10,999 research outputs found
A mathematical characterization of vegetation effect on microwave remote sensing from the Earth
In passive microwave remote sensing of the earth, a theoretical model that utilizes the radiative transfer equations was developed to account for the volume scattering effects of the vegetation canopy. Vegetation canopies such as alfalfa, sorghum, and corn are simulated by a layer of ellipsoidal scatterers and cylindrical structures. The ellipsoidal scatterers represent the leaves of vegetation and are randomly positioned and oriented. The orientation of ellipsoids is characterized by a probability density function of Eulerian angles of rotation. The cylindrical structures represent the stalks of vegetation and their radii are assumed to be much smaller than their lengths. The underlying soil is represented by a half-space medium with a homogeneous permittivity and uniform temperature profile. The radiative transfer quations are solved by a numerical method using a Gaussian quadrature formula to compute both the vertical and horizontal polarized brightness temperature as a function of observation angle. The theory was applied to the interpretation of experimental data obtained from sorghum covered fields near College Station, Texas
Spatial distribution functions of random packed granular spheres obtained by direct particle imaging
We measure the two-point density correlations and Voronoi cell distributions
of cyclically sheared granular spheres obtained with a fluorescence technique
and compare them with random packing of frictionless spheres. We find that the
radial distribution function is captured by the Percus-Yevick equation
for initial volume fraction . However, small but systematic
deviations are observed because of the splitting of the second peak as
is increased towards random close packing. The distribution of the Voronoi free
volumes deviates from postulated distributions, and the orientational
order metric shows disorder compared to numerical results reported for
frictionless spheres. Overall, these measures show significant similarity of
random packing of granular and frictionless spheres, but some systematic
differences as well.Comment: 4 pages, 4 figure
Surface electrical properties experiment. Part 2: Theory of radio-frequency interferometry in geophysical subsurface probing
The radiation fields due to a horizontal electric dipole laid on the surface of a stratified medium were calculated using a geometrical optics approximation, a modal approach, and direct numerical integration. The solutions were obtained from the reflection coefficient formulation and written in integral forms. The calculated interference patterns are compared in terms of the usefulness of the methods used to obtain them. Scattering effects are also discussed and all numerical results for anisotropic and isotropic cases are presented
Cavity quantum electro-optics. II. Input-output relations between traveling optical and microwave fields
In the previous paper [M. Tsang, Phys. Rev. A 81, 063837 (2010), e-print
arXiv:1003.0116], I proposed a quantum model of a cavity electro-optic
modulator, which can coherently couple an optical cavity mode to a microwave
resonator mode and enable novel quantum operations on the two modes, including
laser cooling of the microwave mode, electro-optic entanglement, and
backaction-evading optical measurement of a microwave quadrature. In this
sequel, I focus on the quantum input-output relations between traveling optical
and microwave fields coupled to a cavity electro-optic modulator. With
red-sideband optical pumping, the relations are shown to resemble those of a
beam splitter for the traveling fields, so that in the ideal case of zero
parasitic loss and critical coupling, microwave photons can be coherently
up-converted to "flying" optical photons with unit efficiency, and vice versa.
With blue-sideband pumping, the modulator acts as a nondegenerate parametric
amplifier, which can generate two-mode squeezing and hybrid entangled photon
pairs at optical and microwave frequencies. These fundamental operations
provide a potential bridge between circuit quantum electrodynamics and quantum
optics.Comment: 12 pages, 10 figures, v2: updated and submitte
Expert systems for automated maintenance of a Mars oxygen production system
A prototype expert system was developed for maintaining autonomous operation of a Mars oxygen production system. Normal operation conditions and failure modes according to certain desired criteria are tested and identified. Several schemes for failure detection and isolation using forward chaining, backward chaining, knowledge-based and rule-based are devised to perform several housekeeping functions. These functions include self-health checkout, an emergency shut down program, fault detection and conventional control activities. An effort was made to derive the dynamic model of the system using Bond-Graph technique in order to develop the model-based failure detection and isolation scheme by estimation method. Finally, computer simulations and experimental results demonstrated the feasibility of the expert system and a preliminary reliability analysis for the oxygen production system is also provided
Multifractality and scale invariance in human heartbeat dynamics
Human heart rate is known to display complex fluctuations. Evidence of
multifractality in heart rate fluctuations in healthy state has been reported
[Ivanov et al., Nature {\bf 399}, 461 (1999)]. This multifractal character
could be manifested as a dependence on scale or beat number of the probability
density functions (PDFs) of the heart rate increments. On the other hand, scale
invariance has been recently reported in a detrended analysis of healthy heart
rate increments [Kiyono et al., Phys. Rev. Lett. {\bf 93}, 178103 (2004)]. In
this paper, we resolve this paradox by clarifying that the scale invariance
reported is actually exhibited by the PDFs of the sum of detrended healthy
heartbeat intervals taken over different number of beats, and demonstrating
that the PDFs of detrended healthy heart rate increments are scale dependent.
Our work also establishes that this scale invariance is a general feature of
human heartbeat dynamics, which is shared by heart rate fluctuations in both
healthy and pathological states
Resonant Shattering of Neutron Star Crusts
The resonant excitation of neutron star (NS) modes by tides is investigated
as a source of short gamma-ray burst (sGRB) precursors. We find that the
driving of a crust-core interface mode can lead to shattering of the NS crust,
liberating ~10^46-10^47 erg of energy seconds before the merger of a NS-NS or
NS-black hole binary. Such properties are consistent with Swift/BAT detections
of sGRB precursors, and we use the timing of the observed precursors to place
weak constraints on the crust equation of state. We describe how a larger
sample of precursor detections could be used alongside coincident gravitational
wave detections of the inspiral by Advanced LIGO class detectors to probe the
NS structure. These two types of observations nicely complement one another,
since the former constrains the equation of state and structure near the
crust-core boundary, while the latter is more sensitive to the core equation of
state.Comment: 5 pages, 2 figures. Accepted to PR
Quantum theory of optical temporal phase and instantaneous frequency. II. Continuous time limit and state-variable approach to phase-locked loop design
We consider the continuous-time version of our recently proposed quantum
theory of optical temporal phase and instantaneous frequency [Tsang, Shapiro,
and Lloyd, Phys. Rev. A 78, 053820 (2008)]. Using a state-variable approach to
estimation, we design homodyne phase-locked loops that can measure the temporal
phase with quantum-limited accuracy. We show that post-processing can further
improve the estimation performance, if delay is allowed in the estimation. We
also investigate the fundamental uncertainties in the simultaneous estimation
of harmonic-oscillator position and momentum via continuous optical phase
measurements from the classical estimation theory perspective. In the case of
delayed estimation, we find that the inferred uncertainty product can drop
below that allowed by the Heisenberg uncertainty relation. Although this result
seems counter-intuitive, we argue that it does not violate any basic principle
of quantum mechanics.Comment: 11 pages, 6 figures, v2: accepted by PR
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