1,937 research outputs found
NASA control research overview
An overview of NASA research activities related to the control of aeronautical vehicles is presented. A groundwork is laid by showing the organization at NASA Headquarters for supporting programs and providing funding. Then a synopsis of many of the ongoing activities is presented, some of which will be presented in greater detail elsewhere. A major goal of the workshop is to provide a showcase of ongoing NASA sponsored research. Then, through the panel sessions and conversations with workshop participants, it is hoped to glean a focus for future directions in aircraft controls research
Avionics and controls research and technology
The workshop provided a forum for industry and universities to discuss the state-of-the-art, identify the technology needs and opportunities, and describe the role of NASA in avionics and controls research
Compositional closure for Bayes Risk in probabilistic noninterference
We give a sequential model for noninterference security including probability
(but not demonic choice), thus supporting reasoning about the likelihood that
high-security values might be revealed by observations of low-security
activity. Our novel methodological contribution is the definition of a
refinement order and its use to compare security measures between
specifications and (their supposed) implementations. This contrasts with the
more common practice of evaluating the security of individual programs in
isolation.
The appropriateness of our model and order is supported by our showing that
our refinement order is the greatest compositional relation --the compositional
closure-- with respect to our semantics and an "elementary" order based on
Bayes Risk --- a security measure already in widespread use. We also relate
refinement to other measures such as Shannon Entropy.
By applying the approach to a non-trivial example, the anonymous-majority
Three-Judges protocol, we demonstrate by example that correctness arguments can
be simplified by the sort of layered developments --through levels of
increasing detail-- that are allowed and encouraged by compositional semantics
Nonlinear optical probe of tunable surface electrons on a topological insulator
We use ultrafast laser pulses to experimentally demonstrate that the
second-order optical response of bulk single crystals of the topological
insulator BiSe is sensitive to its surface electrons. By performing
surface doping dependence measurements as a function of photon polarization and
sample orientation we show that second harmonic generation can simultaneously
probe both the surface crystalline structure and the surface charge of
BiSe. Furthermore, we find that second harmonic generation using
circularly polarized photons reveals the time-reversal symmetry properties of
the system and is surprisingly robust against surface charging, which makes it
a promising tool for spectroscopic studies of topological surfaces and buried
interfaces
Algorithmic Analysis of Qualitative and Quantitative Termination Problems for Affine Probabilistic Programs
In this paper, we consider termination of probabilistic programs with
real-valued variables. The questions concerned are:
1. qualitative ones that ask (i) whether the program terminates with
probability 1 (almost-sure termination) and (ii) whether the expected
termination time is finite (finite termination); 2. quantitative ones that ask
(i) to approximate the expected termination time (expectation problem) and (ii)
to compute a bound B such that the probability to terminate after B steps
decreases exponentially (concentration problem).
To solve these questions, we utilize the notion of ranking supermartingales
which is a powerful approach for proving termination of probabilistic programs.
In detail, we focus on algorithmic synthesis of linear ranking-supermartingales
over affine probabilistic programs (APP's) with both angelic and demonic
non-determinism. An important subclass of APP's is LRAPP which is defined as
the class of all APP's over which a linear ranking-supermartingale exists.
Our main contributions are as follows. Firstly, we show that the membership
problem of LRAPP (i) can be decided in polynomial time for APP's with at most
demonic non-determinism, and (ii) is NP-hard and in PSPACE for APP's with
angelic non-determinism; moreover, the NP-hardness result holds already for
APP's without probability and demonic non-determinism. Secondly, we show that
the concentration problem over LRAPP can be solved in the same complexity as
for the membership problem of LRAPP. Finally, we show that the expectation
problem over LRAPP can be solved in 2EXPTIME and is PSPACE-hard even for APP's
without probability and non-determinism (i.e., deterministic programs). Our
experimental results demonstrate the effectiveness of our approach to answer
the qualitative and quantitative questions over APP's with at most demonic
non-determinism.Comment: 24 pages, full version to the conference paper on POPL 201
Correctness by construction for probabilistic programs
The "correct by construction" paradigm is an important component of modern
Formal Methods, and here we use the probabilistic Guarded-Command Language
to illustrate its application to
programming. extends Dijkstra's guarded-command language
with probabilistic choice, and is equipped with a
correctness-preserving refinement relation that enables
compact, abstract specifications of probabilistic properties to be transformed
gradually to concrete, executable code by applying mathematical insights in a
systematic and layered way. Characteristically for "correctness by
construction", as far as possible the reasoning in each refinement-step layer
does not depend on earlier layers, and does not affect later ones. We
demonstrate the technique by deriving a fair-coin implementation of any given
discrete probability distribution. In the special case of simulating a fair
die, our correct-by-construction algorithm turns out to be "within spitting
distance" of Knuth and Yao's optimal solution
Seeding systems and cropping trends in Saskatchewan results of a PFRA survey, 1997-2002
Non-Peer ReviewedFrom 1997 to 2002, Agriculture & Agri-Food Canada’s (AAFC) PFRA Branch conducted a
survey of over 4000 annually cropped fields in Saskatchewan. Each year the same fields were
visited shortly after crop emergence to collect information on crop type, row spacing, opener
type, packing system, amount of previous crop residue, orientation of previous crop stubble, and
adoption of low soil disturbance seeding. Key results are the increasing trend toward lower soil
disturbance seeding, and the high incidence of pulse crops associated with low disturbance
seeding. In depth analysis of trends on individual fields suggest that very few producers are able
to maintain low disturbance seeding every year on the same field. This suggests that some
flexibility is required to allow for periodic soil disturbance to address issues such as perennial
weeds
Wrinkling of a bilayer membrane
The buckling of elastic bodies is a common phenomenon in the mechanics of
solids. Wrinkling of membranes can often be interpreted as buckling under
constraints that prohibit large amplitude deformation. We present a combination
of analytic calculations, experiments, and simulations to understand wrinkling
patterns generated in a bilayer membrane. The model membrane is composed of a
flexible spherical shell that is under tension and that is circumscribed by a
stiff, essentially incompressible strip with bending modulus B. When the
tension is reduced sufficiently to a value \sigma, the strip forms wrinkles
with a uniform wavelength found theoretically and experimentally to be \lambda
= 2\pi(B/\sigma)^{1/3}. Defects in this pattern appear for rapid changes in
tension. Comparison between experiment and simulation further shows that, with
larger reduction of tension, a second generation of wrinkles with longer
wavelength appears only when B is sufficiently small.Comment: 9 pages, 5 color figure
Predicting the Use of Campus Counseling Services for Asian/Pacific Islander, Latino/Hispanic, and White Students: Problem Severity, Gender, and Generational Status
The purpose of the current study was to identify predictors of counseling center use among Asian, Latino/a, and White college students. Findings indicated that females and second generation students report the most severe difficulties. Problem severity and gender predicted counseling center use for White and Asian students, whereas only problem severity predicted use for Latino students. Generational status was not a significant predictor of use for any group
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