1,647 research outputs found
Advanced Sea Clutter Models and their Usefulness for Target Detection
International audienceRobust naval target detection is of significant importance to national security, to navigation safety, and to environmental monitoring. Here we consider the particular case of high resolution coastal radars, working at low grazing angles. The robustness of detection heavily relies on the appropriate knowledge of two classes of backscattered signals: the target echo, and the sea echo. The latter, usually regarded as a noise, is known as the sea clutter. This particular combination, of high resolution and low grazing angles, raises considerable challenges to radar processing algorithms. Specifically, the probability density function governing the sea clutter amplitude is no more Gaussian and a lot of effort has been aimed at characterizing it. Three approaches are reviewed here: the stochastic, texture and chaotic models. While the stochastic models represent an essay to extend classical detection theory to radars operating in marine environment, the other two models represent entirely new paradigms. Since each model has its strengths and weaknesses and more testing on real data is required to credibly validate any of the proposed models, a definitive conclusion is far from reach. However, critical comments, as well as experimentally supported conclusions are presented in the paper
Screening properties of Gaussian electrolyte models, with application to dissipative particle dynamics
We investigate the screening properties of Gaussian charge models of
electrolyte solutions by analysing the asymptotic behaviour of the pair
distribution functions. We use a combination of Monte-Carlo simulations with
the hyper-netted chain integral equation closure, and the random phase
approximation, to establish the conditions under which a screening length is
well defined and the extent to which it matches the expected Debye length. For
practical applications, for example in dissipative particle dynamics, we are
able to summarise our results in succinct rules-of-thumb which can be used for
mesoscale modeling of electrolyte solutions. We thereby establish a solid
foundation for future work, such as the systematic incorporation of specific
ion effects.Comment: 9 pages, 9 figures, 1 table, RevTeX4-
Approach to Asymptotic Behaviour in the Dynamics of the Trapping Reaction
We consider the trapping reaction A + B -> B in space dimension d=1, where
the A and B particles have diffusion constants D_A, D_B respectively. We
calculate the probability, Q(t), that a given A particle has not yet reacted at
time t. Exploiting a recent formulation in which the B particles are eliminated
from the problem we find, for t -> \infty, , where
is the density of B particles and for .Comment: 8 pages, 2 figures; minor change
Physical geomorphometry for elementary land surface segmentation and digital geomorphological mapping
By interpretations related to energy, elementary land surface segmentation can be treated as a physical problem. Many pieces of such a view found in the literature can be combined into a synthetic comprehensive physical approach. The segmentation has to be preceded by defining the character and size of searched units to result from the segmentation. A high-resolution digital elevation model (DEM) is the key input for this task; it should be generalized to the resolution best expressing information about the searched units. Elementary land surface units can be characterized by various parts of potential gravitational energy associated with a set of basic geomorphometric variables. Elevation above sea level (z) represents Global Geomorphic Energy (GGE). Regional and Local Geomorphic Energy (RGE and LGE) are parts of GGE, represented respectively by relative elevation above the local base level (zrel) and local relief (elevation differential in a moving window Δz). Derivation (change) of elevation defines the slope inclination (S), determining the local Potential Energy of Surface (PES) applicable to mass flow. Normal slope line (profile) curvature (kn)s and normal contour (tangential) curvature (kn)c express change in the PES value (ΔPES(kn )s, ΔPES(kn )c), responsible for acceleration/deceleration and convergence/ divergence of flow. Mean curvature (kmean) determines the Potential Energy of Surface applicable to Diffusion (PESD). Energetic interpretation of basic geomorphometric variables enables their direct comparison and systematic evaluation. Consequently, the homogeneity of basic geomorphometric variables defines a hierarchy of states of local geomorphic equilibria: static equilibrium, steady state, and non-steady state dynamic equilibrium. They are local attractors of landform development reflected in the geomorphometric tendency to symmetry (horizontality, various types of linearity, and curvature isotropy, together expressed by gravity concordance). Nonequilibrium and transitional states can be characterized by the PES excess (PESe) determined by difference curvature (kd), by gravity discordant change of the PES characterized by twisting curvature (τg)c, and by Integral Potential Energy of Surface Curvature (IPESC) expressed by Casorati curvature (kC) (general curvedness). Excluding zrel and Δz, all these energy-related geomorphometric variables are local point-based. Local area-based and regional variables such as Glock’s Available Relief, Melton Ruggedness Number, Stream Power Index, Openness, Topographic Position Index, Topographic Wetness Index, and Index of Connectivity also have energetic interpretations although their definition is more complex. Therefore we suggest exclusive use of the local point-based variables in designs of elementary land surface segmentation. The segmentation should take notice of natural interconnections, the hierarchy of geomorphometric variables, elements of Local Geomorphic Energy, and (dis)equilibria states, so that elementary segments are clearly interpretable geomorphologically. This is exemplified by Geographic Object-Based Image Analysis (GEOBIA) segmentation of Sandberg, a territory on the boundary of the Carpathians and Vienna Basin with a complex geomorphic history and marked morphodynamics. Compared with expert-driven field geomorphological mapping, the automatic physically-based segmentation resulted in a more specific delineation and composition of landforms. Physical-geomorphometric characteristics of the elementary forms enabled the formulation of their system and subsequent improvement of the expert-based qualitative genetic analysis, with interpretation leading to a deeper understanding of the development and recent dynamics of the landscape
DL_MG : A Parallel Multigrid Poisson and Poisson–Boltzmann Solver for Electronic Structure Calculations in Vacuum and Solution
The solution of the Poisson equation is a crucial step in electronic structure calculations, yielding the electrostatic potential—a key component of the quantum mechanical Hamiltonian. In recent decades, theoretical advances and increases in computer performance have made it possible to simulate the electronic structure of extended systems in complex environments. This requires the solution of more complicated variants of the Poisson equation, featuring nonhomogeneous dielectric permittivities, ionic concentrations with nonlinear dependencies, and diverse boundary conditions. The analytic solutions generally used to solve the Poisson equation in vacuum (or with homogeneous permittivity) are not applicable in these circumstances, and numerical methods must be used. In this work, we present DL_MG, a flexible, scalable, and accurate solver library, developed specifically to tackle the challenges of solving the Poisson equation in modern large-scale electronic structure calculations on parallel computers. Our solver is based on the multigrid approach and uses an iterative high-order defect correction method to improve the accuracy of solutions. Using two chemically relevant model systems, we tested the accuracy and computational performance of DL_MG when solving the generalized Poisson and Poisson–Boltzmann equations, demonstrating excellent agreement with analytic solutions and efficient scaling to ∼10^9 unknowns and 100s of CPU cores. We also applied DL_MG in actual large-scale electronic structure calculations, using the ONETEP linear-scaling electronic structure package to study a 2615 atom protein–ligand complex with routinely available computational resources. In these calculations, the overall execution time with DL_MG was not significantly greater than the time required for calculations using a conventional FFT-based solver
Generating Function for Particle-Number Probability Distribution in Directed Percolation
We derive a generic expression for the generating function (GF) of the
particle-number probability distribution (PNPD) for a simple reaction diffusion
model that belongs to the directed percolation universality class. Starting
with a single particle on a lattice, we show that the GF of the PNPD can be
written as an infinite series of cumulants taken at zero momentum. This series
can be summed up into a complete form at the level of a mean-field
approximation. Using the renormalization group techniques, we determine
logarithmic corrections for the GF at the upper critical dimension. We also
find the critical scaling form for the PNPD and check its universality
numerically in one dimension. The critical scaling function is found to be
universal up to two non-universal metric factors.Comment: (v1,2) 8 pages, 5 figures; one-loop calculation corrected in response
to criticism received from Hans-Karl Janssen, (v3) content as publishe
TULBURĂRILE DIN SPECTRUL SCHIZOFRENIEI CU DEBUT ÎN COPILĂRIE ŞI ADOLESCENŢĂ – PROVOCAREA DIAGNOSTICĂ
Schizofrenia cu debut precoce şi foarte precoce, mai specifi c, manifestă înainte de 18, respectiv 13 ani, constituie o entitate nosologică încă insufi cient explorată, ce aduce cu sine o veritabilă provocare diagnostică dată
de specifi cităţile vârstei timpurii la care apare. Totodată, schizofrenia cu debut în copilărie şi adolescenţă se
întrepătrunde cu o serie de tulburări ale dezvoltării neurocognitive, având şi o importantă susceptibilitate genetică. Întregul context de apariţie a acesteia, difi cultatea detectării simptomelor psihoproductive în copilăria mică, severitatea simptomelor şi cronicizarea, impun o conduită diagnostică riguroasă şi o abordare multidisciplinară în vederea realizării terapiei optime de lungă durată şi integrării socio-familiale a pacienţilor. Lucrarea de faţă îsi propune sintetizarea datelor din litaratura de specialitate în vederea realizării unui precursor pentru cercetări viitoare ample, care să conducă la crearea unor ghiduri de abordare specifi că a schizofreniei la copil şi adolescent
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