7,801 research outputs found
Model Based Mission Assurance: NASA's Assurance Future
Model Based Systems Engineering (MBSE) is seeing increased application in planning and design of NASAs missions. This suggests the question: what will be the corresponding practice of Model Based Mission Assurance (MBMA)? Contemporaneously, NASAs Office of Safety and Mission Assurance (OSMA) is evaluating a new objectives based approach to standards to ensure that the Safety and Mission Assurance disciplines and programs are addressing the challenges of NASAs changing missions, acquisition and engineering practices, and technology. MBSE is a prominent example of a changing engineering practice. We use NASAs objectives-based strategy for Reliability and Maintainability as a means to examine how MBSE will affect assurance. We surveyed MBSE literature to look specifically for these affects, and find a variety of them discussed (some are anticipated, some are reported from applications to date). Predominantly these apply to the early stages of design, although there are also extrapolations of how MBSE practices will have benefits for testing phases. As the effort to develop MBMA continues, it will need to clearly and unambiguously establish the roles of uncertainty and risk in the system model. This will enable a variety of uncertainty-based analyses to be performed much more rapidly than ever before and has the promise to increase the integration of CRM (Continuous Risk Management) and PRA (Probabilistic Risk Analyses) even more fully into the project development life cycle. Various views and viewpoints will be required for assurance disciplines, and an over-arching viewpoint will then be able to more completely characterize the state of the project/program as well as (possibly) enabling the safety case approach for overall risk awareness and communication
Do Housing Submarkets Really Matter?
We maintain that the appropriate definition of submarkets depends on the use to which they will be put. For mass appraisal purposes, submarkets should be defined so that the accuracy of hedonic predictions will be optimized. Thus we test whether out-of-sample hedonic value predictions can be improved when a large urban housing market is divided into submarkets and we explore the effects of alternative definitions of submarkets on the accuracy of predictions. We compare a set of submarkets based on small geographical areas defined by real estate appraisers with a set of statistically generated submarkets consisting of dwellings that are similar but not necessarily contiguous. The empirical analysis uses a transactions database from Auckland, New Zealand. Price predictions are found to be most accurate when based on the housing market segmentation used by appraisers. We conclude that housing submarkets matter, and location plays the major role in explaining why they matter.Housing; Submarkets; Price Predictions; Mass Appraisal; Hedonic Method
Light-induced switching between singlet and triplet superconducting states
While the search for topological triplet-pairing superconductivity has
remained a challenge, recent developments in optically stabilizing metastable
superconducting states suggest a new route to realizing this elusive phase.
Here, we devise a testable theory of competing superconducting orders that
permits ultrafast switching to an opposite-parity superconducting phase in
centrosymmetric crystals with strong spin-orbit coupling. Using both
microscopic and phenomenological models, we show that dynamical inversion
symmetry breaking with a tailored light pulse can induce odd-parity (spin
triplet) order parameter oscillations in a conventional even-parity (spin
singlet) superconductor, which when driven strongly can send the system to a
competing minimum in its free energy landscape. Our results provide new guiding
principles for engineering unconventional electronic phases using light,
suggesting a fundamentally non-equilibrium route toward realizing topological
superconductivity
The chiral and flavour projection of Dirac-Kahler fermions in the geometric discretization
It is shown that an exact chiral symmetry can be described for Dirac-Kahler
fermions using the two complexes of the geometric discretization. This
principle is extended to describe exact flavour projection and it is shown that
this necessitates the introduction of a new operator and two new structures of
complex. To describe simultaneous chiral and flavour projection, eight
complexes are needed in all and it is shown that projection leaves a single
flavour of chiral field on each.Comment: v2: 17 pages, Latex. 5 images eps. Added references, reformatted and
clarification of some point
Enabling Assurance in the MBSE Environment
A number of specific benefits that fit within the hallmarks of effective development are realized with implementation of model-based approaches to systems and assurance. Model Based Systems Engineering (MBSE) enabled by standardized modeling languages (e.g., SysML) is at the core. These benefits in the context of spaceflight system challenges can include: Improved management of complex development, Reduced risk in the development process, Improved cost management, Improved design decisions. With appropriate modeling techniques the assurance community can improve early oversight and insight into project development. NASA has shown the basic constructs of SysML in an MBSE environment offer several key advantages, within a Model Based Mission Assurance (MBMA) initiative
Model Based Mission Assurance in a Model Based Systems Engineering (MBSE) Framework: State-of-the-Art Assessment
This report explores the current state of the art of Safety and Mission Assurance (S&MA) in projects that have shifted towards Model Based Systems Engineering (MBSE). Its goal is to provide insight into how NASA's Office of Safety and Mission Assurance (OSMA) should respond to this shift. In MBSE, systems engineering information is organized and represented in models: rigorous computer-based representations, which collectively make many activities easier to perform, less error prone, and scalable. S&MA practices must shift accordingly. The "Objective Structure Hierarchies" recently developed by OSMA provide the framework for understanding this shift. Although the objectives themselves will remain constant, S&MA practices (activities, processes, tools) to achieve them are subject to change. This report presents insights derived from literature studies and interviews. The literature studies gleaned assurance implications from reports of space-related applications of MBSE. The interviews with knowledgeable S&MA and MBSE personnel discovered concerns and ideas for how assurance may adapt. Preliminary findings and observations are presented on the state of practice of S&MA with respect to MBSE, how it is already changing, and how it is likely to change further. Finally, recommendations are provided on how to foster the evolution of S&MA to best fit with MBSE
Fusing Quantitative Requirements Analysis with Model-based Systems Engineering
A vision is presented for fusing quantitative
requirements analysis with model-based systems
engineering. This vision draws upon and combines
emergent themes in the engineering milieu.
“Requirements engineering” provides means to
explicitly represent requirements (both functional and
non-functional) as constraints and preferences on
acceptable solutions, and emphasizes early-lifecycle
review, analysis and verification of design and
development plans. “Design by shopping” emphasizes
revealing the space of options available from which to
choose (without presuming that all selection criteria
have previously been elicited), and provides means to
make understandable the range of choices and their
ramifications. “Model-based engineering” emphasizes
the goal of utilizing a formal representation of all
aspects of system design, from development through
operations, and provides powerful tool suites that
support the practical application of these principles.
A first step prototype towards this vision is
described, embodying the key capabilities.
Illustrations, implications, further challenges and
opportunities are outlined
When Can You Fold a Map?
We explore the following problem: given a collection of creases on a piece of
paper, each assigned a folding direction of mountain or valley, is there a flat
folding by a sequence of simple folds? There are several models of simple
folds; the simplest one-layer simple fold rotates a portion of paper about a
crease in the paper by +-180 degrees. We first consider the analogous questions
in one dimension lower -- bending a segment into a flat object -- which lead to
interesting problems on strings. We develop efficient algorithms for the
recognition of simply foldable 1D crease patterns, and reconstruction of a
sequence of simple folds. Indeed, we prove that a 1D crease pattern is
flat-foldable by any means precisely if it is by a sequence of one-layer simple
folds.
Next we explore simple foldability in two dimensions, and find a surprising
contrast: ``map'' folding and variants are polynomial, but slight
generalizations are NP-complete. Specifically, we develop a linear-time
algorithm for deciding foldability of an orthogonal crease pattern on a
rectangular piece of paper, and prove that it is (weakly) NP-complete to decide
foldability of (1) an orthogonal crease pattern on a orthogonal piece of paper,
(2) a crease pattern of axis-parallel and diagonal (45-degree) creases on a
square piece of paper, and (3) crease patterns without a mountain/valley
assignment.Comment: 24 pages, 19 figures. Version 3 includes several improvements thanks
to referees, including formal definitions of simple folds, more figures,
table summarizing results, new open problems, and additional reference
Fluctuating geometries, q-observables, and infrared growth in inflationary spacetimes
Infrared growth of geometrical fluctuations in inflationary spacetimes is
investigated. The problem of gauge-invariant characterization of growth of
perturbations, which is of interest also in other spacetimes such as black
holes, is addressed by studying evolution of the lengths of curves in the
geometry. These may either connect freely falling "satellites," or wrap
non-trivial cycles of geometries like the torus, and are also used in
diffeomorphism- invariant constructions of two-point functions of field
operators. For spacelike separations significantly exceeding the Hubble scale,
no spacetime geodesic connects two events, but one may find geodesics
constrained to lie within constant-time spatial slices. In inflationary
geometries, metric perturbations produce significant and growing corrections to
the lengths of such geodesics, as we show in both quantization on an inflating
torus and in standard slow-roll inflation. These become large, signaling
breakdown of a perturbative description of the geometry via such observables,
and consistent with perturbative instability of de Sitter space. In particular,
we show that the geodesic distance on constant time slices during inflation
becomes non-perturbative a few e-folds after a given scale has left the
horizon, by distances \sim 1/H^3 \sim RS, obstructing use of such geodesics in
constructing IR-safe observables based on the spatial geometry. We briefly
discuss other possible measures of such geometrical fluctuations.Comment: 33 pages, 2 figures, latex; v2: typos corrected, references improve
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