21 research outputs found
Points on nodal lines with given direction
We study of the directional distribution function of nodal lines for
eigenfunctions of the Laplacian on a planar domain. This quantity counts the
number of points where the normal to the nodal line points in a given
direction. We give upper bounds for the flat torus, and compute the expected
number for arithmetic random waves.Comment: The statement and the proof of Theorem 1.2 correcte
Recommended from our members
Percolation, statistical topography, and transport in random-media
A review of classical percolation theory is presented, with an emphasis on novel applications to statistical topography, turbulent diffusion, and heterogeneous media. Statistical topography involves the geometrical properties of the isosets (contour lines or surfaces) of a random potential psi(x). For rapidly decaying correlations of psi, the isopotentials fall into the same universality class as the perimeters of percolation clusters. The topography of long-range correlated potentials involves many length scales and is associated either with the correlated percolation problem or with Mandelbrot's fractional Brownian reliefs. In all cases, the concept of fractal dimension is particularly fruitful in characterizing the geometry of random fields. The physical applications of statistical topography include diffusion in random velocity fields, heat and particle transport in turbulent plasmas, quantum Hall effect, magnetoresistance in inhomogeneous conductors with the classical Hall effect, and many others where random isopotentials are relevant. A geometrical approach to studying transport in random media, which captures essential qualitative features of the described phenomena, is advocated.Physic
Statistical assessment on Non-cooperative Target Recognition using the Neyman-Pearson statistical test
Electromagnetic simulations of a X-target were performed in order to obtain its Radar Cross
Section (RCS) for several positions and frequencies. The software used is the CST MWS©. A 1 : 5
scale model of the proposed aircraft was created in CATIA© V5 R19 and imported directly into
the CST MWS© environment. Simulations on the X-band were made with a variable mesh size
due to a considerable wavelength variation. It is intended to evaluate the Neyman-Pearson (NP)
simple hypothesis test performance by analyzing its Receiver Operating Characteristics (ROCs)
for two different radar detection scenarios - a Radar Absorbent Material (RAM) coated model,
and a Perfect Electric Conductor (PEC) model for recognition purposes.
In parallel the radar range equation is used to estimate the maximum range detection for the
simulated RAM coated cases to compare their shielding effectiveness (SE) and its consequent
impact on recognition. The AN/APG-68(V)9âs airborne radar specifications were used to compute
these ranges and to simulate an airborne hostile interception for a Non-Cooperative Target
Recognition (NCTR) environment. Statistical results showed weak recognition performances
using the Neyman-Pearson (NP) statistical test. Nevertheless, good RCS reductions for most of
the simulated positions were obtained reflecting in a 50:9% maximum range detection gain for
the PAniCo RAM coating, abiding with experimental results taken from the reviewed literature.
The best SE was verified for the PAniCo and CFC-Fe RAMs.SimulaçÔes electromagnéticas do alvo foram realizadas de modo a obter a assinatura radar (RCS)
para vĂĄrias posiçÔes e frequĂȘncias. O software utilizado Ă© o CST MWS©. O modelo proposto Ă
escala 1:5 foi modelado em CATIA© V5 R19 e importado diretamente para o ambiente de trabalho
CST MWS©. Foram efectuadas simulaçÔes na banda X com uma malha de tamanho variåvel
devido à consideråvel variação do comprimento de onda. Pretende-se avaliar estatisticamente
o teste de decisĂŁo simples de Neyman-Pearson (NP), analisando as CaracterĂsticas de Operação
do Receptor (ROCs) para dois cenårios de detecção distintos - um modelo revestido com material
absorvente (RAM), e outro sendo um condutor perfeito (PEC) para fins de detecção.
Em paralelo, a equação de alcance para radares foi usada para estimar o alcance måximo de
detecção para ambos os casos de modo a comparar a eficiĂȘncia de blindagem electromagnĂ©tica
(SE) entre os diferentes revestimentos. As especificaçÔes do radar AN/APG-68(V)9 do F-16 foram
usadas para calcular os alcances para cada material, simulando uma intercepção hostil num
ambiente de reconhecimento de alvos nĂŁo-cooperativos (NCTR). Os resultados mostram performances
de detecção fracas usando o teste de decisão simples de Neyman-Pearson como detector
e uma boa redução de RCS para todas as posiçÔes na gama de frequĂȘncias selecionada. Um ganho
de alcance de detecção måximo 50:9 % foi obtido para o RAM PAniCo, estando de acordo com
os resultados experimentais da bibliografia estudada. JĂĄ a melhor SE foi verificada para o RAM
CFC-Fe e PAniCo
Study of quantitative methods for LEM LANDING-SITE selection Final report
Mathematical, statistical, and optical-Fourier methods for lunar excursion module landing site selectio
The probability of detecting and tracking RADAR targets in clutter at low grazing angles
Modern military acquisition and tracking RADARs are required to operate
against aircraft and missiles specifically designed to have minimal
radar cross section (RCS) and which fly at very low level to take
maximum advantage of terrain screening.
A model for predicting system performance is necessary for a range of
terrain types in varying precipitation and seasonal cultural conditions.
While the main degradation is from surface clutter and denial of sightline
due to terrain and other local obstructions, several other factors such
as multipath propagation, deliberate jamming and even operator performance
contribute to the total model. The possibility that some radars may
track obscured targets, however briefly, by using the diffraction path,
is of particular interest.
Although this report critically examines each of the contributory factors
in order to select optimum values for inclusion in an overall computer
prediction model; a new surface clutter model is specifically developed
for sloped terrain using actual clutter measurements. The model is
validated by comparison with an extensive survey of worldwide clutter
results from both published and unpublished sources.
Certain constraints have been necessary to restrict the study to a
manageable size, while meeting the requirements of the sponsors.
Attention is therefore focussed upon performance prediction for
typical mobile tracking radar systems designed for operation against
small RCS low level targets flying overland
Development and performance evaluation of a multistatic radar system
Multistatic radar systems are of emerging interest as they can exploit spatial diversity, enabling improved
performance and new applications. Their development is being fuelled by advances in enabling technologies
in such fields as communications and Digital Signal Processing (DSP). Such systems differ from
typical modern active radar systems through consisting of multiple spatially diverse transmitter and receiver
sites. Due to this spatial diversity, these systems present challenges in managing their operation as
well as in usefully combining the multiple sources of information to give an output to the radar operator.
In this work, a novel digital Commercial Off-The-Shelf (COTS) based coherent multistatic radar
system designed at University College London, named âNetRadâ, has been developed to produce some
of the first published experimental results, investigating the challenges of operating such a system, and
determining what level of performance might be achievable. Full detail of the various stages involved
in the combination of data from the component transmitter-receiver pairs within a multistatic system is
investigated, and many of the practical issues inherent are discussed.
Simulation and subsequent experimental verification of several centralised and decentralised detection
algorithms in terms of localisation (resolution and parameter estimation) of targets was undertaken.
The computational cost of the DSP involved in multistatic data fusion is also considered. This gave a
clear demonstration of several of the benefits of multistatic radar. Resolution of multiple targets that
would have been unresolvable in a conventional monostatic system was shown. Targets were also shown
to be plotted as two-dimensional vector position and velocities from use of time delay and Doppler shift
information only. A range of targets were used including some such as walking people which were
particularly challenging due to the variability of Radar Cross Section (RCS).
Performance improvements were found to be dependant on the type of multistatic radar, method of
data fusion and target characteristics in question. It is likely that future work will look to further explore
the optimisation of multistatic radar for the various measures of performance identified and discussed in
this work
Mean conservation of nodal volume and connectivity measures for Gaussian ensembles
We study in depth the nesting graph and volume distribution of the nodal
domains of a Gaussian field, which have been shown in previous works to exhibit
asymptotic laws. A striking link is established between the asymptotic mean
connectivity of a nodal domain (i.e.\ the vertex degree in its nesting graph)
and the positivity of the percolation probability of the field, along with a
direct dependence of the average nodal volume on the percolation probability.
Our results support the prevailing ansatz that the mean connectivity and volume
of a nodal domain is conserved for generic random fields in dimension but
not in , and are applied to a number of concrete motivating examples.Comment: 29 pages, 4 figure
COMBAT SYSTEMS Volume 1. Sensor Elements Part I. Sensor Functional Characteristics
This document includes:
CHAPTER 1. SIGNATURES, OBSERVABLES, & PROPAGATORS. CHAPTER 2. PROPAGATION OF ELECTROMAGNETIC RADIATION. I.
â FUNDAMENTAL EFFECTS. CHAPTER 3. PROPAGATION OF ELECTROMAGNETIC RADIATION. II. â WEATHER EFFECTS. CHAPTER 4. PROPAGATION OF ELECTROMAGNETIC RADIATION. III.
â REFRACTIVE EFFECTS. CHAPTER 5. PROPAGATION OF ELECTROMAGNETIC RADIATION IV.
â OTHER ATMOSPHERIC AND UNDERWATER EFFECTS. CHAPTER 6. PROPAGATION OF ACOUSTIC RADIATION. CHAPTER 7. NUCLEAR RADIATION: ITS ORIGIN AND PROPAGATION. CHAPTER 8. RADIOMETRY, PHOTOMETRY, & RADIOMETRIC ANALYSIS. CHAPTER 9. SENSOR FUNCTIONS. CHAPTER 10. SEARCH. CHAPTER 11. DETECTION. CHAPTER 12. ESTIMATION. CHAPTER 13. MODULATION AND DEMODULATION. CHAPTER 14. IMAGING AND IMAGE-BASED PERCEPTION. CHAPTER 15. TRACKING. APPENDIX A. UNITS, PHYSICAL CONSTANTS, AND USEFUL
CONVERSION FACTORS. APPENDIX B. FINITE DIFFERENCE AND FINITE ELEMENT TECHNIQUES. APPENDIX C. PROBABILITY AND STATISTICS. INDEX TO VOLUME 1.
Note by author: Note: Boldface entries in the table of contents are not yet completed