1,105 research outputs found
ARPHA: an FDIR architecture for Autonomous Spacecrafts based on Dynamic Probabilistic Graphical Models
This paper introduces a formal architecture for on-board diagnosis, prognosis and recovery called ARPHA. ARPHA is designed as part of the ESA/ESTEC study called VERIFIM (Veri\ufb01cation of Failure Impact by Model checking). The goal is to allow the design of an innovative on-board FDIR process for autonomous systems, able to deal with uncertain system/environment interactions, uncertain dynamic system evolution, partial observability and detection of recovery actions taking into account imminent failures. We show how the model needed by ARPHA can be built through a standard fault analysis phase, \ufb01nally producing an extended
version of a fault tree called EDFT; we discuss how EDFT can be adopted as a formal language to represent the needed FDIR knowledge, that can be compiled into a corresponding Dynamic Decision Network to be used for the analysis. We also discuss the software architecture we are implementing following this approach, where on-board FDIR can be implemented by exploiting on-line inference based on the junction tree approach typical of probabilisticgraphical models
A GSPN semantics for Continuous Time Bayesian Networks with Immediate Nodes
In this report we present an extension to Continuous Time Bayesian Networks (CTBN) called Generalized Continuous Time Bayesian Networks (GCTBN). The formalism allows one to model, in addition to continuous time delayed variables (with exponentially distributed transition rates), also non delayed or "immediate" variables, which act as standard chance nodes in a Bayesian Network. This allows the modeling of processes having both a continuous-time temporal component and an immediate (i.e. non-delayed) component capturing the logical/probabilistic interactions among the model\u2019s variables. The usefulness of this kind of model is discussed through an example concerning the reliability of a simple component-based system. A semantic model of GCTBNs, based on the formalism of Generalized Stochastic Petri Nets (GSPN) is outlined, whose purpose is twofold: to provide a well-de\ufb01ned semantics for GCTBNs in terms of the underlying stochastic process, and to provide an actual mean to perform inference (both prediction and smoothing) on GCTBNs. The example case study is then used, in order to highlight the exploitation of GSPN analysis for posterior probability computation on the GCTBN model
Optical variability of the BL Lacertae object GC 0109+224. Multiband behaviour and time scales from a 7-years monitoring campaign
We present the most continuous data base of optical
observations ever published on the BL Lacertae object GC 0109+224, collected
mainly by the robotic telescope of the Perugia University Observatory in the
period November 1994-February 2002. These observations have been complemented
by data from the Torino Observatory, collected in the period July 1995-January
1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid
optical variations and six major outbursts were observed at the beginning and
end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the
beginning of 2002. Fast and large-amplitude drops characterized its flux
behaviour. The magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy),
with a mean value of 14.9 (3.38 mJy). In the periods where we collected
multi-filter observations, we analyzed colour and spectral indexes, and the
variability patterns during some flares. The long-term behaviour seems
approximatively achromatic, but during some isolated outbursts we found
evidence of the typical loop-like hysteresis behaviour, suggesting that rapid
optical variability is dominated by non-thermal cooling of a single emitting
particle population. We performed also a statistical analysis of the data,
through the discrete correlation function (DCF), the structure function (SF),
and the Lomb-Scargle periodogram, to identify characteristic times scales, from
days to months, in the light curves, and to quantify the mode of variability.
We also include the reconstruction of the historical light curve and a
photometric calibration of comparison stars, to favour further extensive
optical monitoring of this interesting blazar.Comment: 13 pages, 11 PS figures, 1 EPS figure, 3 tables, accepted by
Astronomy and Astrophysics. Uses A&A documentclass aa.cls, and the package
graphicx.st
Color Variability of the Blazar AO 0235+16
Multicolor (UBVRIJHK) observations of the blazar AO 0235+16 are analyzed. The
light curves were compiled at the Turin Observatory from literature data and
the results of observations obtained in the framework of the WEBT program
(http://www.to.astro/blazars/webt/). The color variability of the blazar was
studied in eight time intervals with a sufficient number of multicolor optical
observations; JHK data are available for only one of these. The spectral energy
distribution (SED) of the variable component remained constant within each
interval, but varied strongly from one interval to another. After correction
for dust absorption, the SED can be represented by a power law in all cases,
providing evidence for a synchrotron nature of the variable component. We show
that the variability at both optical and IR wavelengths is associated with the
same variable source.Comment: 11 pages, 9 figures, 4 tables, accepted for publication in Astronomy
Report
The Optical Emission from Gamma-ray Quasars
We present photometric observations of six radio-loud quasars that were
detected by the COMPTEL gamma-ray telescope. The data encompasses seven
wavebands in the optical and near-infrared. After correction for Galactic
extinction, we find a wide range in optical slopes. Two sources are as blue as
optically-selected quasars, and are likely to be dominated by the accretion
disc emission, while three others show colours consistent with a red
synchrotron component. We discuss the properties of the COMPTEL sample of
quasars, as well as the implications our observations have for multi-wavelength
modelling of gamma-ray quasars.Comment: 12 pages, 3 figures, accepted for publication in P.A.S.A; minor typos
correcte
Decision Networks for modeling and analysis of attack/defense scenarios in critical infrastructures
We propose to exploit Decision Networks (DN) for the analysis of attack/defense scenarios. We show that DN extend both the modeling and the analysis capabilities of formalisms based on Attack Trees, which are the main reference model in such a context. Uncertainty can be addressed at every system level and a decision-theoretic analysis of the risk and of the selection of the best countermeasures can be implemented, by exploiting standard inference algorithms on DN
Remarks on Conserved Quantities and Entropy of BTZ Black Hole Solutions. Part II: BCEA Theory
The BTZ black hole solution for (2+1)-spacetime is considered as a solution
of a triad-affine theory (BCEA) in which topological matter is introduced to
replace the cosmological constant in the model. Conserved quantities and
entropy are calculated via Noether theorem, reproducing in a geometrical and
global framework earlier results found in the literature using local
formalisms. Ambiguities in global definitions of conserved quantities are
considered in detail. A dual and covariant Legendre transformation is performed
to re-formulate BCEA theory as a purely metric (natural) theory (BCG) coupled
to topological matter. No ambiguities in the definition of mass and angular
momentum arise in BCG theory. Moreover, gravitational and matter contributions
to conserved quantities and entropy are isolated. Finally, a comparison of BCEA
and BCG theories is carried out by relying on the results obtained in both
theories.Comment: PlainTEX, 20 page
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