameters:
how well they are coupled to the tidal potential, how strongly they are
dissipated (by the turbulent viscosity), and how densely distributed they are
in frequency. We find that as a function of tidal frequency, the value
exhibits large fluctuations, with its maximum value set by the group of
inertial-modes that have a typical offset from an exact resonance of order
their turbulent damping rates. In our model, inertial-modes shed their tidally
acquired energy very close to the surface within a narrow latitudinal zone (the
'singularity belt'), and the tidal luminosity escapes freely out of the planet.
Strength of coupling between the tidal potential and inertial-modes is
sensitive to the presence of density discontinuities inside Jupiter. In the
case of a discreet density jump (as may be caused by the transition between
metallic and molecular hydrogen), we find a time-averaged . Even
though it remains unclear whether tidal dissipation due to resonant
inertial-modes is the correct answer to the problem, it is impressive that our
simple treatment here already leads to three to five orders of magnitude
stronger damping than that from the equilibrium tide. Moreover, our conclusions
are not affected by the presence of a small solid core, a different
prescription for the turbulent viscosity, or nonlinear mode coupling, but they
depend critically on the static stability in the upper atmosphere of Jupiter.Comment: 27 pages, incl. 11 figures, ApJ in print, expanded discussions
(nonlinearity, radiative envelope
Low Temperature Susceptibility of the Noncentrosymmetric Superconductor CePt_3Si
We report ac susceptibility measurements of polycrystalline CePt_3Si down to
60 mK and in applied fields up to 9 T. In zero field, a full Meissner state
emerges at temperatures T/Tc < 0.3, where Tc=0.65 K is the onset transition
temperature. Though transport measurements show a relatively high upper
critical field Bc2 ~ 4-5 T, the low temperature susceptibility, \chi', is quite
fragile to applied field, with \chi' diminishing rapidly in fields of a few kG.
Interestingly, the field dependence of \chi' is well described by the power
law, 4\pi\chi'=(B/B_c)^{1/2}, where Bc is the field at which the onset of
resistance is observed in transport measurements.Comment: 5 figure
The soft X-ray background: evidence for widespread disruption of the gas halos of galaxy groups
Almost all of the extragalactic X-ray background (XRB) at 0.25 keV can be
accounted for by radio-quiet quasars, allowing us to derive an upper limit of 4
\bgunit\ for the remaining background at 0.25 keV. However, the XRB from the
gas halos of groups of galaxies, with gas removal due to cooling accounted for,
exceeds this upper limit by an order of magnitude if non-gravitational heating
is not included. We calculate this using simulations of halo merger trees and
realistic gas density profiles, which we require to reproduce the observed gas
fractions and abundances of X-ray clusters. In addition, we find that the
entire mass range of groups, from to \Ms,
contributes to the 0.25 keV background in this case. In a further study, we
reduce the luminosities of groups by maximally heating their gas halos while
maintaining the same gas fractions. This only reduces the XRB by a factor of 2
or less. We thus argue that most of the gas associated with groups must be
outside their virial radii. This conclusion is supported by X-ray studies of
individual groups. The properties of both groups and X-ray clusters can be
naturally explained by a model in which the gas is given excess specific
energies of keV/particle by non-gravitational heating. With this
excess energy, the gas is gravitationally unbound from groups, but recollapses
with the formation of a cluster of temperature \ga 1 keV. This is similar to
a model proposed by Pen, but is contrary to the evolution of baryons described
by Cen \& Ostriker. (abridged)Comment: 14 pages, 14 figures, submitted to MNRA
Asymmetric Totally-corrective Boosting for Real-time Object Detection
Real-time object detection is one of the core problems in computer vision.
The cascade boosting framework proposed by Viola and Jones has become the
standard for this problem. In this framework, the learning goal for each node
is asymmetric, which is required to achieve a high detection rate and a
moderate false positive rate. We develop new boosting algorithms to address
this asymmetric learning problem. We show that our methods explicitly optimize
asymmetric loss objectives in a totally corrective fashion. The methods are
totally corrective in the sense that the coefficients of all selected weak
classifiers are updated at each iteration. In contract, conventional boosting
like AdaBoost is stage-wise in that only the current weak classifier's
coefficient is updated. At the heart of the totally corrective boosting is the
column generation technique. Experiments on face detection show that our
methods outperform the state-of-the-art asymmetric boosting methods.Comment: 14 pages, published in Asian Conf. Computer Vision 201
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