26,514 research outputs found
AZB Rectangle Shrinkage Method and Heterogeneous Computing Accelerated Full Image Theory Method Ray Tracing Enabling Complex and Massive Outdoor 6G Propagation Modeling
Until now, despite their high accuracy, the utilization of the conventional
image theory method ray tracers was limited to simple simulation environments
with small number of field observation points and low maximum ray bouncing
order due to their poor computational efficiency. This study presents a novel
full-3D AZB rectangle shrinkage method and heterogeneous computing accelerated
image theory method ray tracing framework for complex and massive outdoor
propagation modeling. The proposed framework is divided into three parts: 1.
Visibility preprocessing part. 2. Visibility tree generation part: in this
part, a novel AZB rectangle shrinkage method that accelerates and reduces
generation speed and size of visibility tree is proposed. 3. Shadow testing and
field calculation part: in this part, a heterogeneous computing algorithm that
can make possible to handle a large amount of field observation points is
proposed. It is demonstrated that the proposed framework is faster more than
651 times than the image theory method solver of WinProp. Also, it is confirmed
that the proposed ray tracing framework can handle 1km x 1km wide and dense
urban outdoor simulation with up to the maximum ray bouncing order of 6 and
thousands of field observation points. The proposed ray tracing framework would
be a cornerstone of future image theory method ray tracing techniques for
complex and massive scenarios that was exclusive to the shooting and bouncing
rays method ray tracers
Cluster Multi-spacecraft Determination of AKR Angular Beaming
Simultaneous observations of AKR emission using the four-spacecraft Cluster
array were used to make the first direct measurements of the angular beaming
patterns of individual bursts. By comparing the spacecraft locations and AKR
burst locations, the angular beaming pattern was found to be narrowly confined
to a plane containing the magnetic field vector at the source and tangent to a
circle of constant latitude. Most rays paths are confined within 15 deg of this
tangent plane, consistent with numerical simulations of AKR k-vector
orientation at maximum growth rate. The emission is also strongly directed
upward in the tangent plane, which we interpret as refraction of the rays as
they leave the auroral cavity. The narrow beaming pattern implies that an
observer located above the polar cap can detect AKR emission only from a small
fraction of the auroral oval at a given location. This has important
consequences for interpreting AKR visibility at a given location. It also helps
re-interpret previously published Cluster VLBI studies of AKR source locations,
which are now seen to be only a subset of all possible source locations. These
observations are inconsistent with either filled or hollow cone beaming models.Comment: 5 pages, 4 figures. Geophys. Res. Letters (accepted
High-frequency asymptotic compression of dense BEM matrices for general geometries without ray tracing
Wave propagation and scattering problems in acoustics are often solved with
boundary element methods. They lead to a discretization matrix that is
typically dense and large: its size and condition number grow with increasing
frequency. Yet, high frequency scattering problems are intrinsically local in
nature, which is well represented by highly localized rays bouncing around.
Asymptotic methods can be used to reduce the size of the linear system, even
making it frequency independent, by explicitly extracting the oscillatory
properties from the solution using ray tracing or analogous techniques.
However, ray tracing becomes expensive or even intractable in the presence of
(multiple) scattering obstacles with complicated geometries. In this paper, we
start from the same discretization that constructs the fully resolved large and
dense matrix, and achieve asymptotic compression by explicitly localizing the
Green's function instead. This results in a large but sparse matrix, with a
faster associated matrix-vector product and, as numerical experiments indicate,
a much improved condition number. Though an appropriate localisation of the
Green's function also depends on asymptotic information unavailable for general
geometries, we can construct it adaptively in a frequency sweep from small to
large frequencies in a way which automatically takes into account a general
incident wave. We show that the approach is robust with respect to non-convex,
multiple and even near-trapping domains, though the compression rate is clearly
lower in the latter case. Furthermore, in spite of its asymptotic nature, the
method is robust with respect to low-order discretizations such as piecewise
constants, linears or cubics, commonly used in applications. On the other hand,
we do not decrease the total number of degrees of freedom compared to a
conventional classical discretization. The combination of the ...Comment: 24 pages, 13 figure
Automatic normal orientation in point clouds of building interiors
Orienting surface normals correctly and consistently is a fundamental problem
in geometry processing. Applications such as visualization, feature detection,
and geometry reconstruction often rely on the availability of correctly
oriented normals. Many existing approaches for automatic orientation of normals
on meshes or point clouds make severe assumptions on the input data or the
topology of the underlying object which are not applicable to real-world
measurements of urban scenes. In contrast, our approach is specifically
tailored to the challenging case of unstructured indoor point cloud scans of
multi-story, multi-room buildings. We evaluate the correctness and speed of our
approach on multiple real-world point cloud datasets
Incorporation of shuttle CCT parameters in computer simulation models
Computer simulations of shuttle missions have become increasingly important during recent years. The complexity of mission planning for satellite launch and repair operations which usually involve EVA has led to the need for accurate visibility and access studies. The PLAID modeling package used in the Man-Systems Division at Johnson currently has the necessary capabilities for such studies. In addition, the modeling package is used for spatial location and orientation of shuttle components for film overlay studies such as the current investigation of the hydrogen leaks found in the shuttle flight. However, there are a number of differences between the simulation studies and actual mission viewing. These include image blur caused by the finite resolution of the CCT monitors in the shuttle and signal noise from the video tubes of the cameras. During the course of this investigation the shuttle CCT camera and monitor parameters are incorporated into the existing PLAID framework. These parameters are specific for certain camera/lens combinations and the SNR characteristics of these combinations are included in the noise models. The monitor resolution is incorporated using a Gaussian spread function such as that found in the screen phosphors in the shuttle monitors. Another difference between the traditional PLAID generated images and actual mission viewing lies in the lack of shadows and reflections of light from surfaces. Ray tracing of the scene explicitly includes the lighting and material characteristics of surfaces. The results of some preliminary studies using ray tracing techniques for the image generation process combined with the camera and monitor effects are also reported
The Submillimeter Bump in Sgr A* from Relativistic MHD Simulations
Recent high resolution observations of the Galactic center black hole allow
for direct comparison with accretion disk simulations. We compare
two-temperature synchrotron emission models from three dimensional, general
relativistic magnetohydrodynamic simulations to millimeter observations of Sgr
A*. Fits to very long baseline interferometry and spectral index measurements
disfavor the monochromatic face-on black hole shadow models from our previous
work. Inclination angles \le 20 degrees are ruled out to 3 \sigma. We estimate
the inclination and position angles of the black hole, as well as the electron
temperature of the accretion flow and the accretion rate, to be i=50+35-15
degrees, \xi=-23+97-22 degrees, T_e=(5.4 +/- 3.0)x10^10 K and
Mdot=(5+15-2)x10^-9 M_sun / yr respectively, with 90% confidence. The black
hole shadow is unobscured in all best fit models, and may be detected by
observations on baselines between Chile and California, Arizona or Mexico at
1.3mm or .87mm either through direct sampling of the visibility amplitude or
using closure phase information. Millimeter flaring behavior consistent with
the observations is present in all viable models, and is caused by magnetic
turbulence in the inner radii of the accretion flow. The variability at
optically thin frequencies is strongly correlated with that in the accretion
rate. The simulations provide a universal picture of the 1.3mm emission region
as a small region near the midplane in the inner radii of the accretion flow,
which is roughly isothermal and has \nu/\nu_c ~ 1-20, where \nu_c is the
critical frequency for thermal synchrotron emission.Comment: 14 pages, 17 figures, accepted by Ap
Isotropic-medium three-dimensional cloaks for acoustic and electromagnetic waves
We propose a generalization of the two-dimensional eikonal-limit cloak
derived from a conformal transformation to three dimensions. The proposed cloak
is a spherical shell composed of only isotropic media; it operates in the
transmission mode and requires no mirror or ground plane. Unlike the well-known
omnidirectional spherical cloaks, it may reduce visibility of an arbitrary
object only for a very limited range of observation angles. In the
short-wavelength limit, this cloaking structure restores not only the
trajectories of incident rays, but also their phase, which is a necessary
ingredient to complete invisibility. Both scalar-wave (acoustic) and transverse
vector-wave (electromagnetic) versions are presented.Comment: 17 pages, 12 figure
Fast ray-tracing algorithm for circumstellar structures (FRACS) I. Algorithm description and parameter-space study for mid-IR interferometry of B[e] stars
The physical interpretation of spectro-interferometric data is strongly
model-dependent. On one hand, models involving elaborate radiative transfer
solvers are too time consuming in general to perform an automatic fitting
procedure and derive astrophysical quantities and their related errors. On the
other hand, using simple geometrical models does not give sufficient insights
into the physics of the object. We propose to stand in between these two
extreme approaches by using a physical but still simple parameterised model for
the object under consideration. Based on this philosophy, we developed a
numerical tool optimised for mid-infrared (mid-IR) interferometry, the fast
ray-tracing algorithm for circumstellar structures (FRACS) which can be used as
a stand-alone model, or as an aid for a more advanced physical description or
even for elaborating observation strategies. FRACS is based on the ray-tracing
technique without scattering, but supplemented with the use of quadtree meshes
and the full symmetries of the axisymmetrical problem to significantly decrease
the necessary computing time to obtain e.g. monochromatic images and
visibilities. We applied FRACS in a theoretical study of the dusty
circumstellar environments (CSEs) of B[e] supergiants (sgB[e]) in order to
determine which information (physical parameters) can be retrieved from present
mid-IR interferometry (flux and visibility). From a set of selected dusty CSE
models typical of sgB[e] stars we show that together with the geometrical
parameters (position angle, inclination, inner radius), the temperature
structure (inner dust temperature and gradient) can be well constrained by the
mid-IR data alone. Our results also indicate that the determination of the
parameters characterising the CSE density structure is more challenging but, in
some cases, upper limits as well as correlations on the parameters
characterising the mass loss can be obtained. Good constraints for the sgB[e]
central continuum emission (central star and inner gas emissions) can be
obtained whenever its contribution to the total mid-IR flux is only as high as
a few percents. Ray-tracing parameterised models such as FRACS are thus well
adapted to prepare and/or interpret long wavelengths (from mid-IR to radio)
observations at present (e.g. VLTI/MIDI) and near-future (e.g. VLTI/MATISSE,
ALMA) interferometers
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