Radar imaging

The article of record as published may be found at http://dx.doi.org/1088/0266-5611/29/5/050301Because of their ability to operate without regard to day, night or weather conditions, radar systems are ubiquitous in remote sensing operations and are used in a wide variety of commercial and military applications. High resolution radar imaging, however, is a remote sensing subcategory that requires raw radar data to be collected over an artificially extended aperture that is much larger than the radar receiving antenna and processed to create a reflectivity image of a scene (typically by backprojection methods). These synthetic aperture radar imaging (SAR) methods have been in use for over 50 years and, while the methodology for simple radar imaging is quite mature, there are still many active research programs seeking to extend the quality of—and information obtained from—SAR images

Microlocal ISAR for Low Signal-to-Noise Environments

The problem of extracting radar target information from multi-aspect high-range-resolution data is examined. We suggest a new non-imaging approach that is based on microlocal analysis, which is a mathematical theory developed to handle highfrequency asymptotics. In essence, we relate features of the target to high-frequency components of the data. To deal with realistic band-limited data, we propose an iterative algorithm (based on the generalized Radon-Hough transform) in which we estimate the high-frequency features of the data, one after another, and subtract out the corresponding band-limited components. The algorithm has been successfully tested on noisy data, and may have a number of advantages over conventional imaging methods.This work was supported by the Office of Naval Research. M.C. also thanks Gary Hewer and the ASEE Summer Faculty Research Program for supporting her stay at China Lake

Construct validation of a language inventory

Language is the all-encompassing term used in many places and having various denotations. For this reason language has uses, too. Oral language is used as a principal factor to determine cultural disadvantage and is the primary medium of instruction in the school setting. Language operates as the intangible aspect in measurements of intelligence. The term \u27language development\u27 is used whenever one refers to the merits of federally funded preschool projects and is accepted without definition while the counter term \u27linguistics\u27 brings confusion in the mind of many classroom teachers and administrators. Commercial materials carry the label linguistic method or a language development program for a specific population. For educators \u27language\u27 is a loose, all powerful term which needs to be limited in meaning to a specific set of principles

Optimal Electromagnetic Measurements

We consider the problem of obtaining information about an inaccessible halfspace from electromagnetic measurements made in the accessible half-space. If the measurements are of limited precision, some scatterers will be undetectable because their scattered fields are below the precision of the measuring instrument. How can we make optimal measurements? In other words, what incident fields should we apply that will result in the biggest measurements? There are many ways to formulate this question, depending on the measuring instruments. In this paper we consider a formulation involving wavesplitting in the accessible half-space: what downgoing wave will result in an upgoing wave of greatest energy? This formulation is most natural for far-field problems. A closely related question arises in the case when we have a guess about the configuration of the inaccessible half-space. What measurements should we make to determine whether our guess is accurate? In this case we compare the scattered field to the field computed from the guessed configuration. Again we look for the incident field that results in the greatest energy difference. We show that the optimal incident field can be found by an iterative process involving time reversal “mirrors”. For band-limited incident fields and compactly supported scatterers, this iterative process converges to a sum of time-harmonic fields

A linearized inverse boundary value problem for Maxwell's equations

AbstractIn this paper, we consider the problem of determining the electromagnetic state of a body from measurements made on the surface of the body. We study the full set of Maxwell's equations that govern time-harmonic electric and magnetic fields. We seek to reconstruct the magnetic permeability μ, the electric permitivity ϵ and the electric conductivity σ in the interior of the body from measurements made on the surface. We exhibit appropriate boundary measurements in the form of a boundary mapping, specifically the mapping from the tangential components of the electric field to the tangential components of the magnetic field. This data can be used to reconstruct μ, ϵ and σ approximately, provided they deviate only slightly from known constants. We also estimate the reconstruction errors

First-Order Statistical Framework for Multi-Channel Passive Detection

In this paper we establish a general first-order statistical framework for the detection of a common signal impinging on spatially distributed receivers. We consider three types of channel models: 1) the propagation channel is completely known, 2) the propagation is known but channel gains are unknown, and 3) the propagation channel is unknown. For each problem, we address the cases of a) known noise variances, b) common but unknown noise variances, and c) different and unknown noise variances. For all 9 cases, we establish generalized-likelihood-ratio (GLR) detectors, and show that each one can be decomposed into two terms. The first term is a weighted combination of the GLR detectors that arise from considering each channel separately. This result is then modified by a fusion or cross-validation term, which expresses the level of confidence that the single-channel detectors have detected a common source. Of particular note are the constant false-alarm rate (CFAR) detectors that allow for scale-invariant detection in multiple channels with different noise powers.Comment: 26 pages, 1 tabl

Connection Between Time- and Frequency-Domain Three-Dimensional Inverse Problems for the Schrödinger Equation

The use of inverse scattering methods in electromagnetic remote sensing, seismic exploration and ultrasonic imaging is rapidly expanding. For these cases which involve classical wave equations with variable velocity,1 no exact inversion methods exists for general three-dimensional (3d) scatterers. However, exact inversion methods (for example, those based on the Born series2 and the Newton-Marchenko equation2) do exist for the 3d Schrödinger equation. In this paper, these inversion methods for Schrödinger’s equation will be rewritten in a form which brings out certain analogies with classical wave equations. It is hoped these analogies will eventually contribute to a common exact inversion method for both types of equations