17,173 research outputs found
Midcourse navigation using statistical filter theory, a manual theodolite, and symbolic computer control
Midcourse navigation using statistical filter theory, manual theodolite, and symbolic computer control applied to manned spacecraf
En route position and time control of aircraft using Kalman filtering of radio aid data
Fixed-time-of-arrival (FTA) guidance and navigation is investigated as a possible technique capable of operation within much more stringent en route separation standards and offering significant advantages in safety, higher traffic densities, and improved scheduling reliability, both en route and in the terminal areas. This study investigated the application of FTA guidance previously used in spacecraft guidance. These FTA guidance techniques have been modified and are employed to compute the velocity corrections necessary to return an aircraft to a specified great-circle reference path in order to exercise en route time and position control throughout the entire flight. The necessary position and velocity estimates to accomplish this task are provided by Kalman filtering of data from Loran-C, VORTAC/TACAN, Doppler radar, radio or barometric altitude,and altitude rate. The guidance and navigation system was evaluated using a digital simulation of the cruise phase of supersonic and subsonic flights between San Francisco and New York City, and between New York City and London
On the asymptotic acoustic-mode phase in red-giant stars and its dependence on evolutionary state
Asteroseismic investigations based on the wealth of data now available,in
particular from the CoRoT and Kepler missions, require a good understanding of
the relation between the observed quantities and the properties of the
underlying stellar structure. Kallinger et al. 2012 found a relation between
their determination of the asymptotic phase of radial oscillations in evolved
stars and the evolutionary state, separating ascending-branch red giants from
helium-burning stars in the `red clump'. Here we provide a detailed analysis of
this relation, which is found to derive from differences between these two
classes of stars in the thermodynamic state of the convective envelope. There
is potential for distinguishing red giants and clump stars based on the phase
determined from observations that are too short to allow distinction based on
determination of the period spacing for mixed modes. The analysis of the phase
may also point to a better understanding of the potential for using the
helium-ionization-induced acoustic glitch to determine the helium abundance in
the envelopes of these stars.Comment: MNRAS, in the pres
High-precision abundances of elements in Kepler LEGACY stars. Verification of trends with stellar age
HARPS-N spectra with S/N > 250 and MARCS model atmospheres were used to
derive abundances of C, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, and Y in ten
stars from the Kepler LEGACY sample (including the binary pair 16 Cyg A and B)
selected to have metallicities in the range -0.15 < [Fe/H] < +0.15 and ages
between 1 and 7 Gyr. Stellar gravities were obtained from seismic data and
effective temperatures were determined by comparing non-LTE iron abundances
derived from FeI and FeII lines. Available non-LTE corrections were also
applied when deriving abundances of the other elements. The results support the
[X/Fe]-age relations previously found for solar twins. [Mg/Fe], [Al/Fe], and
[Zn/Fe] decrease by ~0.1 dex over the lifetime of the Galactic thin disk due to
delayed contribution of iron from Type Ia supernovae relative to prompt
production of Mg, Al, and Zn in Type II supernovae. [Y/Mg] and [Y/Al], on the
other hand, increase by ~0.3 dex, which can be explained by an increasing
contribution of s-process elements from low-mass AGB stars as time goes on. The
trends of [C/Fe] and [O/Fe] are more complicated due to variations of the ratio
between refractory and volatile elements among stars of similar age. Two stars
with about the same age as the Sun show very different trends of [X/H] as a
function of elemental condensation temperature Tc and for 16 Cyg, the two
components have an abundance difference, which increases with Tc. These
anomalies may be connected to planet-star interactions.Comment: 13 pages with 7 figures. Accepted for publication in A&
MODELLING STOCK DYNAMICS IN THE SOUTHERN BENGUELA ECOSYSTEM FOR THE PERIOD 1978–2002
An ecosystem model of the southern Benguela was fitted to available time-series data for the period 1978–2002, to explore how changes in target fish populations in this ecosystem can be attributed to feeding interaction terms and population control patterns, the impact of fishing, and environmental forcing. Fishing patterns were estimated to explain only 2–3% of the variability in the time-series, whereas an estimated productivity forcing pattern applied to phytoplankton explained 4–12% of the variance represented by the sum of squares. Model settings describing prey vulnerability to their predators could explain around 40% of the variability in the time-series. Modelled stock dynamics in the southern Benguela ecosystem more closely represent observed timeseries when wasp-waist control by small pelagic fish is simulated. Overall, model simulations suggest that almost half the variance in the time-series can be explained based on a combination of fishing, vulnerability settings and productivity patterns. Variation in mortalities and prey preferences over time, as well as model fits in relation to available effort series, are discussed. The study advances a model with improved parameterization and credibility to assist with an ecosystem approach to South African fisheries management. Afr. J. mar. Sci. 26: 179–19
Modeling temporal fluctuations in avalanching systems
We demonstrate how to model the toppling activity in avalanching systems by
stochastic differential equations (SDEs). The theory is developed as a
generalization of the classical mean field approach to sandpile dynamics by
formulating it as a generalization of Itoh's SDE. This equation contains a
fractional Gaussian noise term representing the branching of an avalanche into
small active clusters, and a drift term reflecting the tendency for small
avalanches to grow and large avalanches to be constricted by the finite system
size. If one defines avalanching to take place when the toppling activity
exceeds a certain threshold the stochastic model allows us to compute the
avalanche exponents in the continum limit as functions of the Hurst exponent of
the noise. The results are found to agree well with numerical simulations in
the Bak-Tang-Wiesenfeld and Zhang sandpile models. The stochastic model also
provides a method for computing the probability density functions of the
fluctuations in the toppling activity itself. We show that the sandpiles do not
belong to the class of phenomena giving rise to universal non-Gaussian
probability density functions for the global activity. Moreover, we demonstrate
essential differences between the fluctuations of total kinetic energy in a
two-dimensional turbulence simulation and the toppling activity in sandpiles.Comment: 14 pages, 11 figure
Correlations of record events as a test for heavy-tailed distributions
A record is an entry in a time series that is larger or smaller than all
previous entries. If the time series consists of independent, identically
distributed random variables with a superimposed linear trend, record events
are positively (negatively) correlated when the tail of the distribution is
heavier (lighter) than exponential. Here we use these correlations to detect
heavy-tailed behavior in small sets of independent random variables. The method
consists of converting random subsets of the data into time series with a
tunable linear drift and computing the resulting record correlations.Comment: Revised version, to appear in Physical Review Letter
What Fraction of Boron-8 Solar Neutrinos arrive at the Earth as a nu_2 mass eigenstate?
We calculate the fraction of B^8 solar neutrinos that arrive at the Earth as
a nu_2 mass eigenstate as a function of the neutrino energy. Weighting this
fraction with the B^8 neutrino energy spectrum and the energy dependence of the
cross section for the charged current interaction on deuteron with a threshold
on the kinetic energy of the recoil electrons of 5.5 MeV, we find that the
integrated weighted fraction of nu_2's to be 91 \pm 2 % at the 95% CL. This
energy weighting procedure corresponds to the charged current response of the
Sudbury Neutrino Observatory (SNO). We have used SNO's current best fit values
for the solar mass squared difference and the mixing angle, obtained by
combining the data from all solar neutrino experiments and the reactor data
from KamLAND. The uncertainty on the nu_2 fraction comes primarily from the
uncertainty on the solar delta m^2 rather than from the uncertainty on the
solar mixing angle or the Standard Solar Model. Similar results for the
Super-Kamiokande experiment are also given. We extend this analysis to three
neutrinos and discuss how to extract the modulus of the Maki-Nakagawa-Sakata
mixing matrix element U_{e2} as well as place a lower bound on the electron
number density in the solar B^8 neutrino production region.Comment: 23 pages, 8 postscript figures, latex. Dedicated to the memory of
John Bahcall who championed solar neutrinos for many lonely year
2D orbital-like magnetic order in
In high temperature copper oxides superconductors, a novel magnetic order
associated with the pseudogap phase has been identified in two different
cuprate families over a wide region of temperature and doping. We here report
the observation below 120 K of a similar magnetic ordering in the archetypal
cuprate (LSCO) system for x=0.085. In contrast to the
previous reports, the magnetic ordering in LSCO is {\it\bf only} short range
with an in-plane correlation length of 10 \AA\ and is bidimensional
(2D). Such a less pronounced order suggests an interaction with other
electronic instabilities. In particular, LSCO also exhibits a strong tendency
towards stripes ordering at the expense of the superconducting state.Comment: 4 figures, submitted to Phys. Rev. Let
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