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

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

Analysis of the sign-dependent switching observed in a hybrid aligned nematic cell

Copyright © 2009 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 11, article 013045. DOI: 10.1088/1367-2630/11/1/013045An optical waveguide experiment was used to study the influence of dc electric fields on a hybrid aligned nematic liquid crystal cell. This dc switching differed from ac switching in two ways: first, the equilibrium states depended on the sign of the applied voltage, and second, there was transient activity over long (~100 ms) timescales. To understand both of these, a numerical model of the cell's dynamics, which included both the Ericksen–Leslie theory and a drift-diffusion model of mobile ions, has been developed. Comparing modelling with observations, we find that the transients are caused by the motion of tiny concentrations of ionic impurities, and that the sign dependence is caused by an asymmetric distribution of surface charge, rather than the flexoelectric effect

Time-resolved sign-dependent switching in a hybrid aligned nematic liquid crystal cell

Copyright © 2008 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 10, article 083045. DOI: 10.1088/1367-2630/10/8/083045An optical waveguide technique is used to determine the director tilt profile across a hybrid aligned nematic (HAN) liquid crystal cell, in which the optical response is dependent on the sign of the applied voltage. Two physical models are shown that fit the equilibrium experimental data, but with alternative explanations for this sign dependence. Models with either a flexoelectric coefficient of 2.25×10−11 C m−1 or a bound surface charge of 12.2 μC m−2 are shown that fit this equilibrium data. In an attempt to resolve this degeneracy sign-dependent switching data are analysed. However, neither model can explain these switching data, which are affected by slow transients of ~100 ms which are believed to be due to the motion of free ions in the liquid crystal. From the form of these slow transients, it is suggested that the equilibrium position of the ions is next to a cell substrate

Dynamic control of visible radiation by a liquid crystal filled Fabry-Pérot etalon

Copyright © 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 102 (2007) and may be found at http://link.aip.org/link/?JAPIAU/102/093108/1A liquid crystal filled Fabry-Pérot etalon has been constructed to control the resonant transmission of electromagnetic radiation over the visible range of the spectrum. This has been achieved through the use of a 1.5 µm thick homogeneously aligned liquid crystal layer in the core of a silver-clad etalon structure. Applying an electric field across the core reorientates the liquid crystal director and changes the refractive index for incident light polarized parallel to the rubbing direction. By measuring the transmitted intensity as a function of wavelength for a variety of applied voltages shifts in the positions of the resonant transmission modes of up to 80 nm have been observed. In addition, these results have been compared to model data generated using a multilayer optics model to obtain the dispersion of the liquid crystal over the visible range of the electromagnetic spectrum

Amundsen Sea Embayment ice-sheet mass-loss predictions to 2050 calibrated using observations of velocity and elevation change

Mass loss from the Amundsen Sea Embayment of the West Antarctic Ice Sheet is a major contributor to global sea-level rise (SLR) and has been increasing over recent decades. Predictions of future SLR are increasingly modelled using ensembles of simulations within which model parameters and external forcings are varied within credible ranges. Accurately reporting the uncertainty associated with these predictions is crucial in enabling effective planning for, and construction of defences against, rising sea levels. Calibrating model simulations against current observations of ice-sheet behaviour enables the uncertainty to be reduced. Here we calibrate an ensemble of BISICLES ice-sheet model simulations of ice loss from the Amundsen Sea Embayment using remotely sensed observations of surface elevation and ice speed. Each calibration type is shown to be capable of reducing the 90% credibility bounds of predicted contributions to SLR by 34 and 43% respectively

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

Long-term potentiation in neurogliaform interneurons modulates excitation-inhibition balance in the temporoammonic pathway

Apical dendrites of pyramidal neurons integrate information from higher-order cortex and thalamus, and gate signalling and plasticity at proximal synapses. In the hippocampus, neurogliaform cells and other interneurons located within stratum lacunosum-moleculare mediate powerful inhibition of CA1 pyramidal neuron distal dendrites. Is the recruitment of such inhibition itself subject to use-dependent plasticity, and if so, what induction rules apply? Here we show that interneurons in mouse stratum lacunosum-moleculare exhibit Hebbian NMDA receptor-dependent long-term potentiation (LTP). Such plasticity can be induced by selective optogenetic stimulation of afferents in the temporoammonic pathway from the entorhinal cortex, but not by equivalent stimulation of afferents from the thalamic nucleus reuniens. We further show that theta-burst patterns of afferent firing induces LTP in neurogliaform interneurons identified using neuron-derived neurotrophic factor (Ndnf)-Cre mice. Theta-burst activity of entorhinal cortex afferents led to an increase in disynaptic feed-forward inhibition, but not monosynaptic excitation, of CA1 pyramidal neurons. Activity-dependent synaptic plasticity in stratum lacunosum-moleculare interneurons thus alters the excitation-inhibition balance at entorhinal cortex inputs to the apical dendrites of pyramidal neurons, implying a dynamic role for these interneurons in gating CA1 dendritic computations. Abstract figure legend Hebbian LTP of excitatory transmission onto interneurons located within hippocampal stratum lacunosum moleculare (SLM) can be induced by electrical stimulation protocols involving pairing of pre-and post-synaptic activity. Using Ndnf-Cre mice, we show that hippocampal neurogliaform (NGF) cells express this form of LTP. These cells receive glutamatergic afferents from both the nucleus reuniens of the thalamus and the entorhinal cortex (EC), but selective optogenetic activation of either set of fibers reveals LTP at EC inputs only. Using an optogenetic theta-burst stimulation (OptoTBS) protocol to stimulate EC fibers in a physiologically relevant way, we show that NGF interneuron LTP translates to an increase in disynaptic inhibition onto CA1 pyramidal cell distal dendrites. Monosynaptic EC-CA1 pyramidal cell inputs do not undergo equivalent potentiation, leading to a net decrease in the excitation/inhibition (E/I) ratio of this pathway

Flow-driven transition and associated velocity profiles in a nematic liquid-crystal cell

S. A. Jewell, S. L. Cornford, F. Yang, P. S. Cann, and J. R. Sambles, Physical Review E, Vol. 80, article 041706 (2009) "Copyright © 2009 by the American Physical Society."The alignment properties and distribution of flow speed during Poiseuille flow through a microchannel of a nematic liquid crystal in a cell with homeotropic surface alignment has been measured using a combination of conoscopy, fluorescence confocal polarizing microscopy, and time-lapse imaging. Two topologically distinct director profiles, with associated fluid velocity fields, are found to exist with the preferred state dictated by the volumetric flow rate of the liquid crystal. The results show excellent agreement with model data produced using the Ericksen-Leslie nematodynamics theory