Model-based Extrapolation Research Articles

ω-κ 알고리즘을 이용한 SAR 영상의 방위각 방향 외삽 기법 연구

본 논문에서는 고해상도 SAR(Synthetic Aperture Radar) 영상 획득을 위해 방위각 방향 해상도를 향상시키기 위한 방법을 소개하였다. SAR 영상 획득을 위해 <TEX>${\omega}-k$</TEX>(omega-k) 알고리즘을 이용하였으며, 2차원 주파수 영역에서 방위각 방향으로 AR(Auto-Regressive) 방법을 이용한 외삽을 이용하여 해상도를 향상시켰다. AR 방법은 선형 예측(linear prediction) 모델을 기반으로 한 외삽 기법이다. AR 방법을 이용한 외삽 기법 중에서 Burg 알고리즘을 이용하여 예측 차수(prediction order)와 표적의 거리에 따른 성능 비교 결과를 보여 준다. In this paper, we introduce a method which enhances the azimuth resolution to obtain the high-resolution SAR image. We used <TEX>${\omega}-k$</TEX> algorithm to obtain the SAR image and extrapolation using auto-regressive(AR) method to enhance the azimuth resolution in the 2-D frequency domain. The AR method is a linear prediction model-based extrapolation technique. In the result, we showed the performance comparison with respect to the target range and the prediction order of Burg algorithm which is one of AR method.

Elections and the Regression Discontinuity Design: Lessons from Close U.S. House Races, 1942–2008

Following David Lee's pioneering work, numerous scholars have applied the regression discontinuity (RD) design to popular elections. Contrary to the assumptions of RD, however, we show that bare winners and bare losers in U.S. House elections (1942–2008) differ markedly on pretreatment covariates. Bare winners possess largeex antefinancial, experience, and incumbency advantages over their opponents and are usually the candidates predicted to win byCongressional Quarterly's pre-election ratings. Covariate imbalance actually worsens in the closest House elections. National partisan tides help explain these patterns. Previous works have missed this imbalance because they rely excessively on model-based extrapolation. We present evidence suggesting that sorting in close House elections is due mainly to activities on or before Election Day rather than postelection recounts or other manipulation. The sorting is so strong that it is impossible to achieve covariate balance between matched treated and control observations, making covariate adjustment a dubious enterprise. Although RD is problematic for postwar House elections, this example does highlight the design's advantages over alternatives: RD's assumptions are clear and weaker than model-based alternatives, and their implications are empirically testable.

Transient fields for Mg ions traversing gadolinium hosts at velocities above and below the K-shell electron velocity

Transient-field strengths for Mg ions traversing Gd hosts have been measured above the K-shell electron velocity. The results support a recent model-based extrapolation of the transient-field strength and show sufficient magnitude for g-factor measurements on exotic nuclei near the island of inversion around 32Mg produced as intermediate-energy fragments.

Molecular-dynamics simulation of thermal conductivity of silicon crystals

We investigate the thermal conductivity of bulk silicon crystals based on molecular-dynamics (MD) simulations. If it is taken that the system size must be larger than the phonon mean free path, several hundreds of millions of atoms must be computed for crystals with large thermal conductivity values such as Si. We demonstrate in this work that the thermal conductivity of Si crystals can be simulated by MD techniques using several thousands of atoms with periodic boundary conditions. We identify that the key issues generating size artifacts in small molecular-dynamics systems are the frequency cutoff imposed by the simulation domain length and the correlation artifacts caused by the periodic boundary conditions. Our method relies on the spectral Green-Kubo formulation combined with a model-based extrapolation. The obtained thermal conductivity results are in good agreement with the reference data. Both the Green-Kubo formulation and the Boltzmann transport equation lead to the prediction that the thermal conductivities of bulk crystals depend on the frequency of the thermal disturbance. This result has important implications for high-frequency electronic devices.

A model-based angular extrapolation technique for iterative method-of-moments solvers

A model-based angular extrapolation technique for iterative method-of-moments solvers

Recent results regarding turbulent wall pressure and shear stress in boundary-layer flow and the role of the viscous wall condition.

Recently, the relation between fluctuating wall pressure and shear stress at low streamwise (subconvective) wave numbers, and between these and their fluctuating Reynolds-stress ‘‘sources,’’ has been examined analytically for nominally homogeneous, slightly compressible turbulent boundary-layer flow. At low-wave-vector magnitude (but within the incompressive domain), the contributions to the wall pressure and to the wave-vector-directed component of shear stress associated with imposition of the viscous wall condition prove to be equal, perfectly coherent, and differing in phase by π/2. Assuming apparently plausible source scaling and properties, this viscous contribution is then expected to be nearly wave-vector-white and of such magnitude as to dominate the low-wave-number spectrum of wall pressure as well as shear stress and account for currently measured levels. The lack of experimental evidence for the decrease in the spectrum of turbulent wall pressure toward low wave numbers that would be expected if the Kraichnan–Phillips theorem applies as in inviscid flow might thereby be accounted for. Likewise, the level of turbulent wall-shear stress at low wave numbers obtained by model-based extrapolation of measured sublayer velocity spectra is comparable with the level of wall pressure, as predicted. But a recent treatment seems to imply that the spectrum of wall pressure vanishes toward low wave numbers even for real, viscous flow. This result is suggested to relate only to wave numbers on the order of the reciprocal streamwise extent of the turbulent boundary layer, not to wave numbers which, though perhaps smaller than the reciprocal boundary-layer thickness, are great enough to lie in the domain of interest where the condition of streamwise homogeneity is met. Controversy and conflicting views of the role of the viscous wall condition in determining turbulent boundary-layer pressure at low wave numbers thus endure. [Work supported by ONR, Code 1132F.]