Scattering from coated targets using a frequency-dependent, surfaceimpedance boundary condition in FDTD

One method for reducing the radar cross section of objects such as aircraft and missiles is the application of a lossy coating. Computing scattering from targets coated with dielectric/magnetic materials is challenging due to the reduced wavelengths of an incident field inside the coating. These smaller wavelengths require finer sampling of the fields. A technique for implementing this calculation without greatly increased memory requirements or computation times has previously been developed using a finite-difference time-domain (FDTD) code which has been tested in one, two, and three dimensions. The method requires knowledge of the frequency behaviour of the complex permittivity and permeability, and the thickness of the dielectric coating and is applicable to thin coatings when one or more reflections from the conducting surface are significant. The impedance at the surface of the coating is computed based on the given information and then approximated using a summation of causal functions. The approximated impedance is Z-transformed and added to the FDTD code in special update equations for the fields at the surface of the coating. No computations are required inside the coatings so the FDTD grid can be sized based on the free-space wavelength. The result obtained is valid over the entire frequency range of interest, assuming that the approximated surface impedance is a good match over the entire range. Comparisons with measurements of a scale model coated missile show good agreement and almost no increase in resource requirements over a standard FDTD calculation for an uncoated metal target     

Noniterative compensation for the distance-dependent detector response and photon attenuation in SPECT imaging

A filtering approach is described, which accurately compensates for the 2D distance-dependent detector response, as well as for photon attenuation in a uniform attenuating medium. The filtering method is based on the frequency distance principle (FDP) which states that points in the object at a specific source-to-detector distance provide the most significant contribution to specified frequency regions in the discrete Fourier transform (DFT) of the sinogram. By modeling the detector point spread function as a 2D Gaussian function whose width is dependent on the source-to-detector distance, a spatially variant inverse filter can be computed and applied to the 3D DFT of the set of all sinogram slices. To minimize noise amplification the inverse filter is rolled off at high frequencies by using a previously published Wiener filter strategy. Attenuation compensation is performed with Bellini's method. It was observed that the tomographic point response, after distance-dependent filtering with the FDP, was approximately isotropic and varied substantially less with position than that obtained with other correction methods. Furthermore, it was shown that processing with this filtering technique provides reconstructions with minimal degradation in image fidelity.     

The Software Architecture for a Large Telephone Switch

This paper describes the software and the software structure used in a family of high-capacity digital telephone switches. The call processing task is complex, and has great diversity. It must support existing and future features, including some not yet envisioned. The software must also provide man-machine interfaces, maintain the office database, and offer automatic recovery from hardware and software errors. It must also meet critical real-time performance specifications. The techniques adopted to meet these requirements include the use of a strongly typed high-level language, independent design and compilation of software modules, and the provision of an automated program library system and problem reporting system. Considerable experience in the development and support of large software systems has been accumulated, and is described in the paper.     

Laser generation of narrow-band and directed ultrasound

An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element.     

Dipolar interactions and origin of spin ice in ising pyrochlore magnets

Recent experiments suggest that the Ising pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 display qualitative properties of the nearest-neighbor "spin ice" model. We discuss the dipolar energy scale present in both these materials and discuss how spin-ice behavior can occur despite the presence of long-range dipolar interactions. We present results of numerical simulations and a mean field analysis of Ising pyrochlore systems. Based on our quantitative theory, we suggest that the spin-ice behavior in these systems is due to long-range dipolar interactions, and that the nearest-neighbor exchange in Dy2Ti2O7 is antiferromagnetic.     

The Oshima-Sekiguchi and Liouville theorems on Heintze groups

Let L be an elliptic operator on a Riemannian manifold M. A function F annihilated by L is said to be L-harmonic. F is said to have moderate growth if and only if F grows at most exponentially in the Riemannian distance. If M is a rank-one symmetric space and L is the Laplace-Beltrami operator for M, the Oshima-Sekiguchi theorem [T. Oshima, J. Sekiguchi, Eigenspaces of invariant differential operators on an affine symmetric space, Invent. Math. 57 (1980) 1-81] states that a L-harmonic function F has moderate growth if and only if F is the Poisson integral of a distribution on the Furstenberg boundary. In this work we prove that this result generalizes to a very large class of homogeneous Riemannian manifolds of negative curvature. We also (i) prove a Liouville type theorem that characterizes the “polynomial-like” harmonic functions which vanish on the boundary in terms of their growth properties, (ii) describe all “polynomial-like” harmonic functions, and (iii) give asymptotic expansions for the Poisson kernel. One consequence of this work is that every Schwartz distribution on the boundary is the boundary value for a L-harmonic function F which is uniquely determined modulo “polynomial-like” harmonic functions.     

On a class of relatively prime sequences

For each natural number n, let a₀(n) = n, and if a₀(n),…,a_i(n) have already been defined, let a_i+1(n) > a_i(n) be minimal with (a_i+1(n), a₀(n) … a_i(n)) = 1. Let g(n) be the largest a_i(n) not a prime or the square of a prime. We show that g(n) ～ n and that $\text{g(n) > n + cn}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{log}\text{(n)}$ for some c > 0. The true order of magnitude of g(n) ? n seems to be connected with the fine distribution of prime numbers. We also show that “most” a_i(n) that are not primes or squares of primes are products of two distinct primes. A result of independent interest comes of one of our proofs: For every sufficiently large n there is a prime $\text{p < n}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ with [$\text{n}\text{p}$] composite.     

Standardized evaluation methodology for 2-D-3-D registration

In the past few years, a number of two-dimensional (2-D) to three-dimensional (3-D) (2-D-3-D) registration algorithms have been introduced. However, these methods have been developed and evaluated for specific applications, and have not been directly compared. Understanding and evaluating their performance is therefore an open and important issue. To address this challenge we introduce a standardized evaluation methodology, which can be used for all types of 2-D-3-D registration methods and for different applications and anatomies. Our evaluation methodology uses the calibrated geometry of a 3-D rotational X-ray (3DRX) imaging system (Philips Medical Systems, Best, The Netherlands) in combination with image-based 3-D-3-D registration for attaining a highly accurate gold standard for 2-D X-ray to 3-D MR/CT/3DRX registration. Furthermore, we propose standardized starting positions and failure criteria to allow future researchers to directly compare their methods. As an illustration, the proposed methodology has been used to evaluate the performance of two 2-D-3-D registration techniques, viz. a gradient-based and an intensity-based method, for images of the spine. The data and gold standard transformations are available on the internet (http://www.isi.uu.nl/Research/Databases/).