5,067 research outputs found
Analytical low-thrust satellite maneuvers for rapid ground target revisit
This paper presents an analytical solution for a low-thrust maneuver to reduce the flyover time of a given terrestrial target. The work extends the general solution previously developed by the authors for a 3-phase spiral transfer that results in a change in the relative right ascension of the ascending node and argument of latitude of satellites in a constellation, by varying the orbital period and the J2 effect experienced by each satellite. This work improves the accuracy of the existing method by including the periodic effects of J2 in the analytical solution. Using these improved equations, a calculation of the flyover time of a given latitude can be determined, and the passes for which the target longitude is in view identified. Validation against a numerical orbit propagator shows the analytical method to accurately predict the sub-satellite point of the satellite to within ±1° of longitude after 15 days. A case study is performed showing that the method can successfully be used to reduce the time of flyover of Los Angeles from 14 days to just 1.97 days, with a change of velocity (ΔV) of 63m/s. The full exploration of the solution space shows the problem to be highly complex, such that an increase in the ΔV used for a maneuver will not necessarily reduce the time of flyover, potentially making optimization using a numerical solution challenging. It also shows that very similar flyover times can be achieved with very different ΔV usage. As such, an overview of the solution space is extremely valuable in allowing an informed trade-off between the time of flyover and maneuver ΔV
HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede
We report new observations of the spectrum of Ganymede in the spectral range
1160 - 1720 A made with the Space Telescope Imaging Spectrograph (STIS) on HST
on 1998 October 30. The observations were undertaken to locate the regions of
the atomic oxygen emissions at 1304 and 1356 A, previously observed with the
GHRS on HST, that Hall et al. (1998) claimed indicated the presence of polar
aurorae on Ganymede. The use of the 2" wide STIS slit, slightly wider than the
disk diameter of Ganymede, produced objective spectra with images of the two
oxygen emissions clearly separated. The OI emissions appear in both
hemispheres, at latitudes above 40 degrees, in accordance with recent Galileo
magnetometer data that indicate the presence of an intrinsic magnetic field
such that Jovian magnetic field lines are linked to the surface of Ganymede
only at high latitudes. Both the brightness and relative north-south intensity
of the emissions varied considerably over the four contiguous orbits (5.5
hours) of observation, presumably due to the changing Jovian plasma environment
at Ganymede. However, the observed longitudinal non-uniformity in the emission
brightness at high latitudes, particularly in the southern hemisphere, and the
lack of pronounced limb brightening near the poles are difficult to understand
with current models. In addition to observed solar HI Lyman-alpha reflected
from the disk, extended Lyman-alpha emission resonantly scattered from a
hydrogen exosphere is detected out to beyond two Ganymede radii from the limb,
and its brightness is consistent with the Galileo UVS measurements of Barth et
al. (1997).Comment: 7 pages, 4 figures, accepted for publication in ApJ, June 1, 200
Production of crystallizable human chymase from a Bacillus subtilis system
AbstractA Bacillus subtilis strain deficient in seven extracellular proteases was used to produce human mast cell chymase and is a viable expression system for serine proteases and other classes of proteins. Chymase is produced at 0.3–0.5 mg/l and is purified by three chromatography steps. Two crystal forms of PMSF-treated chymase were optimized. The first is C2 with a=47.94 Å, b=85.23 Å, c=174.18 Å, β=96.74°, and diffracts to at least 2.1 Å, while the second is P212121, with cell dimensions a=43.93 Å, b=58.16 Å, and c=86.09 Å, and a diffraction limit of approximately 1.9 Å. The first crystal form has either three or four molecules/asymmetric unit, while the second has one molecule/asymmetric unit
Study of ATLAS sensitivity to FCNC top decays
The ATLAS experiment sensitivity to top quark Flavour Changing Neutral
Current (FCNC) decays was studied at LHC using ttbar events. While one of the
top quarks is expected to follow the dominant Standard Model decay t->bW, the
other decays through a FCNC channel, i.e. t-> Z u(c), t-> gamma u(c) or t-> g
u(c). Different types of analyses, applied to each FCNC decay mode, were
compared. The FCNC branching ratio sensitivity (assuming a 5sigma signal
significance) and 95% confidence level limits on the branching ratios (in the
hypothesis of signal absence) were obtained
Aberration-free ultra-thin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces
The concept of optical phase discontinuities is applied to the design and
demonstration of aberration-free planar lenses and axicons, comprising a phased
array of ultrathin subwavelength spaced optical antennas. The lenses and
axicons consist of radial distributions of V-shaped nanoantennas that generate
respectively spherical wavefronts and non-diffracting Bessel beams at telecom
wavelengths. Simulations are also presented to show that our aberration-free
designs are applicable to high numerical aperture lenses such as flat
microscope objectives
The moderating effect of brand orientation on inter-firm market orientation and performance
While prior research has shown that market and brand orientation
are key contributors to successful business performance, research to
date has not fully explored how inter firm collaboration for these two
key orientations can enhance business performance. The purpose of
the paper is to investigate the relationship between inter-firm market
and performance; to test for the moderating role of brand orientation
in that relationship. A total of 169 completed pairs of surveys were
collected of small and medium enterprises operating internationally
in a variety of industries in Switzerland. The results show that inter-firm
market and brand orientation are two antecedents of marketing and
financial performance. The impact of inter-firm market on marketing
and financial performance is significant when the brand orientation
is favorable. This study extends previous research by examining the
moderating role of brand orientation on inter firm market orientation,
which is important, especially for firms wanting to increase their brand
reputation by entering into partnerships with other firms. Further
research is indicated, to identify the key moderators of the driving
force of inter-firm market in relation to business performance and
the reason why maintaining a strong brand presence is important in
the international marketplace
NF-κB-mediated effects on behavior and cartilage pathology in a non-invasive loading model of post-traumatic osteoarthritis
OBJECTIVE: This study aimed to examine the temporal activation of NF-κB and its relationship to the development of pain-related sensitivity and behavioral changes in a non-invasive murine knee loading model of PTOA.
METHOD: Following knee injury NF-κB activity was assessed longitudinally via in vivo imaging in FVB. Cg-Tg (HIV-EGFP,luc)8Tsb/J mice. Measures of pain-related sensitivity and behavior were also assessed longitudinally for 16 weeks. Additionally, we antagonized NF-κB signaling via intra-articular delivery of an IκB kinase two antagonist to understand how local NF-κB inhibition might alter disease progression.
RESULTS: Following joint injury NF-κB signaling within the knee joint was transiently increased and peaked on day 3 with an estimated 1.35 p/s/cm
CONCLUSION: These findings underscore the development of behavioral changes in this non-invasive loading model of PTOA and their relationships to NF-κB activation and pathology. They also highlight the potential chondroprotective effects of NF-κB inhibition shortly following joint injury despite limitations in preventing the long-term development of joint degeneration in this model of PTOA
Disappearance of Elliptic Flow: A New Probe for the Nuclear Equation of State
Using a relativistic hadron transport model, we investigate the utility of
the elliptic flow excitation function as a probe for the stiffness of nuclear
matter and for the onset of a possible quark-gluon-plasma (QGP)
phase-transition at AGS energies 1 < E_Beam < 11 AGeV. The excitation function
shows a strong dependence on the nuclear equation of state, and exhibits
characteristic signatures which could signal the onset of a phase transition to
the QGP.Comment: 11 pages, 4 Postscript figures, uses epsf.sty, submitted to Physical
Review Letter
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