27,676 research outputs found
Evaluation of the use of on-board spacecraft energy storage for electric propulsion missions
On-board spacecraft energy storage represents an under utilized resource for some types of missions that also benefit from using relatively high specific impulse capability of electric propulsion. This resource can provide an appreciable fraction of the power required for operating the electric propulsion subsystem in some missions. The most probable mission requirement for utilization of this energy is that of geostationary satellites which have secondary batteries for operating at high power levels during eclipse. The study summarized in this report selected four examples of missions that could benefit from use of electric propulsion and on-board energy storage. Engineering analyses were performed to evaluate the mass saved and economic benefit expected when electric propulsion and on-board batteries perform some propulsion maneuvers that would conventionally be provided by chemical propulsion. For a given payload mass in geosynchronous orbit, use of electric propulsion in this manner typically provides a 10% reduction in spacecraft mass
An updated maximum likelihood approach to open cluster distance determination
An improved method for estimating distances to open clusters is presented and
applied to Hipparcos data for the Pleiades and the Hyades. The method is
applied in the context of the historic Pleiades distance problem, with a
discussion of previous criticisms of Hipparcos parallaxes. This is followed by
an outlook for Gaia, where the improved method could be especially useful.
Based on maximum likelihood estimation, the method combines parallax, position,
apparent magnitude, colour, proper motion, and radial velocity information to
estimate the parameters describing an open cluster precisely and without bias.
We find the distance to the Pleiades to be pc, in accordance
with previously published work using the same dataset. We find that error
correlations cannot be responsible for the still present discrepancy between
Hipparcos and photometric methods. Additionally, the three-dimensional space
velocity and physical structure of Pleiades is parametrised, where we find
strong evidence of mass segregation. The distance to the Hyades is found to be
pc, also in accordance with previous results. Through the use
of simulations, we confirm that the method is unbiased, so will be useful for
accurate open cluster parameter estimation with Gaia at distances up to several
thousand parsec.Comment: 14 pages, 8 figures, 6 tables, 3 appendices. Accepted in A&
Modelling Hen Harrier Dynamics to Inform Human-Wildlife Conflict Resolution : A Spatially-Realistic, Individual-Based Approach
Peer reviewedPublisher PD
Functional biases in GRB's spectral parameter correlations
Gamma Ray Bursts (GRBs) show evidence of different spectral shapes, light
curves, duration, host galaxies and they explode within a wide redshift range.
However, the most of them seems to follow very tight correlations among some
observed quantities relating to their energetic. If true, these correlations
have significant implications on burst physics, giving constraints on
theoretical models. Moreover, several suggestions have been made to use these
correlations in order to calibrate GRBs as standard candles and to constrain
the cosmological parameters. We investigate the cosmological relation between
low energy index in GRBs prompt spectra and the redshift . We
present a statistical analysis of the relation between the total isotropic
energy and the peak energy (also known as Amati relation) in
GRBs spectra searching for possible functional biases. Possible implications on
the vs relation of the vs correlation are
evaluated. We used MonteCarlo simulations and the boostrap method to evaluate
how large are the effects of functional biases on the vs . We
show that high values of the linear correlation coefficent, up to about 0.8, in
the vs relation are obtained for random generated samples of
GRBs, confirming the relevance of functional biases. Astrophysical consequences
from vs relation are then to be revised after a more accurate
and possibly bias free analysis.Comment: 6 pages, 6 figures, conference poster session: "070228: The Next
Decade of Gamma-Ray Burst Afterglows", Amsterdam, March 2007, MNRAS submitte
SVOM pointing strategy: how to optimize the redshift measurements?
The Sino-French SVOM mission (Space-based multi-band astronomical Variable
Objects Monitor) has been designed to detect all known types of gamma-ray
bursts (GRBs) and to provide fast and reliable GRB positions. In this study we
present the SVOM pointing strategy which should ensure the largest number of
localized bursts allowing a redshift measurement. The redshift measurement can
only be performed by large telescopes located on Earth. The best scientific
return will be achieved if we are able to combine constraints from both space
segment (platform and payload) and ground telescopes (visibility).Comment: Proceedings of Gamma-Ray Bursts 2007 conference, Santa Fe, USA, 5-9
November 2007. Published in AIP conf. proc. 1000, 585-588 (2008
Protein Supplementation as Related to Sources of Protein for Growing-Finishing Lambs
The objective of this study was to evaluate commercially available sources of supplemental protein in diets for growing and finishing lambs. Supplements were selected to differ in protein in diets for growing and finishing lambs. Supplements were selected to differ in protein solubility and amino acid profile and were expected to be degraded at different rates in the rumen to furnish protein post-ruminally varying in dietary and bacterial origin. If the principles concerning these parameters reported by other investigators would be applicable, response in terms of growth and feed utilization would be a measure of the value of the supplements for productive purposes
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
Developmental design, fabrication, and test of acoustic suppressors for fans of high bypass turbofan engines
An analysis procedure was developed for design of acoustically treated nacelles for high bypass turbofan engines. The plan was applied to the conceptual design of a nacelle for the quiet engine typical of a 707/DC-8 airplane installation. The resultant design was modified to a test nacelle design for the NASA Lewis quiet fan. The acoustic design goal was a 10 db reduction in effective perceived fan noise levels during takoff and approach. Detailed nacelle designs were subsequently developed for both the quiet engine and the quiet fan. The acoustic design goal for each nacelle was 15 db reductions in perceived fan noise levels from the inlet and fan duct. Acoustically treated nacelles were fabricated for the quiet engine and quiet fan for testing. Performance of selected inlet and fan duct lining configurations was experimentally evaluated in a flow duct. Results of the tests show that the linings perform as designed
Oceanic stochastic parametrizations in a seasonal forecast system
We study the impact of three stochastic parametrizations in the ocean
component of a coupled model, on forecast reliability over seasonal timescales.
The relative impacts of these schemes upon the ocean mean state and ensemble
spread are analyzed. The oceanic variability induced by the atmospheric forcing
of the coupled system is, in most regions, the major source of ensemble spread.
The largest impact on spread and bias came from the Stochastically Perturbed
Parametrization Tendency (SPPT) scheme - which has proven particularly
effective in the atmosphere. The key regions affected are eddy-active regions,
namely the western boundary currents and the Southern Ocean. However, unlike
its impact in the atmosphere, SPPT in the ocean did not result in a significant
decrease in forecast error. Whilst there are good grounds for implementing
stochastic schemes in ocean models, our results suggest that they will have to
be more sophisticated. Some suggestions for next-generation stochastic schemes
are made.Comment: 24 pages, 3 figure
FEP covers for silicon solar cells
Feasibility of fluorinated ethylene propylene as replacement for conventional silicon solar cell cover
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