3,560 research outputs found
The motion of throw away detectors relative to the space shuttle
The motions of throw away detectors (TAD's) are analyzed using the linearized relative motion equations. The TAD's are to be used in the amps program as diagnostic instruments for making various measurements near the shuttle. The TAD's are ejected from the shuttle in arbitrary directions with small relative velocities (0.1 to 1.0 m/s) their subsequent trajectories relative to the shuttle are analyzed. Initial conditions that are likely to result in recontact between the TAD and the shuttle are identified. The sensitivity of the motion to variations in the initial conditions, possibly resulting from inaccuracy in the ejection mechanism, are analyzed as are effects of atmospheric drag. A targeting method, a method of giving the TAD correct initial conditions such that it will pass through a given point relative to the shuttle at a given time, is developed. The results of many specific cases are presented in graphical form
A plan for spacecraft automated rendezvous
An automated rendezvous approach has been developed that utilizes advances in technology to reduce real-time/near real-time flight operations support personnel to an acceptable level that is near the minimum without jeopardizing the success of the mission. The on-board flight targeting uses a rule-based system to select the pursuit vehicle phasing orbits and uses precise navigation updates from the pursuit/target spacecraft made possible by the global positioning system receivers/processors on both spacecraft to adjust the phasing orbits and achieve rendezvous. The ascent-to-orbit targeting for the pursuit vehicle has been successfully decoupled from the on-orbit orbit transfer phasing targeting. Typical launch window data have been developed for the heavy lift launch vehicle and cargo transfer vehicle for a Space Station Freedom rendezvous mission
Mopra CO Observations of the Bubble HII Region RCW120
We use the Mopra radio telescope to test for expansion of the molecular gas
associated with the bubble HII region RCW120. A ring, or bubble, morphology is
common for Galactic HII regions, but the three-dimensional geometry of such
objects is still unclear. Detected near- and far-side expansion of the
associated molecular material would be consistent with a three-dimensional
spherical object. We map the transitions of CO,
CO, CO, and CO, and detect emission from all
isotopologues. We do not detect the masing lines of
CHOH at 108.8939 GHz. The strongest CO emission is from the
photodissociation region (PDR), and there is a deficit of emission toward the
bubble interior. We find no evidence for expansion of the molecular material
associated with RCW120 and therefore can make no claims about its geometry. The
lack of detected expansion is roughly in agreement with models for the
time-evolution of an HII region like RCW120, and is consistent with an
expansion speed of . Single-position CO spectra show
signatures of expansion, which underscores the importance of mapped spectra for
such work. Dust temperature enhancements outside the PDR of RCW120 coincide
with a deficit of emission in CO, confirming that these temperature
enhancements are due to holes in the RCW120 PDR. H emission shows that
RCW120 is leaking of the ionizing photons into the interstellar
medium (ISM) through PDR holes at the locations of the temperature
enhancements. H-alpha emission also shows a diffuse "halo" from leaked photons
not associated with discrete holes in the PDR. Overall of all
ionizing photons are leaking into the nearby ISM.Comment: 35 pages, 14 figures. Accepted to Ap
Scheduling aircraft landings - the static case
This is the publisher version of the article, obtained from the link below.In this paper, we consider the problem of scheduling aircraft (plane) landings at an airport. This problem is one of deciding a landing time for each plane such that each plane lands within a predetermined time window and that separation criteria between the landing of a plane and the landing of all successive planes are respected. We present a mixed-integer zero–one formulation of the problem for the single runway case and extend it to the multiple runway case. We strengthen the linear programming relaxations of these formulations by introducing additional constraints. Throughout, we discuss how our formulations can be used to model a number of issues (choice of objective function, precedence restrictions, restricting the number of landings in a given time period, runway workload balancing) commonly encountered in practice. The problem is solved optimally using linear programming-based tree search. We also present an effective heuristic algorithm for the problem. Computational results for both the heuristic and the optimal algorithm are presented for a number of test problems involving up to 50 planes and four runways.J.E.Beasley. would like to acknowledge the financial support of the Commonwealth Scientific and Industrial Research Organization, Australia
Risk perceptions, sexual attitudes, and sexual behavior after HPV vaccination in 11-12 year-old girls
OBJECTIVES: Among 11-12 year-old girls who received the human papillomavirus (HPV) vaccine, we explored, over the subsequent 30 months: (1) trajectories of knowledge about HPV/HPV vaccines and vaccine-related risk perceptions; (2) whether knowledge and risk perceptions impacted sexual attitudes and sexual experience; (3) whether mothers, clinicians, and media influenced girls' risk perceptions, attitudes, and behavior.
METHODS: Girls and mothers (n=25dyads) completed separate, semi-structured interviews within 2 days of, and 6, 18, and 30 months after, their first HPV vaccine dose. Knowledge, risk perceptions related to HPV and other sexually transmitted infections (STIs), and attitudes about sexual behaviors were assessed. Sexual experience was assessed at girls' 30 month interviews. Clinicians completed interviews at baseline. Transcribed interviews were analyzed using framework analysis.
RESULTS: Girls' baseline knowledge was poor but often improved with time. Most girls (n=18) developed accurate risk perceptions about HPV but only half (n=12) developed accurate risk perceptions about other STIs by 30 months. The vast majority of girls thought that safer sex was still important, regardless of knowledge, risk perceptions, or sexual experience. Girls whose HPV knowledge was high at baseline or increased over time tended to articulate accurate risk perceptions; those who were able to articulate accurate risk perceptions tended to report not having initiated sexual activity. Girls whose mothers demonstrated higher knowledge and/or communication about HPV vaccination tended to articulate accurate risk perceptions, whereas clinicians and media exposure did not appear to influence risk perceptions.
CONCLUSIONS: Higher knowledge about HPV vaccines among mothers and girls was linked with more accurate risk perceptions among girls. Clinicians may play an important role in providing education about HPV vaccines to mothers and girls
Meteorological radar facility. Part 1: System design
A compilation of information regarding systems design of space shuttles used in meteorological radar probes is presented. Necessary radar equipment is delineated, while space system elements, calibration techniques, antenna systems and other subsystems are reviewed
Early stages of ramified growth in quasi-two-dimensional electrochemical deposition
I have measured the early stages of the growth of branched metal aggregates
formed by electrochemical deposition in very thin layers. The growth rate of
spatial Fourier modes is described qualitatively by the results of a linear
stability analysis [D.P. Barkey, R.H. Muller, and C.W. Tobias, J. Electrochem.
Soc. {\bf 136}, 2207 (1989)]. The maximum growth rate is proportional to
where is the current through the electrochemical cell,
the electrolyte concentration, and . Differences
between my results and the theoretical predictions suggest that
electroconvection in the electrolyte has a large influence on the instability
leading to ramified growth.Comment: REVTeX, four ps figure
Fluctuations, line tensions, and correlation times of nanoscale islands on surfaces
We analyze in detail the fluctuations and correlations of the (spatial)
Fourier modes of nano-scale single-layer islands on (111) fcc crystal surfaces.
We analytically show that the Fourier modes of the fluctuations couple due to
the anisotropy of the crystal, changing the power spectrum of the fluctuations,
and that the actual eigenmodes of the fluctuations are the appropriate linear
combinations of the Fourier modes. Using kinetic Monte Carlo simulations with
bond-counting parameters that best match realistic energy barriers for hopping
rates, we deduce absolute line tensions as a function of azimuthal orientation
from the analyses of the fluctuation of each individual mode. The
autocorrelation functions of these modes give the scaling of the correlation
times with wavelength, providing us with the rate-limiting kinetics driving the
fluctuations, here step-edge diffusion. The results for the energetic
parameters are in reasonable agreement with available experimental data for
Pb(111) surfaces, and we compare the correlation times of island-edge
fluctuations to relaxation times of quenched Pb crystallites.Comment: 11 pages, 8 figures; to appear in PRB 70, xxx (15 Dec 2004), changes
in MC and its implication
Changing shapes in the nanoworld
What are the mechanisms leading to the shape relaxation of three dimensional
crystallites ? Kinetic Monte Carlo simulations of fcc clusters show that the
usual theories of equilibration, via atomic surface diffusion driven by
curvature, are verified only at high temperatures. Below the roughening
temperature, the relaxation is much slower, kinetics being governed by the
nucleation of a critical germ on a facet. We show that the energy barrier for
this step linearly increases with the size of the crystallite, leading to an
exponential dependence of the relaxation time.Comment: 4 pages, 5 figures. Accepted by Phys Rev Let
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