11,013 research outputs found
SEASAT synthetic-aperture radar data user's manual
The SEASAT Synthetic-Aperture Radar (SAR) system, the data processors, the extent of the image data set, and the means by which a user obtains this data are described and the data quality is evaluated. The user is alerted to some potential problems with the existing volume of SEASAT SAR image data, and allows him to modify his use of that data accordingly. Secondly, the manual focuses on the ultimate focuses on the ultimate capabilities of the raw data set and evaluates the potential of this data for processing into accurately located, amplitude-calibrated imagery of high resolution. This allows the user to decide whether his needs require special-purpose data processing of the SAR raw data
The Making of Citizenship
Nach der Unabhängigkeit Tansanias und Ugandas verhandelten lokale Akteure Staatsbürgerschaft neu. Als Außenseiter mussten die asiatischen Minderheiten Ostafrikas ihre Rolle in einer neuen Welt finden. Doch während der 1960er verengten sich die sozialen und ökonomischen Räume für sie, was letztlich in der Ausweisung der Asiaten aus Uganda gipfelte
How an Unfunded Pension System looks like Defined Benefits but works like Defined Contributions: The German Pension Reform
This paper describes the German pension reform process 1992-2007 with a stress on a remark-able development: the public pay-as-you-go-financed pension system has almost silently moved from a traditional defined benefit system to a system which works in many respects like a defined contribution system. The paper combines economic with political considerations, hopefully offering a few lessons that are useful also for other countries.
The Cerium volume collapse: Results from the LDA+DMFT approach
The merger of density-functional theory in the local density approximation
(LDA) and many-body dynamical mean field theory (DMFT) allows for an ab initio
calculation of Ce including the inherent 4f electronic correlations. We solve
the DMFT equations by the quantum Monte Carlo (QMC) technique and calculate the
Ce energy, spectrum, and double occupancy as a function of volume. At low
temperatures, the correlation energy exhibits an anomalous region of negative
curvature which drives the system towards a thermodynamic instability, i.e.,
the -to- volume collapse, consistent with experiment. The
connection of the energetic with the spectral evolution shows that the physical
origin of the energy anomaly and, thus, the volume collapse is the appearance
of a quasiparticle resonance in the 4f-spectrum which is accompanied by a rapid
growth in the double occupancy.Comment: 4 pages, 3 figure
Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches
A semi-classical Monte Carlo model for studying three-dimensional carrier
dynamics in photoconductive switches is presented. The model was used to
simulate the process of photoexcitation in GaAs-based photoconductive antennas
illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed
the power and frequency bandwidth of THz radiation emitted from these devices
as a function of bias voltage, pump pulse duration and pump pulse location. We
show that the mechanisms limiting the THz power emitted from photoconductive
switches fall into two regimes: when illuminated with short duration (<40 fs)
laser pulses the energy distribution of the Gaussian pulses constrains the
emitted power, while for long (>40 fs) pulses, screening is the primary
power-limiting mechanism. A discussion of the dynamics of bias field screening
in the gap region is presented. The emitted terahertz power was found to be
enhanced when the exciting laser pulse was in close proximity to the anode of
the photoconductive emitter, in agreement with experimental results. We show
that this enhancement arises from the electric field distribution within the
emitter combined with a difference in the mobilities of electrons and holes.Comment: 7 pages, 7 figure
Analyzing veterinary surveillance data: Approaches to model the relationship between disease incidence and cattle trade
Two approaches to the analysis of registry data for bovine diseases with regard to the relationship between disease incidence and cattle trade are proposed. Firstly, a parameter-driven spatio-temporal disease mapping model
formulated in a hierarchical Bayesian framework is used. Various cattle movement parameters, e.g. the number and proportion of in-movements from infected regions, can be included as potential covariates. Within this context problems of such an endogenous covariate are discussed. Since a purely parameter-driven approach is often not adequate to depict local epidemics, a so-called observationdriven infectious disease model is proposed as a second possibility. It includes an autoregressive part for counts in the region of interest in the past. Additionally,
the sum of previous cases in other regions weighted by cattle movements is added to assess the spread of the disease by trading. Both models are applied to cases
of Coxiellosis in Switzerland, 2005 to 2009
Homogeneous metallicities and radial velocities for Galactic globular clusters. II. New CaT metallicities for 28 distant and reddened globular clusters
Although the globular clusters in the Milky Way have been studied for a long
time, a significant fraction of them lack homogeneous metallicity and radial
velocity measurements. In an earlier paper we presented the first part of a
project to obtain metallicities and radial velocities of Galactic globular
clusters from multiobject spectroscopy of their member stars using the ESO Very
Large Telescope. In this paper we add metallicities and radial velocities for a
new sample of 28 globular clusters, including in particular globular clusters
in the MW halo and the Galactic bulge. Together with our previous results, this
study brings the number of globular clusters with homogeneous measurements to
\% of those listed in the W. Harris' catalogue. As in our previous
work, we have used the CaII triplet lines to derive metallicities and radial
velocities. For most of the clusters in this study, this is the first analysis
based on spectroscopy of individual member stars. The metallicities derived
from the CaII triplet are then compared to the results of our parallel study
based on spectral fitting in the optical region and the implications for
different calibrations of the CaII triplet line strengths are discussed. We
also comment on some interesting clusters and investigate the presence of an
abundance spread in the globular clusters here. A hint of a possible intrinsic
spread is found for NGC 6256, which therefore appears to be a good candidate
for further study.Comment: 13 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
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Forced versus coupled dynamics in Earth system modelling and prediction
We compare coupled nonlinear climate models and their simplified forced counterparts with respect to predictability and phase space topology. Various types of uncertainty plague climate change simulation, which is, in turn, a crucial element of Earth System modelling. Since the currently preferred strategy for simulating the climate system, or the Earth System at large, is the coupling of sub-system modules (representing, e.g. atmosphere, oceans, global vegetation), this paper explicitly addresses the errors and indeterminacies generated by the coupling procedure. The focus is on a comparison of forced dynamics as opposed to fully, i.e. intrinsically, coupled dynamics. The former represents a particular type of simulation, where the time behaviour of one complex systems component is prescribed by data or some other external information source. Such a simplifying technique is often employed in Earth System models in order to save computing resources, in particular when massive model inter-comparisons need to be carried out. Our contribution to the debate is based on the investigation of two representative model examples, namely (i) a low-dimensional coupled atmosphere-ocean simulator, and (ii) a replica-like simulator embracing corresponding components. Whereas in general the forced version (ii) is able to mimic its fully coupled counterpart (i), we show in this paper that for a considerable fraction of parameter- and state-space, the two approaches qualitatively differ. Here we take up a phenomenon concerning the predictability of coupled versus forced models that was reported earlier in this journal: the observation that the time series of the forced version display artificial predictive skill. We present an explanation in terms of nonlinear dynamical theory. In particular we observe an intermittent version of artificial predictive skill, which we call on-off synchronization, and trace it back to the appearance of unstable periodic orbits. We also find it to be governed by a scaling law that allows us to estimate the probability of artificial predictive skill. In addition to artificial predictability we observe artificial bistability for the forced version, which has not been reported so far. The results suggest that bistability and intermittent predictability, when found in a forced model set-up, should always be cross-validated with alternative coupling designs before being taken for granted
On the Number of Iterations for Dantzig-Wolfe Optimization and Packing-Covering Approximation Algorithms
We give a lower bound on the iteration complexity of a natural class of
Lagrangean-relaxation algorithms for approximately solving packing/covering
linear programs. We show that, given an input with random 0/1-constraints
on variables, with high probability, any such algorithm requires
iterations to compute a
-approximate solution, where is the width of the input.
The bound is tight for a range of the parameters .
The algorithms in the class include Dantzig-Wolfe decomposition, Benders'
decomposition, Lagrangean relaxation as developed by Held and Karp [1971] for
lower-bounding TSP, and many others (e.g. by Plotkin, Shmoys, and Tardos [1988]
and Grigoriadis and Khachiyan [1996]). To prove the bound, we use a discrepancy
argument to show an analogous lower bound on the support size of
-approximate mixed strategies for random two-player zero-sum
0/1-matrix games
Forced versus coupled dynamics in Earth system modelling and prediction
International audienceWe compare coupled nonlinear climate models and their simplified forced counterparts with respect to predictability and phase space topology. Various types of uncertainty plague climate change simulation, which is, in turn, a crucial element of Earth System modelling. Since the currently preferred strategy for simulating the climate system, or the Earth System at large, is the coupling of sub-system modules (representing, e.g. atmosphere, oceans, global vegetation), this paper explicitly addresses the errors and indeterminacies generated by the coupling procedure. The focus is on a comparison of forced dynamics as opposed to fully, i.e. intrinsically, coupled dynamics. The former represents a particular type of simulation, where the time behaviour of one complex systems component is prescribed by data or some other external information source. Such a simplifying technique is often employed in Earth System models in order to save computing resources, in particular when massive model inter-comparisons need to be carried out. Our contribution to the debate is based on the investigation of two representative model examples, namely (i) a low-dimensional coupled atmosphere-ocean simulator, and (ii) a replica-like simulator embracing corresponding components.Whereas in general the forced version (ii) is able to mimic its fully coupled counterpart (i), we show in this paper that for a considerable fraction of parameter- and state-space, the two approaches qualitatively differ. Here we take up a phenomenon concerning the predictability of coupled versus forced models that was reported earlier in this journal: the observation that the time series of the forced version display artificial predictive skill. We present an explanation in terms of nonlinear dynamical theory. In particular we observe an intermittent version of artificial predictive skill, which we call on-off synchronization, and trace it back to the appearance of unstable periodic orbits. We also find it to be governed by a scaling law that allows us to estimate the probability of artificial predictive skill. In addition to artificial predictability we observe artificial bistability for the forced version, which has not been reported so far. The results suggest that bistability and intermittent predictability, when found in a forced model set-up, should always be cross-validated with alternative coupling designs before being taken for granted
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