44,241 research outputs found
An RF interference mitigation methodology with potential applications in scheduling
Software tools for interference analysis and mitigation were developed in the Communications Link Analysis and Simulation System (CLASS) environment for: communications performance evaluation; and mission planning. Potential applications are seen in analysis, evaluation, and optimization of user schedules. Tools producing required separation angles and potential interference intervals can be used as an aid to mutual interference mitigation within a scheduling system
Password Cracking and Countermeasures in Computer Security: A Survey
With the rapid development of internet technologies, social networks, and
other related areas, user authentication becomes more and more important to
protect the data of the users. Password authentication is one of the widely
used methods to achieve authentication for legal users and defense against
intruders. There have been many password cracking methods developed during the
past years, and people have been designing the countermeasures against password
cracking all the time. However, we find that the survey work on the password
cracking research has not been done very much. This paper is mainly to give a
brief review of the password cracking methods, import technologies of password
cracking, and the countermeasures against password cracking that are usually
designed at two stages including the password design stage (e.g. user
education, dynamic password, use of tokens, computer generations) and after the
design (e.g. reactive password checking, proactive password checking, password
encryption, access control). The main objective of this work is offering the
abecedarian IT security professionals and the common audiences with some
knowledge about the computer security and password cracking, and promoting the
development of this area.Comment: add copyright to the tables to the original authors, add
acknowledgement to helpe
A method for interference mitigation in space communications scheduling
Increases in the number of user spacecraft and data rates supported by NASA's Tracking and Data Relay Satellite System (TDRSS) in the S and Ku bands could result in communications conflicts due to mutual interference. A method to mitigate interference while minimizing unnecessary scheduling restrictions on both TDRSS network and user resources, based on consideration of all relevant communications parameters, was developed. The steps of this method calculate required separation angles at TDRS and produce interference intervals, which can be used in the production of schedules free of unacceptable interference. The method can also be used as a basis for analysis, evaluation, and optimization of user schedules with respect to communications performance. Described here are the proposed method and its potential application to scheduling in space communications. Test cases relative to planned missions, including the Earth Observing System, the Space Station Manned Base, and the Space Shuttle are discussed
Coherent transport in Nb/delta-doped-GaAs hybrid microstructures
Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is
presented. The structures fabrication procedure is based on delta-doped layers
grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap
layer to protect the active semiconductor layers during ex situ transfer. The
superconductor is then sputter deposited in situ after thermal desorption of
the protective layer. Two types of structures in particular will be discussed,
i.e., a reference junction and the engineered one that contains an additional
insulating AlGaAs barrier inserted during the growth in the semiconductor. This
latter configuration may give rise to controlled interference effects and
realizes the model introduced by de Gennes and Saint-James in 1963. While both
structures show reflectionless tunneling-dominated transport, only the
engineered junction shows additionally a low-temperature single marked
resonance peaks superimposed to the characteristic Andreev-dominated subgap
conductance. The analysis of coherent magnetotransport in both microstructures
is successfully performed within the random matrix theory of Andreev transport
and ballistic effects are included by directly solving the Bogoliubov-de Gennes
equations. The impact of junction morphology on reflectionless tunneling and
the application of the employed fabrication technique to the realization of
complex semiconductor-superconductor systems are furthermore discussed.Comment: 9 pages, 8 figures, invited review paper, to be published in Mod.
Phys. Lett.
Non-equilibrium chemistry and dust formation in AGB stars as probed by SiO line emission
We have performed high spatial resolution observations of SiO line emission
for a sample of 11 AGB stars using the ATCA, VLA and SMA interferometers.
Detailed radiative transfer modelling suggests that there are steep chemical
gradients of SiO in their circumstellar envelopes. The emerging picture is one
where the radial SiO abundance distribution starts at an initial high
abundance, in the case of M-stars consistent with LTE chemistry, that
drastically decreases at a radius of ~1E15 cm. This is consistent with a
scenario where SiO freezes out onto dust grains. The region of the wind with
low abundance is much more extended, typically ~1E16 cm, and limited by
photodissociation. The surpisingly high SiO abundances found in carbon stars
requires non-equilibrium chemical processes.Comment: 2 pages, 1 figure. To be published in the proceedings of the
conference "Why Galaxies Care about AGB Stars", held in Vienna, August 7-11,
2006; F. Kerschbaum, C. Charbonnel, B. Wing eds, ASP Conf.Ser. in pres
Toxicological approach to setting spacecraft maximum allowable concentrations for carbon monoxide
The Spacecraft Maximum Allowable Concentrations (SMACs) are exposure limits for airborne chemicals used by NASA in spacecraft. The aim of these SMACs is to protect the spacecrew against adverse health effects and performance decrements that would interfere with mission objectives. Because of the 1 and 24 hr SMACs are set for contingencies, minor reversible toxic effects that do not affect mission objectives are acceptable. The 7, 30, or 180 day SMACs are aimed at nominal operations, so they are established at levels that would not cause noncarcinogenic toxic effects and more than one case of tumor per 1000 exposed individuals over the background. The process used to set the SMACs for carbon monoxide (CO) is described to illustrate the approach used by NASA. After the toxicological literature on CO was reviewed, the data were summarized and separated into acute, subchronic, and chronic toxicity data. CO's toxicity depends on the formation of carboxyhemoglobin (COHb) in the blood, reducing the blood's oxygen carrying capacity. The initial task was to estimate the COHb levels that would not produce toxic effects in the brain and heart
The Effect of Spectral Composition on the Photochemical Production of Hydrogen Peroxide in Lake Water
Hydrogen peroxide was produced when samples of lake water were exposed to direct or filtered sunlight in which UV or UV(B+C) light was selectively removed. In all cases, the concentration of hydrogen peroxide increased linearly with time-integrated irradiance. While both visible and UV light can induce the formation of hydrogen peroxide, the contribution from the latter was disproportionately large as it was responsible for about two-thirds of the formation of hydrogen peroxide. Among the UV lights, the contributions from UV-A and UV-(B+C) light were 70% and 30% respectively. The contribution from UV-A light was equivalent to about one half of the total production of hydrogen peroxide. Thus, relative to its contribution to the total irradiance in the solar spectrum, UV-A light is the most efficient type of light for the formation of hydrogen peroxide in lake waters
Large amplitude MHD waves upstream of the Jovian bow shock: Reinterpretation
Observations of large amplitude magnetohydrodynamic (MHD) waves upstream of the Jovian bow shock were previously interpreted as arising from a resonant electromagnetic ion beam instability. That interpretation was based on the conclusion that the observed fluctuations were predominantly right elliptically polarized in the solar wind rest frame. Because it was noted that the fluctuations are, in fact, left elliptically polarized, a reanalysis of the observations was necessary. Several mechanisms for producing left hand polarized MHD waves in the observed frequency range were investigated. Instabilities excited by protons appear unlikely to account for the observations. A resonant instability excited by relativistic electrons escaping from the Jovian magnetosphere is a likely source of free energy consistent with the observations. Evidence for the existence of such a population of electrons was found in both the Low Energy Charged Particle experiments and Cosmic Ray experiments on Voyager 2
JPL Energy Consumption Program (ECP) documentation: A computer model simulating heating, cooling and energy loads in buildings
The engineering manual provides a complete companion documentation about the structure of the main program and subroutines, the preparation of input data, the interpretation of output results, access and use of the program, and the detailed description of all the analytic, logical expressions and flow charts used in computations and program structure. A numerical example is provided and solved completely to show the sequence of computations followed. The program is carefully structured to reduce both user's time and costs without sacrificing accuracy. The user would expect a cost of CPU time of approximately $5.00 per building zone excluding printing costs. The accuracy, on the other hand, measured by deviation of simulated consumption from watt-hour meter readings, was found by many simulation tests not to exceed + or - 10 percent margin
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