1,517 research outputs found
An expert system for wind shear avoidance
A study of intelligent guidance and control concepts for protecting against the adverse effects of wind shear during aircraft takeoffs and landings is being conducted, with current emphasis on developing an expert system for wind shear avoidance. Principal objectives are to develop methods for assessing the likelihood of wind shear encounter (based on real-time information in the cockpit), for deciding what flight path to pursue (e.g., takeoff abort, landing go-around, or normal climbout or glide slope), and for using the aircraft's full potential for combating wind shear. This study requires the definition of both deterministic and statistical techniques for fusing internal and external information , for making go/no-go decisions, and for generating commands to the manually controlled flight. The program has begun with the development of the WindShear Safety Advisor, an expert system for pilot aiding that is based on the FAA Windshear Training Aid; a two-volume manual that presents an overview , pilot guide, training program, and substantiating data provides guidelines for this initial development. The WindShear Safety Advisor expert system currently contains over 200 rules and is coded in the LISP programming language
Digging for Dark Matter: Spectral Analysis and Discovery Potential of Paleo-Detectors
Paleo-detectors are a recently proposed method for the direct detection of
Dark Matter (DM). In such detectors, one would search for the persistent damage
features left by DM--nucleus interactions in ancient minerals. Initial
sensitivity projections have shown that paleo-detectors could probe much of the
remaining Weakly Interacting Massive Particle (WIMP) parameter space. In this
paper, we improve upon the cut-and-count approach previously used to estimate
the sensitivity by performing a full spectral analysis of the background- and
DM-induced signal spectra. We consider two scenarios for the systematic errors
on the background spectra: i) systematic errors on the normalization only, and
ii) systematic errors on the shape of the backgrounds. We find that the
projected sensitivity is rather robust to imperfect knowledge of the
backgrounds. Finally, we study how well the parameters of the true WIMP model
could be reconstructed in the hypothetical case of a WIMP discovery.Comment: 14 pages, 5 figures, code available at
https://github.com/tedwards2412/paleo_detectors/ . v2: Added additional
analysis theory details, matches version published in PR
Nash Codes for Noisy Channels
This paper studies the stability of communication protocols that deal with
transmission errors. We consider a coordination game between an informed sender
and an uninformed decision maker, the receiver, who communicate over a noisy
channel. The sender's strategy, called a code, maps states of nature to
signals. The receiver's best response is to decode the received channel output
as the state with highest expected receiver payoff. Given this decoding, an
equilibrium or "Nash code" results if the sender encodes every state as
prescribed. We show two theorems that give sufficient conditions for Nash
codes. First, a receiver-optimal code defines a Nash code. A second, more
surprising observation holds for communication over a binary channel which is
used independently a number of times, a basic model of information
transmission: Under a minimal "monotonicity" requirement for breaking ties when
decoding, which holds generically, EVERY code is a Nash code.Comment: More general main Theorem 6.5 with better proof. New examples and
introductio
Time Domain Simulations of Arm Locking in LISA
Arm locking is a technique that has been proposed for reducing laser
frequency fluctuations in the Laser Interferometer Space Antenna (LISA), a
gravitational-wave observatory sensitive in the milliHertz frequency band. Arm
locking takes advantage of the geometric stability of the triangular
constellation of three spacecraft that comprise LISA to provide a frequency
reference with a stability in the LISA measurement band that exceeds that
available from a standard reference such as an optical cavity or molecular
absorption line. We have implemented a time-domain simulation of arm locking
including the expected limiting noise sources (shot noise, clock noise,
spacecraft jitter noise, and residual laser frequency noise). The effect of
imperfect a priori knowledge of the LISA heterodyne frequencies and the
associated 'pulling' of an arm locked laser is included. We find that our
implementation meets requirements both on the noise and dynamic range of the
laser frequency.Comment: Revised to address reviewer comments. Accepted by Phys. Rev.
Carrier-mediated magnetoelectricity in complex oxide heterostructures
While tremendous success has been achieved to date in creating both single
phase and composite magnetoelectric materials, the quintessential
electric-field control of magnetism remains elusive. In this work, we
demonstrate a linear magnetoelectric effect which arises from a novel
carrier-mediated mechanism, and is a universal feature of the interface between
a dielectric and a spin-polarized metal. Using first-principles density
functional calculations, we illustrate this effect at the SrRuO/SrTiO
interface and describe its origin. To formally quantify the magnetic response
of such an interface to an applied electric field, we introduce and define the
concept of spin capacitance. In addition to its magnetoelectric and spin
capacitive behavior, the interface displays a spatial coexistence of magnetism
and dielectric polarization suggesting a route to a new type of interfacial
multiferroic
New Projections for Dark Matter Searches with Paleo-Detectors
Paleo-detectors are a proposed experimental technique to search for dark
matter (DM). In lieu of the conventional approach of operating a tonne-scale
real-time detector to search for DM-induced nuclear recoils, paleo-detectors
take advantage of small samples of naturally occurring rocks on Earth that have
been deep underground ( km), accumulating nuclear damage tracks from
recoiling nuclei for Gyr. Modern microscopy techniques promise
the capability to read out nuclear damage tracks with nanometer resolution in
macroscopic samples. Thanks to their Gyr integration times,
paleo-detectors could constitute nuclear recoil detectors with keV recoil
energy thresholds and 100 kilotonne-yr exposures. This combination would allow
paleo-detectors to probe DM-nucleon cross sections orders of magnitude below
existing upper limits from conventional direct detection experiments. In this
article, we use improved background modeling and a new spectral analysis
technique to update the sensitivity forecast for paleo-detectors. We
demonstrate the robustness of the sensitivity forecast to the (lack of)
ancillary measurements of the age of the samples and the parameters controlling
the backgrounds, systematic mismodeling of the spectral shape of the
backgrounds, and the radiopurity of the mineral samples. Specifically, we
demonstrate that even if the uranium concentration in paleo-detector samples is
(per weight), many orders of magnitude larger than what we expect in
the most radiopure samples obtained from ultra basic rock or marine evaporite
deposits, paleo-detectors could still probe DM-nucleon cross sections below
current limits. For DM masses GeV/, the sensitivity of
paleo-detectors could still reach down all the way to the conventional neutrino
floor in a Xe-based direct detection experiment.Comment: Invited contribution to Instruments "Special Issue Innovative
Experimental Techniques for Direct Dark Matter Detection)". 30 pages, 5
figures, 1 table. Code available at https://github.com/sbaum90/paleoSpec and
https://github.com/sbaum90/paleoSen
Berry-phase theory of polar discontinuities at oxide-oxide interfaces
In the framework of the modern theory of polarization, we rigorously
establish the microscopic nature of the electric displacement field D. In
particular, we show that the longitudinal component of D is preserved at a
coherent and insulating interface. To motivate and elucidate our derivation, we
use the example of LAO/STO interfaces and superlattices, where the validity of
the above conservation law is not immediately obvious. Our results generalize
the "locality principle" of constrained-D density functional theory to the
first-principles modeling of charge-mismatched systems.Comment: 4 pages, 2 figure
Generalizing movements with information-theoretic stochastic optimal control
Stochastic optimal control is typically used to plan a movement for a specific situation. Although most stochastic optimal control methods fail to generalize this movement plan to a new situation without replanning, a stochastic optimal control method is presented that allows reuse of the obtained policy in a new situation, as the policy is more robust to slight deviations from the initial movement plan. To improve the robustness of the policy, we employ information-theoretic policy updates that explicitly operate on trajectory distributions instead of single trajectories. To ensure a stable and smooth policy update, the ”distance” is limited between the trajectory distributions of the old and the new control policies. The introduced bound offers a closed-form solution for the resulting policy and extends results from recent developments in stochastic optimal control. In contrast to many standard stochastic optimal control algorithms, the current approach can directly infer the system dynamics from data points, and hence can also be used for model-based reinforcement learning. This paper represents an extension of the paper by Lioutikov et al. (“Sample-Based Information-Theoretic Stochastic Optimal Control,” Proceedings of 2014 IEEE International Conference on Robotics and Automation (ICRA), IEEE, Piscataway, NJ, 2014, pp. 3896–3902). In addition to revisiting the content, an extensive theoretical comparison is presented of the approach with related work, additional aspects of the implementation are discussed, and further evaluations are introduced
Managing for change: October 11, 1989
Bi-weekly newsletter of University Hospital's Change Project, provided to managers at the hospital
- …