17,408 research outputs found
Receding horizon control of vectored thrust flight experiment
Abstract:
The application of a constrained receding horizon control technique to stabilise an indoor vectored-thrust flight experiment, known as the Caltech ducted fan, is given. The receding horizon control problem is formulated as a constrained optimal control problem and solved in real time with an efficient, computational method that combines nonlinear control theory, B-spline basis functions, and nonlinear programming. Characteristic issues, including non-zero computational times, convergence properties, choice of horizon length and terminal cost are discussed. The study validates the applicability of real-time receding horizon control for constrained systems with fast dynamics
Electromagnetic interferences from plasmas generated in meteoroids impacts
It is shown that the plasma, generated during an impact of a meteoroid with
an artificial satellite, can produce electromagnetic radiation below the
microwave frequency range. This interference is shown to exceed local noise
sources and might disturb regular satellite operations.Comment: 6 pages, no figures. This version macthes the published versio
Determination of the Far-Infrared Cosmic Background Using COBE/DIRBE and WHAM Data
Determination of the cosmic infrared background (CIB) at far infrared
wavelengths using COBE/DIRBE data is limited by the accuracy to which
foreground interplanetary and Galactic dust emission can be modeled and
subtracted. Previous determinations of the far infrared CIB (e.g., Hauser et
al. 1998) were based on the detection of residual isotropic emission in skymaps
from which the emission from interplanetary dust and the neutral interstellar
medium were removed. In this paper we use the Wisconsin H-alpha Mapper (WHAM)
Northern Sky Survey as a tracer of the ionized medium to examine the effect of
this foreground component on determination of the CIB. We decompose the DIRBE
far infrared data for five high Galactic latitude regions into H I and H-alpha
correlated components and a residual component. We find the H-alpha correlated
component to be consistent with zero for each region, and we find that addition
of an H-alpha correlated component in modeling the foreground emission has
negligible effect on derived CIB results. Our CIB detections and 2 sigma upper
limits are essentially the same as those derived by Hauser et al. and are given
by nu I_nu (nW m-2 sr-1) < 75, < 32, 25 +- 8, and 13 +- 3 at 60, 100, 140, and
240 microns, respectively. Our residuals have not been subjected to a detailed
anisotropy test, so our CIB results do not supersede those of Hauser et al. We
derive upper limits on the 100 micron emissivity of the ionized medium that are
typically about 40% of the 100 micron emissivity of the neutral atomic medium.
This low value may be caused in part by a lower dust-to-gas mass ratio in the
ionized medium than in the neutral medium, and in part by a shortcoming of
using H-alpha intensity as a tracer of far infrared emission.Comment: 38 pages, 8 figures. Accepted for publication in Ap
Ras-mediated phosphorylation of a conserved threonine residue enhances the transactivation activities of c-Ets1 and c-Ets2
The Ras oncogene products regulate the expression of genes in transformed cells, and members of the Ets family of transcription factors have been implicated in this process. To determine which Ets factors are the targets of Ras signaling pathways, the abilities of several Ets factors to activate Ras-responsive enhancer (RRE) reporters in the presence of oncogenic Ras were examined. In transient transfection assay, reporters containing RREs composed of Ets-AP-1 binding sites could be activated 30-fold in NIH 3T3 fibroblasts and 80-fold in the macrophage-like line RAW264 by the combination of Ets1 or Ets2 and Ras but not by several other Ets factors that were tested in the assay. Ets2 and Ras also superactivated an RRE composed of Ets-Ets binding sites, but the Ets-responsive promoter of the c-fms gene was not superactivated. Mutation of a threonine residue to alanine in the conserved amino-terminal regions of Ets1 and Ets2 (threonine 38 and threonine 72, respectively) abrogated the ability of each of these proteins to superactivate reporter gene expression. Phosphoamino acid analysis of radiolabeled Ets2 revealed that Ras induced normally absent threonine-specific phosphorylation of the protein. The Ras-dependent increase in threonine phosphorylation was not observed in Ets2 proteins that had the conserved threonine 72 residue mutated to alanine or serine. These data indicate that Ets1 and Ets2 are specific nuclear targets of Ras signaling events and that phosphorylation of a conserved threonine residue is a necessary molecular component of Ras-mediated activation of these transcription factors
Effect of spin orbit scattering on the magnetic and superconducting properties of nearly ferromagnetic metals: application to granular Pt
We calculate the effect of scattering on the static, exchange enhanced, spin
susceptibility and show that in particular spin orbit scattering leads to a
reduction of the giant moments and spin glass freezing temperature due to
dilute magnetic impurities. The harmful spin fluctuation contribution to the
intra-grain pairing interaction is strongly reduced opening the way for BCS
superconductivity. We are thus able to explain the superconducting and magnetic
properties recently observed in granular Pt as due to scattering effects in
single small grains.Comment: 9 pages 3 figures, accepted for publication in Phys. Rev. Letter
Imperfect Homoclinic Bifurcations
Experimental observations of an almost symmetric electronic circuit show
complicated sequences of bifurcations. These results are discussed in the light
of a theory of imperfect global bifurcations. It is shown that much of the
dynamics observed in the circuit can be understood by reference to imperfect
homoclinic bifurcations without constructing an explicit mathematical model of
the system.Comment: 8 pages, 11 figures, submitted to PR
Recommended from our members
Simple method for sub-diffraction resolution imaging of cellular structures on standard confocal microscopes by three-photon absorption of quantum dots
This study describes a simple technique that improves a recently developed 3D sub-diffraction imaging method based on three-photon absorption of commercially available quantum dots. The method combines imaging of biological samples via tri-exciton generation in quantum dots with deconvolution and spectral multiplexing, resulting in a novel approach for multi-color imaging of even thick biological samples at a 1.4 to 1.9-fold better spatial resolution. This approach is realized on a conventional confocal microscope equipped with standard continuous-wave lasers. We demonstrate the potential of multi-color tri-exciton imaging of quantum dots combined with deconvolution on viral vesicles in lentivirally transduced cells as well as intermediate filaments in three-dimensional clusters of mouse-derived neural stem cells (neurospheres) and dense microtubuli arrays in myotubes formed by stacks of differentiated C2C12 myoblasts
Discovery of VHE gamma-rays from RGB J0152+017
The BL Lac object RGB J0152+017 (z = 0.080) was predicted to be a very
high-energy (VHE; > 100 GeV) gamma-ray source, due to its high X-ray and radio
fluxes.
We report recent observations of this source made in late October and
November 2007 with the H.E.S.S. array consisting of four imaging atmospheric
Cerenkov telescopes. Contemporaneous observations were made in X-rays with the
Swift and RXTE satellites, in the optical band with the ATOM telescope, and in
the radio band with the Nancay Radio Telescope.
As a result, RGB J0152+017 is discovered as a source of VHE gamma-rays by
H.E.S.S. A signal of 173 gamma-ray photons corresponding to a statistical
significance of 6.6 sigmas was found in the data. The energy spectrum of the
source can be described by a powerlaw with a spectral index of = 2.95 +-
0.36stat +- 0.20syst. The integral flux above 300 GeV corresponds to ~2% of the
flux of the Crab nebula. The source spectral energy distribution (SED) can be
described using a two-component (extended jet and blob in jet) non-thermal
synchrotron self-Compton (SSC) leptonic model, plus a thermal host galaxy
component. The parameters that are found are very close to those found for TeV
blazars in similar SSC studies.
The location of its synchrotron peak, as derived from the SED in Swift data,
allows clear classification as a high-frequency-peaked BL Lac (HBL).Comment: 6 pages, 5 figures; accepted to appear in the proceedings of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, 22-25
April 2008, Palaiseau, Franc
- …