7,912 research outputs found
Study of fault-tolerant software technology
Presented is an overview of the current state of the art of fault-tolerant software and an analysis of quantitative techniques and models developed to assess its impact. It examines research efforts as well as experience gained from commercial application of these techniques. The paper also addresses the computer architecture and design implications on hardware, operating systems and programming languages (including Ada) of using fault-tolerant software in real-time aerospace applications. It concludes that fault-tolerant software has progressed beyond the pure research state. The paper also finds that, although not perfectly matched, newer architectural and language capabilities provide many of the notations and functions needed to effectively and efficiently implement software fault-tolerance
PROSET — A Language for Prototyping with Sets
We discuss the prototyping language PROSET(Prototyping with Sets) as a language for experimental and evolutionary prototyping, focusing its attention on algorithm design. Some of PROSET’s features include generative communication, flexible exception handling and the integration of persistence. A discussion of some issues pertaining to the compiler and the programming environment conclude the pape
Observational Implications of Precessing Protostellar Discs and Jets
We consider the dynamics of a protostellar disc in a binary system where the
disc is misaligned with the orbital plane of the binary, with the aim of
determining the observational consequences for such systems. The disc wobbles
with a period approximately equal to half the binary's orbital period and
precesses on a longer timescale. We determine the characteristic timescale for
realignment of the disc with the orbital plane due to dissipation. If the
dissipation is determined by a simple isotropic viscosity then we find, in line
with previous studies, that the alignment timescale is of order the viscous
evolution timescale. However, for typical protostellar disc parameters, if the
disc tilt exceeds the opening angle of the disc, then tidally induced shearing
within the disc is transonic. In general, hydrodynamic instabilities associated
with the internally driven shear result in extra dissipation which is expected
to drastically reduce the alignment timescale. For large disc tilts the
alignment timescale is then comparable to the precession timescale, while for
smaller tilt angles , the alignment timescale varies as . We discuss the consequences of the wobbling, precession and
rapid realignment for observations of protostellar jets and the implications
for binary star formation mechanisms.Comment: MNRAS, in press. 10 pages. Also available at
http://www.ast.cam.ac.uk/~mbat
Deep Synoptic Array Science: Polarimetry of 25 New Fast Radio Bursts Provides Insights into their Origins
We report on a full-polarization analysis of the first 25 as yet
non-repeating FRBs detected at 1.4 GHz by the 110-antenna Deep Synoptic Array
(DSA-110) during commissioning observations. We present details of the data
reduction, calibration, and analysis procedures developed for this novel
instrument. The data have 32 s time resolution and sensitivity to Faraday
rotation measures (RMs) between rad m. RMs are detected for
20 FRBs with magnitudes ranging from rad m. FRBs are
found to have high () linear-polarization fractions. The remaining
FRBs exhibit significant circular polarization (), or are either
partially depolarized () or unpolarized (). We investigate the
mechanism of depolarization, disfavoring stochastic RM variations within a
scattering screen as a dominant cause. Polarization-state and possible RM
variations are observed in the four FRBs with multiple sub-components, but only
one other FRB shows a change in polarization state. We combine the DSA-110
sample with polarimetry of previously published FRBs, and compare the
polarization properties of FRB sub-populations and FRBs with Galactic pulsars.
Although FRBs are typically far more polarized than the average profiles of
Galactic pulsars, and exhibit greater spread in polarization fractions than
pulsar single pulses, we find a remarkable similarity between FRB polarization
fractions and the youngest (characteristic ages yr) pulsars. Our
results support a scenario wherein FRB emission is intrinsically highly
linearly polarized, and where propagation effects within progenitor
magnetospheres can result in conversion to circular polarization and
depolarization. Young pulsar emission and magnetospheric-propagation geometries
may form a useful analogy for the origin of FRB polarization.Comment: 43 pages, 17 figure
HST astrometry in the 30 Doradus region: II. Runaway stars from new proper motions in the Large Magellanic Cloud
We present a catalog of relative proper motions for 368,787 stars in the 30
Doradus region of the Large Magellanic Cloud (LMC), based on a dedicated
two-epoch survey with the Hubble Space Telescope (HST) and supplemented with
proper motions from our pilot archival study. We demonstrate that a relatively
short epoch difference of 3 years is sufficient to reach a 0.1 mas
yr level of precision or better. A number of stars have relative proper
motions exceeding a 3-sigma error threshold, representing a mixture of Milky
Way denizens and 17 potential LMC runaway stars. Based upon 183 VFTS OB-stars
with the best proper motions, we conclude that none of them move faster than
0.3 mas yr in each coordinate -- equivalent to 70 km
s. Among the remaining 351 VFTS stars with less accurate proper motions,
only one candidate OB runaway can be identified. We rule out any OB star in our
sample moving at a tangential velocity exceeding 120 km s. The
most significant result of this study is finding 10 stars over wide range of
masses, which appear to be ejected from the massive star cluster R136 in the
tangential plane to angular distances from out to
, equivalent to 8-98 pc. The tangential velocities of these
runaways appear to be correlated with apparent magnitude, indicating a possible
dependence on the stellar mass.Comment: 45 pages (in referee format), 12 figures, 3 tables. Submitted to AJ.
Comments are welcom
Component-based software engineering
To solve the problems coming with the current software development methodologies, component-based software engineering has caught many researchers\u27 attention recently. In component-based software engineering, a software system is considered as a set of software components assembled together instead of as a set of functions from the traditional perspective. Software components can be bought from third party vendors as off-the-shelf components and be assembled together.
Component-based software engineering, though very promising, needs to solve several core issues before it becomes a mature software development strategy. The goal of this dissertation is to establish an infrastructure for component-based software development. The author identifies and studies some of the core issues such as component planning, component building, component assembling, component representation, and component retrieval.
A software development process model is developed in this dissertation to emphasize the reuse of existing software components. The software development process model addresses how a software system should be planned and built to maximize the reuse of software components. It conducts domain engineering and application engineering simultaneously to map a software system to a set of existing components in such a way that the development of a software system can reuse the existing software components to the full extent. Besides the planning of software development based on component technology, the migration and integration of legacy systems, most of which are non-component-based systems, to the component-based software systems are studied. A framework and several methodologies are developed to serve as the guidelines of adopting component technology in legacy systems.
Component retrieval is also studied in this dissertation. One of the most important issues in component-based software engineering is how to find a software component quickly and accurately in a component repository. A component representation framework is developed in this dissertation to represent software components. Based on the component representation framework, an efficient searching method that combines neural network, information retrieval, and Bayesian inference technology is developed. Finally a prototype component retrieval system is implemented to demonstrate the correctness and feasibility of the proposed method
Evaluation of meterological rocket data
Meteorological rocket data compared with rawinsonde observation
Near-inertial internal wave field in the Canada Basin from ice-tethered profilers
Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 413–426, doi:10.1175/JPO-D-13-0117.1.Salinity and temperature profiles from drifting ice-tethered profilers in the Beaufort gyre region of the Canada Basin are used to characterize and quantify the regional near-inertial internal wave field over one year. Vertical displacements of potential density surfaces from the surface to 750-m depth are tracked from fall 2006 to fall 2007. Because of the time resolution and irregular sampling of the ice-tethered profilers, near-inertial frequency signals are marginally resolved. Complex demodulation is used to determine variations with a time scale of several days in the amplitude and phase of waves at a specified near-inertial frequency. Characteristics and variability of the wave field over the course of the year are investigated quantitatively and related to changes in surface wind forcing and sea ice cover.The ITP program and J. Toole’s contributions were supported
by the National Science Foundation Office of Polar
Programs Arctic Observing Network. We acknowledge
the support of the Office of Naval Research (Grant
N00014-11-1-0454) for this study. Support for H. Dosser was also provided by the Natural Sciences and Engineering
Research Council of Canada.2014-08-0
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