465 research outputs found
Apollo experience report: Development of guidance targeting techniques for the command module and launch vehicle
The development of the guidance targeting techniques for the Apollo command module and launch vehicle is discussed for four types of maneuvers: (1) translunar injection, (2) translunar midcourse, (3) lunar orbit insertion, and (4) return to earth. The development of real-time targeting programs for these maneuvers and the targeting procedures represented are discussed. The material is intended to convey historically the development of the targeting techniques required to meet the defined target objectives and to illustrate the solutions to problems encountered during that development
Are You Tampering With My Data?
We propose a novel approach towards adversarial attacks on neural networks
(NN), focusing on tampering the data used for training instead of generating
attacks on trained models. Our network-agnostic method creates a backdoor
during training which can be exploited at test time to force a neural network
to exhibit abnormal behaviour. We demonstrate on two widely used datasets
(CIFAR-10 and SVHN) that a universal modification of just one pixel per image
for all the images of a class in the training set is enough to corrupt the
training procedure of several state-of-the-art deep neural networks causing the
networks to misclassify any images to which the modification is applied. Our
aim is to bring to the attention of the machine learning community, the
possibility that even learning-based methods that are personally trained on
public datasets can be subject to attacks by a skillful adversary.Comment: 18 page
Transport control by coherent zonal flows in the core/edge transitional regime
3D Braginskii turbulence simulations show that the energy flux in the
core/edge transition region of a tokamak is strongly modulated - locally and on
average - by radially propagating, nearly coherent sinusoidal or solitary zonal
flows. The flows are geodesic acoustic modes (GAM), which are primarily driven
by the Stringer-Winsor term. The flow amplitude together with the average
anomalous transport sensitively depend on the GAM frequency and on the magnetic
curvature acting on the flows, which could be influenced in a real tokamak,
e.g., by shaping the plasma cross section. The local modulation of the
turbulence by the flows and the excitation of the flows are due to wave-kinetic
effects, which have been studied for the first time in a turbulence simulation.Comment: 5 pages, 5 figures, submitted to PR
Feature pyramid transformer
Feature interactions across space and scales underpin modern visual
recognition systems because they introduce beneficial visual contexts.
Conventionally, spatial contexts are passively hidden in the CNN's increasing
receptive fields or actively encoded by non-local convolution. Yet, the
non-local spatial interactions are not across scales, and thus they fail to
capture the non-local contexts of objects (or parts) residing in different
scales. To this end, we propose a fully active feature interaction across both
space and scales, called Feature Pyramid Transformer (FPT). It transforms any
feature pyramid into another feature pyramid of the same size but with richer
contexts, by using three specially designed transformers in self-level,
top-down, and bottom-up interaction fashion. FPT serves as a generic visual
backbone with fair computational overhead. We conduct extensive experiments in
both instance-level (i.e., object detection and instance segmentation) and
pixel-level segmentation tasks, using various backbones and head networks, and
observe consistent improvement over all the baselines and the state-of-the-art
methods.Comment: Published at the European Conference on Computer Vision, 202
Automatic Classification of Roof Shapes for Multicopter Emergency Landing Site Selection
Geographic information systems (GIS) now provide accurate maps of terrain,
roads, waterways, and building footprints and heights. Aircraft, particularly
small unmanned aircraft systems, can exploit additional information such as
building roof structure to improve navigation accuracy and safety particularly
in urban regions. This paper proposes a method to automatically label building
roof shape types. Satellite imagery and LIDAR data from Witten, Germany are fed
to convolutional neural networks (CNN) to extract salient feature vectors.
Supervised training sets are automatically generated from pre-labeled buildings
contained in the OpenStreetMap database. Multiple CNN architectures are trained
and tested, with the best performing networks providing a condensed feature set
for support vector machine and decision tree classifiers. Satellite and LIDAR
data fusion is shown to provide greater classification accuracy than through
use of either data type individually
Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection
conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta
(ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic
shear (strong guide field). Changing the simulation domain size, we find that
the ion response varies greatly. For reconnecting regions with scales
comparable to the ion Larmor radius, the ions do not respond to the
reconnection dynamics leading to ''electron-only'' reconnection with very large
quasi-steady reconnection rates. The transition to more traditional
''ion-coupled'' reconnection is gradual as the reconnection domain size
increases, with the ions becoming frozen-in in the exhaust when the magnetic
island width in the normal direction reaches many ion inertial lengths. During
this transition, the quasi-steady reconnection rate decreases until the ions
are fully coupled, ultimately reaching an asymptotic value. The scaling of the
ion outflow velocity with exhaust width during this electron-only to
ion-coupled transition is found to be consistent with a theoretical model of a
newly reconnected field line. In order to have a fully frozen-in ion exhaust
with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width
of at least several ion inertial lengths is needed. In turbulent systems with
reconnection occurring between magnetic bubbles associated with fluctuations,
using geometric arguments we estimate that fully ion-coupled reconnection
requires magnetic bubble length scales of at least several tens of ion inertial
lengths
TBI lesion segmentation in head CT: impact of preprocessing and data augmentation
Automatic segmentation of lesions in head CT provides keyinformation for patient management, prognosis and disease monitoring.Despite its clinical importance, method development has mostly focusedon multi-parametric MRI. Analysis of the brain in CT is challengingdue to limited soft tissue contrast and its mono-modal nature. We studythe under-explored problem of fine-grained CT segmentation of multiplelesion types (core, blood, oedema) in traumatic brain injury (TBI). Weobserve that preprocessing and data augmentation choices greatly impactthe segmentation accuracy of a neural network, yet these factors arerarely thoroughly assessed in prior work. We design an empirical studythat extensively evaluates the impact of different data preprocessing andaugmentation methods. We show that these choices can have an impactof up to 18% DSC. We conclude that resampling to isotropic resolutionyields improved performance, skull-stripping can be replaced by using theright intensity window, and affine-to-atlas registration is not necessaryif we use sufficient spatial augmentation. Since both skull-stripping andaffine-to-atlas registration are susceptible to failure, we recommend theiralternatives to be used in practice. We believe this is the first work toreport results for fine-grained multi-class segmentation of TBI in CT. Ourfindings may inform further research in this under-explored yet clinicallyimportant task of automatic head CT lesion segmentation
Rewriting a Deep Generative Model
A deep generative model such as a GAN learns to model a rich set of semantic
and physical rules about the target distribution, but up to now, it has been
obscure how such rules are encoded in the network, or how a rule could be
changed. In this paper, we introduce a new problem setting: manipulation of
specific rules encoded by a deep generative model. To address the problem, we
propose a formulation in which the desired rule is changed by manipulating a
layer of a deep network as a linear associative memory. We derive an algorithm
for modifying one entry of the associative memory, and we demonstrate that
several interesting structural rules can be located and modified within the
layers of state-of-the-art generative models. We present a user interface to
enable users to interactively change the rules of a generative model to achieve
desired effects, and we show several proof-of-concept applications. Finally,
results on multiple datasets demonstrate the advantage of our method against
standard fine-tuning methods and edit transfer algorithms.Comment: ECCV 2020 (oral). Code at https://github.com/davidbau/rewriting. For
videos and demos see https://rewriting.csail.mit.edu
Online interventions to prevent mental health problems implemented in school settings: the perspectives from key stakeholders in Austria and Spain
Background: Schools are key settings for delivering mental illness prevention in adolescents. Data on stakeholdersâ attitudes and factors relevant for the implementation of Internet-based prevention programmes are scarce. Methods: Stakeholders in the school setting from Austria and Spain were consulted. Potential facilitators (e.g. teachers and school psychologists) completed an online questionnaire (N=50), policy makers (e.g. representatives of the ministry of education and health professional associations) participated in semi-structured interviews (N=9) and pupils (N=29, 14â19âyears) participated in focus groups. Thematic analysis was used to identify experiences with, attitudes and needs towards Internet-based prevention programmes, underserved groups, as well as barriers and facilitators for reach, adoption, implementation and maintenance. Results: Experiences with Internet-based prevention programmes were low across all stakeholder groups. Better reach of the target groups was seen as main advantage whereas lack of personal contact, privacy concerns, risk for misuse and potential stigmatization when implemented during school hours were regarded as disadvantages. Relevant needs towards Internet-based programmes involved attributes of the development process, general requirements for safety and performance, presentation of content, media/tools and contact options of online programmes. Positive attitudes of school staff, low effort for schools and compatibility to schoolsâ curriculum were seen as key factors for successful adoption and implementation. A sound implementation of the programme in the school routine and continued improvement could facilitate maintenance of online prevention initiatives in schools. Conclusions: Attitudes towards Internet-based mental illness prevention programmes in school settings are positive across all stakeholder groups. However, especially safety concerns have to be considered
- âŠ