Showing papers by "United States Naval Research Laboratory published in 1981"
TL;DR: This paper develops a statistical technique to define a noise model, and then successfully applies a local statistics noise filtering algorithm to a set of actual SEASAT SAR images, resulting in smoothed images that permit observers to resolve fine detail with an enhanced edge effect.
880 citations
TL;DR: A self-starting, distributed algorithm is proposed and developed that establishes and maintains a reliable structure that is especially suited to the needs of the HF Intra-Task Force (ITF) communication network, which is discussed in the paper.
Abstract: In this paper we consider the problem of organizing a set of mobile, radio-equipped nodes into a connected network. We require that a reliable structure be acquired and maintained in the face of arbitrary topological changes due to node motion and/or failure. We also require that such a structure be achieved without the use of a central controller. We propose and develop a self-starting, distributed algorithm that establishes and maintains such a connected architecture. This algorithm is especially suited to the needs of the HF Intra-Task Force (ITF) communication network, which is discussed in the paper.
870 citations
TL;DR: In this article, an extension of Lee's local statistics method modified to utilize local gradient information is presented, where the local mean and variance are computed from a reduced set of pixels depending on the orientation of the edge.
819 citations
TL;DR: The highly ordered crystal structure of crambin has been solved at 1.5 Å resolution directly from the diffraction data of a native crystal at a wavelength remote from the sulphur absorption edge.
Abstract: The highly ordered crystal structure of crambin has been solved at 15 A resolution directly from the diffraction data of a native crystal at a wavelength remote from the sulphur absorption edge The molecule has three disulphide bridges among its 46 amino acid residues, of which 46% are in helices and 17% are in a β-sheet Crambin is shown to be an amphipathic protein, inasmuch as its six charged groups are segregated from hydrophobic surface elements Phasing methods used here will also apply elsewhere
621 citations
TL;DR: The need for formal security models is described, the structure and operation of military security controls are described, how automation has affected security problems is considered, and possible models that have been proposed and applied to date are surveyed.
Abstract: Efforts to build "secure" computer systems have now been underway for more than a decade. Many designs have been proposed, some prototypes have been constructed, and a few systems are approaching the production stage. A small number of systems are even operating in what the Department of Defense calls the "multilevel" mode some information contained m these computer systems may have a clasmfication higher than the clearance of some of the users of those systems. This paper revmws the need for formal security models, describes the structure and operation of military security controls, considers how automation has affected security problems, surveys models that have been proposed and applied to date, and suggests possible d~rectlons for future models
470 citations
TL;DR: In this paper, the authors investigated important 13C line-broadening mechanisms in organic solids and their dependences on experimental parameters, notably static and rf magnetic field strength, and showed that no significant gain in resolution will be enjoyed at high magnetic fields, especially when variable-temperature operation is available.
395 citations
TL;DR: The principle of minimum cross-entropy (minimum directed divergence, minimum discrimination information) is a general method of inference about an unknown probability density when there exists a prior estimate of the density and new information in the form of constraints on expected values.
Abstract: The principle of minimum cross-entropy (minimum directed divergence, minimum discrimination information) is a general method of inference about an unknown probability density when there exists a prior estimate of the density and new information in the form of constraints on expected values. Various fundamental properties of cross-entropy minimization are proven and collected in one place. Cross-entropy's well-known properties as an information measure are extended and strengthened when one of the densities involved is the result of cross-entropy minimization. The interplay between properties of cross-entropy minimization as an inference procedure and properties of cross-entropy as an information measure is pointed out. Examples are included and general analytic and computational methods of finding minimum cross-entropy probability densities are discussed.
384 citations
TL;DR: In this paper, the authors present theoretical and numerical simulation studies supporting these ideas on equatorial spread-F (ESF) phenomena and show that these irregularities are formed by a two-step process in which they are driven by the steep plasma density gradients created by the primary long wavelength Rayleigh-Taylor type plasma fluid instabilities.
375 citations
TL;DR: The observations of a number of objects by the Voyager EUV instruments are summarized in this article, and the summary is considered to demonstrate the wide ranging application of the EUV spectroscopy and emphasizes the continuing importance of the search and discovery nature of spectroscopic techniques.
Abstract: The observations of a number of objects by the Voyager EUV instruments are summarized. The summary is considered to demonstrate the wide ranging application of the EUV spectroscopy. It also marks an important step forward in spectrography and emphasizes the continuing importance of the search and discovery nature of spectroscopic techniques.
249 citations
TL;DR: In this paper, the authors show that crystal structure is a basic factor in the grain-size dependence of the critical fracture energy of ceramics, and that materials of noncubic crystal structure show broader variations in fracture energy with grain size.
Abstract: Evaluation of literature data combined with new experimental measurements shows that crystal structure is a basic factor in the grain-size dependence of the critical fracture energy of ceramics. Notch beam (NB), double cantilever beam (DCB), and work-of-fracture (WOF) tests of materials having a cubic crystal structure generally agree and show, at most, a limited variation of fracture energy with grain size. However, maxima of the order of 25% in γ at intermediate grain size and 25% decreases in γ at larger grain size may occur. Materials of noncubic crystal structure show broader variations in γ with G. Both DCB and WOF tests show γ passing through maxima that are typically 100 to >400% of values at fine or large grain size. Limited DT data are consistent with this trend but various results of NB tests are mixed. Some NB test data agree with the DCB and WOF trends, whereas others taken from the same materials show no change or only a limited continuous decrease of γ as G increases.
218 citations
TL;DR: An information theory approach to the theory and practice of linear predictive coded speech compression systems is developed and it is shown that a traditional LPC system can be viewed as a minimum distortion or nearest-neighbor system where the distortion measure is a minimum discrimination information between a speech process model and an observed frame of actual speech.
Abstract: An information theory approach to the theory and practice of linear predictive coded (LPC) speech compression systems is developed. It is shown that a traditional LPC system can be viewed as a minimum distortion or nearest-neighbor system where the distortion measure is a minimum discrimination information between a speech process model and an observed frame of actual speech. This distortion measure is used in an algorithm for computer-aided design of block source codes subject to a fidelity criterion to obtain a 750-bits/s speech compression system that resembles an LPC system but has a much lower rate, a larger memory requirement, and requires no on-line LPC analysis. Quantitative and informal subjective comparisons are made among our system and LPC systems.
TL;DR: In this article, the selfconsistent electronic structures of Si, Ge, and zinc-blende GaP, GaAs, ZnS, and ZnSe have been determined using the linear combination of Gaussian orbitals method with a local-density form of the exchange-correlation functional.
Abstract: The self-consistent electronic structures of Si, Ge, and zinc-blende GaP, GaAs, ZnS, and ZnSe have been determined using the linear combination of Gaussian orbitals method with a local-density form of the exchange-correlation functional. A completely general form of the spatial dependence of the potential has been used to describe accurately the bonding character in the tetrahedral environment. Results are presented for the valence- and conduction-band energies, densities of states, effective masses, and charge densities. Comparisons are made with previous calculations and with photoemission measurements. A striking result is that the local-density theory underestimates the optical band gaps by approximately 30% or more, although the general conduction-band topology is good. The theoretical valence-band energies, charge densities, and electron and hole effective masses are also in good agreement with experiment. The energies and wave functions presented here are used to determine the optical properties of these materials in the following paper.
TL;DR: In this paper, progress toward high laser fusion can be measured by a set of five critical elements: (1) the laser-to-fuel coupling efficiency, (2) the cold fuel isentrope, (3) the implosion symmetry, (4) the ablation pressure and (5) the ignition concept.
Abstract: Progress toward high laser fusion can be measured by a set of five critical elements. They are: (1) the laser-to-fuel coupling efficiency, (2) the cold fuel isentrope, (3) the implosion symmetry, (4) the ablation pressure, and (5) the ignition concept.
TL;DR: This new method differs from previous methods in its explicit inclusion of a prior estimate of the power spectrum, and it reduces to maximum entropy spectral analysis as a special case.
Abstract: The principle of minimum cross-entropy (minimum directed divergence, minimum discrimination information, minimum relative entropy) is summarized, discussed, and applied to the classical problem of estimating power spectra given values of the autocorrelation function. This new method differs from previous methods in its explicit inclusion of a prior estimate of the power spectrum, and it reduces to maximum entropy spectral analysis as a special case. The prior estimate can be viewed as a means of shaping the spectral estimator. Cross-entropy minimization yields a family of shaped spectral estimators consistent with known autocorrelations. Results are derived in two equivalent ways: once by minimizing the cross-entropy of underlying probability densities, and once by arguments concerning the cross-entropy between the input and output of linear filters. Several example minimum cross-entropy spectra are included.
TL;DR: In this paper, the interband optical properties of Si, Ge, GaP, GaAs, ZnS, and ZnSe were calculated using ab initio self-consistent energy bands and wave functions obtained from the previous paper.
Abstract: The interband optical properties of Si, Ge, GaP, GaAs, ZnS, and ZnSe were calculated using ab initio self-consistent energy bands and wave functions obtained from the previous paper (paper I). Qualitatively good agreement with experiment is found, but all peak positions are shifted to lower energies since the local-density approximation underestimates the optical band gaps. Agreement with experiment with regard to line shape and peak position can be improved using an empirical energy-dependent self-energy correction as appears in the Sham-Kohn local-density theory of excitation. After examining the possible effects of lifetime broadening, our results indicate that additional many-body, excitonic, and local-field corrections must be included to achieve quantitative agreement in the intensity of certain features in the optical spectra.
TL;DR: In this paper, the mass spectra (ion intensity versus n) show anomalous behavior which is correlated with hypothesized dominance of cubiclike clusters having low surface energies, which is the case in our work.
Abstract: Ultrahigh-mass cluster ions (m/z>18 000) of the type (M(MX)/sub n/)/sup +/ have been produced by xenon-ion bombardment of CsI and detected by a high-performance secondary-ion mass spectrometer. The mass spectra (ion intensity versus n) show anomalous behavior which is correlated with hypothesized dominance of cubiclike clusters having low surface energies.
TL;DR: Larvae of the cosmopolitan fouling bryozoan Bugula neritina (Linnaeus) were offered various substrata for attachment and metamorphosis and settlement preference changes did not correlate with the changes in wettability observed on these substrata.
TL;DR: In this paper, a finite difference analysis describing the transient thermal response of fiber-reinforced organic matrix composte plates subjected to intense surface heating is presented, and a one-dimensional finite difference analyzer is presented.
Abstract: A one-dimensional finite difference analysis describing the transient ther mal response of fiber-reinforced organic matrix composte plates subjected to intense surface heating is presented The eff
TL;DR: In this article, it was shown that the oxygen(s) trapping the unpaired spin are bonded to only one silicon in the glass network, which placed important constraints on the supposed precursors of the peroxy radical in high-purity amorphous silica.
Abstract: $^{29}\mathrm{Si}$ hyperfine structure determinations for two fundamental oxygen-associated paramagnetic defect centers in high-purity amorphous silica complete the characterization of the nonbridging oxygen hole center and the peroxy radical by demonstrating in each case that the oxygen(s) trapping the unpaired spin are bonded to only one silicon in the glass network. It is shown that these findings place important constraints on the supposed precursors of the peroxy radical in $a$-Si${\mathrm{O}}_{2}$.
TL;DR: In this article, a mathematical model of the grain-size dependence of fracture energy of noncubic ceramics is developed and discussed, and the model predicts results similar to those experimentally observed.
Abstract: A mathematical model of the grain-size dependence of fracture energy of noncubic ceramics is developed and discussed. This model attributes the maxima in fracture energy of noncubic materials to countertrends of increasing numbers of microcracks and decreasing energy absorption per microcrack as grain size increases. The model predicts results similar to those experimentally observed. Agreement between calculated and measured fracture energy values is generally good, except for materials with extreme anisotropy. Sources of these differences and methods of improving calculations are discussed.
TL;DR: In this article, the fracture energies of elastomer-modified epoxy polymers have been determined over a range of strain rates from 10−2 to 103 sec−1.
Abstract: The fracture energies of elastomer-modified epoxy polymers have been determined over a range of strain rates from 10−2 to 103 sec−1. The modifiers included a liquid carboxyterminated butadiene acrylonitrile and a solid rubber. They were used alone and also in combination. In all cases, the modifiers increased the toughness of the base resin by orders of magnitude and one combination of liquid and solid rubber increased toughness by 60 times. There was a general decrease in fracture energy with increasing strain rate but even during impact testing the modified epoxys were 10 to 20 times tougher than the base polymer. Scanning electron microscopy revealed that, when combined with the liquid rubber, the solid rubber induced a localized shear yielding.
TL;DR: It is shown that bandlimiting the new codes prior to pulse compression acts as a waveform amplitude weighting which has the effect of increasing the mainlobe to sidelobe ratios.
Abstract: A new class of symmetric radar pulse compression polyphase codes is introduced which is compatible with digital signal processing. These codes share many of the useful properties of the Frank polyphase code. In contrast with the Frank code, the new codes are not subject to mainlobe to sidelobe ratio degradation caused by bandlimiting prior to sampling and digital pulse compression. It is shown that bandlimiting the new codes prior to pulse compression acts as a waveform amplitude weighting which has the effect of increasing the mainlobe to sidelobe ratios.
TL;DR: In this paper, the axial wavelength and the characteristic frequency of oscillation at the onset of absolute instability are given. And preliminary experimental results on both types of oscillations are reported and are found to be in good agreement with the theory.
Abstract: The operation of the gyrotron travelling wave amplifier is based on the convective cyclotron maser instability. It is found that this convective instability may become absolute (nonconvective) at a sufficiently high current level, resulting in oscillation instead of amplification. This threshold current for the transition depends sensitively on the applied magnetic field. The axial wavelength and the characteristic frequency of oscillation at the onset of absolute instability are given. It is found that momentum spread has virtually no effect on the threshold current. A small amount of resistive wall loss, however, raises the threshold current significantly. Oscillations due to partial reflection at the ends of the system are also examined. Preliminary experimental results on both types of oscillations are reported and are found to be in good agreement with the theory.
TL;DR: The static thermal sensitivity of the optical phase in bare and jacketed fibers has been studied both analytically and experimentally and the results are found to be in agreement with experimental results obtained from measurements employing a Mach-Zehnder fiber interferometer.
Abstract: The static thermal sensitivity of the optical phase in bare and jacketed fibers has been studied both analytically and experimentally. Taking into account the exact fiber composition and geometry, the strains have been determined from the thermally induced stresses using the appropriate boundary conditions, and the resulting phase shift has been calculated. The results of this analysis are found to be in agreement with experimental results obtained from measurements employing a Mach-Zehnder fiber interferometer.
TL;DR: In this paper, it was found that during 1978 and 1979, coronal holes reflected the influence of differential rotation, and were present within a slowly evolving large-scale pattern in spite of the relatively high level of sunspot activity.
Abstract: It was found that, during 1978 and 1979, coronal holes reflected the influence of differential rotation, and were present within a slowly evolving large-scale pattern in spite of the relatively high level of sunspot activity. The long-lived 28.5-day pattern is not produced by a rigidly rotating quasi-stationary structure on the sun, but appears to be produced by a nonstationary migratory process associated with solar differential rotation. The association between coronal holes and solar wind speed enhancements at earth continues to depend on the latitude of the holes (relative to the heliographic latitude of earth), but even the best associations since 1976 have speeds of only 500-600 km/s rather than the values of 600-700 km/s that usually occurred during the declining phase of sunspot cycle 20.
TL;DR: In this article, the initial strength of (σi) and thermal shock resistances (ΔTc and σr/σi), as determined by quench tests, of Al2O3-ZrO2 composites are increased by increasing amounts of tetragonal ZrO 2 second phase for contents of up to ∼15 vol%.
Abstract: The initial strength of (σi) and thermal shock resistances (ΔTc and σr/σi), as determined by quench tests, of Al2O3-ZrO2 composites are increased by increasing amounts of tetragonal ZrO2 second phase for contents of up to ∼15 vol%. For composites with ≤9 vol% ZrO2 the increases in σr and ΔTc reflect the increase in γIC with addition of ZrO2 However, for ZrO2contents >9 vol%, the thermal shock resistances (ΔTc and σr/σi) and σi are also affected by machining-induced microcracking in the surface of the samples. For ZrO2 contents >14 vol%, bulk microcracking can become extensive and result in a degradation of σi and ΔTc.
01 Jan 1981
TL;DR: In this paper, the main operating parameters of various free-electron lasers (FEL's) are discussed in a comparative way, providing a useful tool for laser design and a comparative evaluation of the various lasers.
Abstract: This paper discusses in a comparative way the main operating parameters of various free-electron lasers (FEL's), providing a useful tool for laser design and a comparative evaluation of the various lasers. We show that the various kinds of FEL's satisfy the same gain-dispersion relation and differ only in a single coupling parameter k . The different gain regimes which are common to all FEL's are delineated. We find the small signal gain in all the gain regimes (warm and cold beam, low- or high-gain, single electron, collective or strong coupling interaction). The laser gain parameter, radiation extraction efficiency, maximum power generation, and spectral width are given and compared in the various kinds of FEL's and gain regimes. The maximum power generation of all FEL's (except Compton-Raman scattering) is shown to be limited by an interaction region width parameter. This parameter and, consequently, the laser power are larger in the highly relativistic limit by a factor \sim \gamma_{0} in all bremsstrahlung FEL's, in comparison to Cerenkov-Smith-Purcell FEL's. Some expressions which were derived earlier for the magnetic bremsstrahlung FEL, like the expression for gain in the low-gain regime with the space charge effect correction and the low-gain expression for efficiency, are shown to be special cases of more general expressions.
TL;DR: Friction measurements were performed on 52100 steel implanted with Ti to fluences between 5 and 50 × 10 16 /cm 2. Auger analysis and transmission electron microscopy (TEM) were used to identify the composition and microstructure of implanted surfaces as mentioned in this paper.