5,253 research outputs found
Single-scatter channel impulse response model of non-line-of-sight ultraviolet communications
Previous studies on the temporal characteristics of single-scatter
transmission in non-line-of-sight (NLOS) ultraviolet communications (UVC) were
based on the prolate-spheroidal coordinate system. In this work, a novel
single-scatter channel impulse response (CIR) model is proposed in the
spherical coordinate system, which is more natural and comprehensible than the
prolate-spheroidal coordinate system in practical applications. Additionally,
the results of the widely accepted Monte-Carlo (MC)-based channel model of NLOS
UVC are provided to verify the proposed single-scatter CIR model. Results
indicate that the computational time costed by the proposed single-scatter CIR
model is decreased to less than 0.7% of the MC-based one with comparable
accuracy in assessing the temporal characteristics of NLOS UVC channels.Comment: 10 pages, 4 figure
Photodissociation of HBr/LiF(001): A quantum mechanical model
The photodissociation dynamics of HBr adsorbed on a LiF(001) surface is studied using time-independent quantum mechanics. The photodissociation lineshape and the Br(P(sub 1/2)-2)/Br(P(sub 3/2)-2) yield ratio are computed and compared with the corresponding quantities for gas phase photodissociation. The angular distribution of the hydrogen photofragments following excitation of adsorbed HBr is computed and found to agree qualitatively with experimental data. The effect of polarization of the photon is illustrated and discussed. The field polarization is found to affect significantly the magnitude of the photodissociation signal but not the angular dependence of the photofragment distribution, in agreement with experiment and in accord with expectations for a strongly aligned adsorbed phase
Hartree-Fock-Bogoliubov Model and Simulation of Attractive and Repulsive Bose-Einstein Condensates
We describe a model of dynamic Bose-Einstein condensates near a Feshbach
resonance that is computationally feasible under assumptions of spherical or
cylindrical symmetry. Simulations in spherical symmetry approximate the
experimentally measured time to collapse of an unstably attractive condensate
only when the molecular binding energy in the model is correct, demonstrating
that the quantum fluctuations and atom-molecule pairing included in the model
are the dominant mechanisms during collapse. Simulations of condensates with
repulsive interactions find some quantitative disagreement, suggesting that
pairing and quantum fluctuations are not the only significant factors for
condensate loss or burst formation. Inclusion of three-body recombination was
found to be inconsequential in all of our simulations, though we do not
consider recent experiments [1] conducted at higher densities
Apparatus and process for freeform fabrication of composite reinforcement preforms
A solid freeform fabrication process and apparatus for making a three-dimensional reinforcement shape. The process comprises the steps of (1) operating a multiple-channel material deposition device for dispensing a liquid adhesive composition and selected reinforcement materials at predetermined proportions onto a work surface; (2) during the material deposition process, moving the deposition device and the work surface relative to each other in an X-Y plane defined by first and second directions and in a Z direction orthogonal to the X-Y plane so that the materials are deposited to form a first layer of the shape; (3) repeating these steps to deposit multiple layers for forming a three-dimensional preform shape; and (4) periodically hardening the adhesive to rigidize individual layers of the preform. These steps are preferably executed under the control of a computer system by taking additional steps of (5) creating a geometry of the shape on the computer with the geometry including a plurality of segments defining the preform shape and each segment being preferably coded with a reinforcement composition defining a specific proportion of different reinforcement materials; (6) generating programmed signals corresponding to each of the segments in a predetermined sequence; and (7) moving the deposition device and the work surface relative to each other in response to these programmed signals. Preferably, the system is also operated to generate a support structure for any un-supported feature of the 3-D preform shape
Overcoming the Challenges Associated with Image-based Mapping of Small Bodies in Preparation for the OSIRIS-REx Mission to (101955) Bennu
The OSIRIS-REx Asteroid Sample Return Mission is the third mission in NASA's
New Frontiers Program and is the first U.S. mission to return samples from an
asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is
the selection of a prime sample-site on the surface of asteroid (101955) Bennu.
Mission success hinges on identifying a site that is safe and has regolith that
can readily be ingested by the spacecraft's sampling mechanism. To inform this
mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx
Camera Suite and the images are used to develop several foundational data
products. Acquiring the necessary inputs to these data products requires
observational strategies that are defined specifically to overcome the
challenges associated with mapping a small irregular body. We present these
strategies in the context of assessing candidate sample-sites at Bennu
according to a framework of decisions regarding the relative safety,
sampleability, and scientific value across the asteroid's surface. To create
data products that aid these assessments, we describe the best practices
developed by the OSIRIS-REx team for image-based mapping of irregular small
bodies. We emphasize the importance of using 3D shape models and the ability to
work in body-fixed rectangular coordinates when dealing with planetary surfaces
that cannot be uniquely addressed by body-fixed latitude and longitude.Comment: 31 pages, 10 figures, 2 table
Information content of ozone retrieval algorithms
The algorithms are characterized that were used for production processing by the major suppliers of ozone data to show quantitatively: how the retrieved profile is related to the actual profile (This characterizes the altitude range and vertical resolution of the data); the nature of systematic errors in the retrieved profiles, including their vertical structure and relation to uncertain instrumental parameters; how trends in the real ozone are reflected in trends in the retrieved ozone profile; and how trends in other quantities (both instrumental and atmospheric) might appear as trends in the ozone profile. No serious deficiencies were found in the algorithms used in generating the major available ozone data sets. As the measurements are all indirect in someway, and the retrieved profiles have different characteristics, data from different instruments are not directly comparable
NASA/MSFC FY88 Global Scale Atmospheric Processes Research Program Review
Interest in environmental issues and the magnitude of the environmental changes continues. One way to gain more understanding of the atmosphere is to make measurements on a global scale from space. The Earth Observation System is a series of new sensors to measure globally atmospheric parameters. Analysis of satellite data by developing algorithms to interpret the radiance information improves the understanding and also defines requirements for these sensors. One measure of knowledge of the atmosphere lies in the ability to predict its behavior. Use of numerical and experimental models provides a better understanding of these processes. These efforts are described in the context of satellite data analysis and fundamental studies of atmospheric dynamics which examine selected processes important to the global circulation
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