Showing papers on "Point source published in 1977"

Estimation of time delay in the presence of source or receiver motion

Abstract: Signals, received at two (or more) locations, and originating from a remote acoustic point source, exhibit time delay(s) that are useful for estimating source location or receiver geometries. Consequently, numerous procedures have been proposed for passive time‐delay estimation in such cases. Past investigations apply, however, only when source and receiver motion is negligible. This paper extends previous results for maximum‐likelihood (ML) estimation of time delay to the situation where the source, or the receivers, or both are in motion. It is shown that in the simplest case with two receivers, one of the signals must be appropriately time scaled prior to computing a generalized cross correlation of the received signals.

Final SAS-2 gamma-ray results on sources in the galactic anticenter region

Abstract: Analysis of SAS-2 high energy Gamma ray data from the direction of the galactic anticenter shows that this region is characterized by: a diffuse emission from the galactic plane which has a maximum along b=0 deg and an enhancement toward negative latitudes associated with Gould's Belt, a strong point source in the direction of the Crab nebula, and a second intense localized source near galactic coordinates 195 deg, +5 deg. Gamma ray emission from the Crab source is dominated by a pulsed flux from PSR 0531+21. The total flux above 100MeV is 3.7 + or - 0.8 million/sq cm s. The source near 195 deg, + 5 deg has a flux above 100 MeV of 4.3 + or - 0.9 million/sq cm s. Its spectrum appears flatter than that of the Crab. The diffuse galactic plane emission at negative lattitudes shows a general correlation with the local matter distribution associated with Gould's Belt. The calculated Gamma ray intensity agrees well with the SAS-2 observations.

Diffractions for arbitrary source-receiver locations

Abstract: The author’s earlier diffraction paper (Trorey, 1970) for coincident source and receiver locations is extended to the case of arbitrary source and receiver locations. The extension requires no additional assumptions and, as in the earlier paper, is in closed form suitable for calculations. The new results permit modeling of normal‐moveout effects and can thus, for example, be used to study migration before and after common‐depth‐point stacking as well as to study common‐depth‐point stacking itself. In general, the point source/receiver response of an arbitrary subsurface with arbitrary source/receiver locations can be calculated.

Spatial distribution of compton-produced X-ray flux from rich and regular clusters of galaxies

Abstract: The spatial distribution of radio and X-ray fluxes from rich and regular clusters of galaxies is discussed. A diffusion model is investigated in which high-energy electrons are ejected from a point source at the center of the cluster. While diffusing out, the electrons lose energy by Compton scattering with the universal blackbody photons, thus producing the X-ray emission. Traversing intracluster magnetic fields, the electrons emit synchrotron radio emission. For the Coma cluster of galaxies good agreement is found between the observed and predicted X-ray counting-rate distribution for the Compton model. From the observed radio data a value of about 3 by 10 to the -8th power gauss is deduced for the average magnetic-field strength in the central region of Coma.

White light optical processing and holography.

Abstract: A method is described wherein optical processing and holography using transparencies are performed using point source white light. The method is coherent, in that the optical systems are linear in amplitude rather than intensity. Perfect wavelength compensation is achieved, with no restriction on the source spectral width.

Determination of the decameter wavelength spectrum of the quiet Sun.

Abstract: The Teepee Tee array of the Clark Lake Radio Observatory has been used to compare the flux of the Sun with that of the sidereal sources Tau A and Vir A at several frequencies in the range 109.0–19.0 MHz. Only the two central banks of the E-W arm of the array were used as elements of a phase switched interferometer so that the Sun could be observed as a point source and compared directly to the sidereal sources. The Sun was still partially resolved however, and appropriate corrections for this effect were made. The observations were taken at times when the Sun and either Tau A or Vir A were at the same declination. We have therefore been able to derive the values for the solar flux, without having to resort to a gain vs zenith distance correction. The observations, combined with those available in the literature, allow us to derive an accurate meter and decameter wavelength spectrum of the quiet Sun.

The Generation of Time-Dependent Neutron Transport Solutions in Infinite Media

Abstract: The multiple collision technique as applied to the monoenergetic time-dependent neutron transport equation for pulsed plane source emission in an infinite medium is used to obtain the flux due to a pulsed point source in the same medium This result is then integrated to determine the flux due to the corresponding pulsed line source problem The semi-infinite albedo problem is also shown to be solvable using the multiple collision approach A generalization to include delayed neutrons follows directly from the multiple collision treatment, as does an equivalence between a monoenergetic time-dependent problem and a particular stationary slowing down problem in infinite geometry Results are tabulated and comparisons are made to provide benchmark solutions to the fundamental time-dependent transport problems considered and thus bridge the gap between theory and practice

Structure of the X-ray source in the Virgo cluster of galaxies

Abstract: High-angular-resolution observations in the 0.15-1.5-keV band with an imaging X-ray telescope shows the extended X-ray source in the Virgo cluster of galaxies to be a diffuse halo of about 15 arcmin core radius surrounding M87. The angular structure of the surface brightness is marginally consistent with either of two simple models: (1) an isothermal (or adiabatic or hydrostatic) sphere plus a point source at M87 accounting for 12% of the total 0.5-1.5-keV intensity or (2) a power-law function without a discrete point source. No evidence for a point source is seen in the 0.15-0.28-keV band, which is consistent with self-absorption by about 10 to the 21st power per sq cm of matter having a cosmic abundance. The power-law models are motivated by the idea that radiation losses regulate the accretion of matter onto M87 and can account for the observed difference in the size of the X-ray source as seen in the present measurements and at higher energies.

Turbulent buoyant plumes

Abstract: The velocity and temperature distributions in turbulent buoyant induced by a line source or point source of heat are calculated by assuming the eddy viscosity and eddy diffusivity to be constant in any cross section of the plume. Two solutions in closed forms are obtained for the two‐dimensional plume, corresponding to turbulent Prandtl number σ equal to 2/3 and 2. Two such solutions are also obtained for the round plume, corresponding to σ equal to 1.1 and 2. The solution for σ=2/3 is compared with previous measurements for two‐dimensional plumes, and the solution for σ=1.1 is compared with previous measurements for the axisymmetric plume. The analytical and experimental results agree well in the two‐dimensional case, and satisfactorily in the axisymmetric case.

Propagation in Slowly-Varying Wave-Guides

Abstract: It is shown that a slight modification to the ray ansatz of short wave asymptotics can lead to a considerable improvement in the uniformity of the solution. A superposition of ray solutions is used to obtain a description of the sound waves generated by a multi-pole point source in a weakly stratified ocean.

Radiometric errors caused by diffraction from circular apertures: edge effects.

Abstract: The diffraction corrections associated with a circular aperture are calculated for the case of a point source and a detector aperture having a diameter approximately equal to that of the illuminated region. This investigation is made for monochromatic and complex (tungsten) radiation; two types of detectors are considered: a typical silicon diode and a neutral detector. The intensity distribution near the edge is calculated for the same cases. Some experimental results are also presented to corroborate the calculations and to suggest the behavior with extended sources. We show that strong diffraction effects are present close to the shadow edge even with a source–detector combination having a very extended wavelength band. In radio-metric measurements, a suitable compromise between diffraction effects, stray light, and vignetting effects can be achieved by having the detector aperture diameter approximately half that of the illuminated region. An extended source will reduce diffraction effects. Finally, some suggestions are made with respect to making diffraction corrections for a series of apertures.

Ultrasonic pest repellent method and system

Abstract: A method of and system for sonifying a portion of a given area to rid the area of unwanted pests comprising providing at least one ultrasonic point source each producing a set of pulsed signals having a randomly varying frequency in the range of about 18-30 KHz and having an intensity of at least 90 dB at the periphery of a cone shaped area extending outwardly therefrom and having a predetermined maximum length and a predetermined maximum width. The point sources are spaced in the given area to maintain the minimum sound intensity level at any point in the sonified portion at not less than 90 dB by disposing point sources opposite one another when the distance therebetween is greater the maximum length, disposing point sources adjacent one another to effect a longitudinal overlapping of adjacent cones when the room width is greater than the maximum width and disposing point sources perpendicular to the adjacent point sources to sonify any dead space therebetween.

A New Method for Computing Pollutant Concentrations in the Presence of Limited Vertical Mixing

Abstract: A new method for computing pollutant concentrations in the presence of a totally reflecting inversion lid and ground surface is presented. The method is applicable to gaussian plume treatments of line and area sources, in addition to point source cases. The maximum error quoted occurs at the algorithm crossover point, which itself was chosen so as to minimize the maximum error.

Distance measuring based on caustics.

Abstract: A distance measuring technique based on caustics formed by illuminating an ellipsoid reflector with a point light source was developed. A study of the influence of the ellipticity of the mirror and the relative positions of the screen where the caustic is formed, the point source, and the mirror on the limits and accuracy of the range measured was undertaken. It was found that the accuracy in measuring a distance may be greater than the accuracy obtained by a direct measurement of the same distance by any conventional method used in measuring the diameter of the caustic on the screen.

Search for microarcsecond structure in low-frequency variable radio sources.

Abstract: A search for interstellar scintillation (ISS) of low-frequency variable radio sources is reported. Observations of 28 confirmed or suspected low-frequency variables, 21 nonvariable sources, and two pulsars were made at 408 MHz. As expected, the pulsars showed ISS, but scintillation was not detected in any other source. A typical upper limit to the rms modulation due to ISS is 150 mJy, giving lower limits to the apparent angular diameter of about 10 to the -6th arcsec. The possibilities that a true point source is broadened to an apparent angular diameter greater than or roughly equal to 10 to the -6th arcsec by scattering local to the source or in a general intergalactic medium are discussed.

Analytical Model of Dispersion in Tidal Fjords

Abstract: Using the integral transforms technique, the unsteady three-dimensional advective-diffusion equation was solved analytically for the dispersion of pollutants from an instantaneous point source in open channels of finite width. The flow velocity through the channel was the mean current velocity which was assumed to vary sinusoidally with time but linearly with water depth. The effect of first-order reactions is to reduce the concentration level throughout the channel. The spread of pollutants in the direction of mean flow is accelerated as a result of the existence of a vertical shear. The width of the channel is important, as is the boundary in the y direction that profoundly affects the horizontal dispersion when the source is close to the bank, but its effect diminishes as the source is moving farther away from the bank. These findings, as well as others, are examined and illustrated graphically.

The Inverse Fast Field Program (IFFP): An Application to the Determination of the Acoustic Parameters of the Ocean Bottom

Abstract: : The problem of estimating the acoustic field produced by a point source located in an environment which is arbitrarily stratified with depth only has received considerable attention. Various models exist which provide reasonable results provided sufficient information about the environment and source characteristic is available. This paper is concerned with solving for the inverse of this problem, i.e., information about the environment, in particular the impedance at the ocean-bottom interface is sought from a knowledge of the received acoustic field and the source which produced it. The theoretical formulation of the scheme is presented and its numerical implementation is discussed and illustrated for three examples of successive orders of environmental complexity.

Characterization of the AURORA FXR Photon Radiation Field for SREMP Studies

Abstract: Radiation transport and electromagnetic field calculations were performed in order to characterize the field sources for source region EMP experiments performed in the AURORA flash x-ray facility. Electron-photon transport calculations were performed to obtain the energy-angular dependent photon output of the AURORA bremsstrahlung converters. Four spatial representations of the AURORA photon source were investigated: plane wave, point source at the AURORA hot spot, 4 point sources at the converter centers, and 4 sources uniformly distributed over the converter surfaces. Photon transport and electron transport calculations were performed to obtain the photo-Compton current and air ionization within the HDL Blue Cylinder EMP test chamber. Field calculations were performed for air-filled and evacuated test chamber conditions. Based on examination of the fields sensitivities to the photon source representation, we conclude that the 4-point or 4-distributed spatial source representation with our calculated energy-angular distribution is satisfactory for EMP calculations. The plane wave and hot spot representations we find to be unsatisfactory.

Angular spectrum measurements of atmospheric turbulence

Abstract: The unique property of the theoretical formula for the angular spectrum of a point source seen through a turbulent medium is that it is related in simple closed form to the integrated structure function of the turbulence. It therefore provides a reliable measure of the state of turbulence. Rudimentary measurements over a 5 km path show a remarkable confirmation, over a range 1000:1 in intensity, of the Kolmogoroff two-thirds law, and of the underlying theory of wave propagation through a turbulent medium.

Theoretical aspects of thermal blooming compensation

Abstract: In this paper we present the theoretical background required for evaluating phase compensation as a technique for reducing the effects of thermal blooming. By using a time-dependent approach for thermal blooming, we can define and evaluate the instantaneous Green’s function for the heated atmosphere. The phase at the mirror is determined by propagating a point source from the focal point to the laser. The effectiveness of this technique is determined by propagating point sources within a diffraction-limited spot to the laser and comparing the resulting phase to the phase of a point source at the focal point. The phase difference will be small and the increase in irradiance large if the heating occurs where geometric optics is valid. The phase difference is large and the increase in irradiance small if atmospheric heating occurs where significant diffractive spreading is also occurring. The resulting gradients lie in the geometric shadow of the aperture and are not accessible to correction by contouring the mirror. Numerical results illustrating the analysis are presented for both cw and pulsed lasers.

Computation of velocity in the wave train of a point source

Abstract: : The velocity potential and the components of velocity in the wave train of a point source are derived from Havelock's integral. Analysis and documentation are given for the computation of velocity by trapezoidal integration, by asymptotic approximation, and by integration by parts. Accuracy and efficiency are least dependent upon the depth of the source in the integration by parts. The three methods of computation are available for application in a set of subroutines.