15,847 research outputs found
Mercury in the environment
Problems in assessing mercury concentrations in environmental materials are discussed. Data for situations involving air, water, rocks, soils, sediments, sludges, fossil fuels, plants, animals, foods, and man are drawn together and briefly evaluated. Details are provided regarding the toxicity of mercury along with tentative standards and guidelines for mercury in air, drinking water, and food
Pseudo-noise test set for communication system evaluation
A test set for communications systems is described which includes a pseudo noise sequence generator providing a test signal that is fed to a pair of signal channels. The first channel includes a spectrum shaping filter and a conditioning amplifier. The second channel includes a variable delay circuit, a spectrum shaping filter matched to the first filter, and an amplifier. The output of the first channel was applied to the system under test. The output of the system and the output of the second channel are compared to determine the degree of distortion suffered by the test signal due to the communications system
Method of and means for testing a tape record/playback system
A tape record/playback system was tested by first deriving an analog test signal and a band-limited digital reference signal from a pseudo-noise sequence generator driven by a clock signal. It recorded the signals on respective tracks of the system during operation in a record mode. During the playback mode of operation of the system, a delayed analog reference signal without time base variations was reconstructed from the played back reference signal. It was compared with the played back test signal in order to obtain an error signal that was a measure of the performance of the system
Millimeter wave satellite concepts, volume 1
The identification of technologies necessary for development of millimeter spectrum communication satellites was examined from a system point of view. Development of methodology based on the technical requirements of potential services that might be assigned to millimeter wave bands for identifying the viable and appropriate technologies for future NASA millimeter research and development programs, and testing of this methodology with selected user applications and services were the goals of the program. The entire communications network, both ground and space subsystems was studied. Cost, weight, and performance models for the subsystems, conceptual design for point-to-point and broadcast communications satellites, and analytic relationships between subsystem parameters and an overall link performance are discussed along with baseline conceptual systems, sensitivity studies, model adjustment analyses, identification of critical technologies and their risks, and brief research and development program scenarios for the technologies judged to be moderate or extensive risks. Identification of technologies for millimeter satellite communication systems, and assessment of the relative risks of these technologies, was accomplished through subsystem modeling and link optimization for both point-to-point and broadcast applications
Two-Band-Type Superconducting Instability in MgB2
Using the tight-binding method for the -bands in MgB, the Hubbard
on-site Coulomb interaction on two inequivalent boron -orbitals is
transformed into expressions in terms of -band operators. For scattering
processes relevant to the problemin which a wave vector {\bf q} is parallel to
, it is found to take a relatively simple form consisting of
intra-band Coulomb scattering, interband pair scattering etc. with large
constant coupling constants. This allows to get a simple expression for the
amplitude of interband pair scattering between two -bands, which diverges
if the interband polarization function in it becomes large enough.The latter
was approximately evaluated and found to be largely enhanced in the band
structure in MgB. These results lead to a divergent interband pair
scattering, meaning two-band-type superconducting instability with enhanced
. Adding a subsidiary BCS attractive interaction in each band into
consideration, a semi-quantitative gap equation is given, and and isotope
exponent are derived. The present instability is asserted to be the
origin of high in MgB.Comment: 4 pages, to be published in J. Phys. Soc. Jpn. vol. 70, No.
Ka-band study: 1988
The Ka-band study team was chartered in late 1987 to bring together all the planning elements for establishing 32 GHz (Ka-band) as the primary downlink frequency for deep-space operation, and to provide a stable baseline from which to pursue that development. This article summarizes the results of that study at its conclusion in mid-1988, and corresponds to material presented to NASA's Office of Space Operations on July 14, 1988. For a variety of reasons, Ka-band is the right next major step in deep-space communications. It offers improved radio metric accuracy through reduced plasma sensitivity and increased bandwidth. Because of these improvements, it offers the opportunity to reduce costs in the flight radio system or in the DSN by allocating part of the overall benefits of Ka-band to this cost reduction. A mission scenario is being planned that can drive at least two and possibly all three of the DSN subnets to provide a Ka-band downlink capability by the turn of the century. The implementation scenario devised by the study team is believed to be feasible within reasonable resource expectations, and capable of providing the needed upgrade as a natural follow-on to the technology development which is already underway
Comment on `Renormalization-Group Calculation of the Dependence on Gravity of the Surface Tension and Bending Rigidity of a Fluid Interface'
It is shown that the interface model introduced in Phys. Rev. Lett. 86, 2369
(2001) violates fundamental symmetry requirements for vanishing gravitational
acceleration , so that its results cannot be applied to critical properties
of interfaces for .Comment: A Comment on a recent Letter by J.G. Segovia-L\'opez and V.
Romero-Roch\'{\i}n, Phys. Rev. Lett.86, 2369 (2001). Latex file, 1 page
(revtex
Millimeter wave satellite concepts. Volume 1: Executive summary
The objectives of the program were: (1) development of methodology based on the technical requirements of potential services that might be assigned to millimeter wave bands for identifying the viable and appropriate technologies for future NASA millimeter research and development programs, and (2) testing of this methodology with user applications and services. The scope of the program included the entire communications network, both ground and space subsystems. The reports include: (1) cost, weight, and performance models for the subsystems, (2) conceptual design for point-to-point and broadcast communications satellites, (3) analytic relationships between subsystem parameters and an overall link performance, (4) baseline conceptual systems, (5) sensitivity studies, (6) model adjustment analyses, (7) identification of critical technologies and their risks, (8) brief R&D program scenarios for the technologies judged to be moderate or extensive risks
Dynamics of monatomic liquids
We present a theory of the dynamics of monatomic liquids built on two basic
ideas: (1) The potential surface of the liquid contains three classes of
intersecting nearly-harmonic valleys, one of which (the ``random'' class)
vastly outnumbers the others and all whose members have the same depth and
normal mode spectrum; and (2) the motion of particles in the liquid can be
decomposed into oscillations in a single many-body valley, and nearly
instantaneous inter-valley transitions called transits. We review the
thermodynamic data which led to the theory, and we discuss the results of
molecular dynamics (MD) simulations of sodium and Lennard-Jones argon which
support the theory in more detail. Then we apply the theory to problems in
equilibrium and nonequilibrium statistical mechanics, and we compare the
results to experimental data and MD simulations. We also discuss our work in
comparison with the QNM and INM research programs and suggest directions for
future research.Comment: 53 pages, 16 figures. Differs from published version in using
American English spelling and grammar (published version uses British
English
- …