406 research outputs found
Element gain drifts as an imaging dynamic range limitation in PAF-based interferometers
Interferometry with phased-array feeds (PAFs) presents new calibration
challenges in comparison with single-pixel feeds. In particular, temporal
instability of the compound beam patterns due to element gain drifts (EGDs) can
produce calibration artefacts in interferometric images. To translate imaging
dynamic range requirements into PAF hardware and calibration requirements, we
must learn to relate EGD levels to imaging artefact levels. We present a
MeqTrees-based simulations framework that addresses this problem, and apply it
to the APERTIF prototype currently in development for the WSRT.Comment: 4 pages, 3 figures, poster presentation at the XXX URSI General
Assembly and Scientific Symposium (Istanbul, Turkey, August 13-20, 2011
Numerical Results for the System Noise Temperature of an Aperture Array Tile and Comparison with Measurements
The purpose of this report is to document the noise performance of a complex
beamforming array antenna system and to characterize the recently developed
noise measurement facility called THACO, which was developed at ASTRON. The
receiver system includes the array antenna of strongly coupled 144 TSA
elements, 144 Low Noise Amplifiers (LNAs) (Tmin =35-40K) and the data
recording/storing facilities of the initial test station that allow for
off-line digital beamforming. The primary goal of this study is to compare the
measured receiver noise temperatures with the simulated values for several
practical beamformers, and to predict the associated receiver noise coupling
contribution, antenna thermal noise and ground noise pick-up (due to the back
radiation).Comment: ASTRON repor
An Axi-Symmetric Segmented Composite SKA Dish Design: Performance and Production Analysis
A concept of an axi-symmetric dish as antenna reflector for the next
generation radio telescope - the Square Kilometre Array (SKA) - is presented.
The reflector is based on the use of novel thermoplastic composite material
(reinforced with carbon fibre) in the context of the telescope design with wide
band single pixel feeds. The baseline of this design represents an array of
100's to 1000's reflector antennas of 15-m diameter and covers frequencies from
<1 to 10 GHz. The purpose of our study is the analysis of the production cost
of the dish and its performance in combination with a realistic wideband feed
(such as the 'Eleven Antenna' feed) over a wide frequency band and a range of
elevation angles. The presented initial simulation results inidicate the
potential of the proposed dish concept for low-cost and mass production and
demonstrate sensitivity comparable to that of the presently considered off-set
Gregorian reflector antenna with the same projected aperture area. We expect
this observation to be independent of the choice of the feed, as several other
single-pixel wideband feeds (that have been reported in the literature) have
similar beamwidth and phase center location, both being rather constant with
frequency.Comment: Invited paper for the Asia-Pacific Microwave Conference 2011 (APMC
2011), Melbourne, 5-8 Dec., Australia, 201
APERTIF, a focal plane array for the WSRT
In this paper we describe a focal plane array (FPA) prototype, based on
Vivaldi elements, developed for the Westerbork Synthesis Radio Telescope (WSRT)
to increase its instantaneous field of view by a factor 25 and double its
current bandwidth. This prototype is the first step in a project that has the
ambition to equip most of the WSRT antennas with FPAs to improve the survey
speed of the telescope. Examples of scientific applications are surveys of the
northern sky in polarised continuum and HI emission, and efficient searches for
pulsars and transients.Comment: 7 pages, 6 figures. Presented at "The Evolution of Galaxies through
the Neutral Hydrogen Window", Feb 1-3, 2008, Arecibo Observatory, Puerto
Rico. High resolution versions of figures available in jpg-forma
Characterising the Apertif primary beam response
Context. Phased Array Feeds (PAFs) are multi element receivers in the focal
plane of a telescope that make it possible to form simultaneously multiple
beams on the sky by combining the complex gains of the individual antenna
elements. Recently the Westerbork Synthesis Radio Telescope (WSRT) was upgraded
with PAF receivers and carried out several observing programs including two
imaging surveys and a time domain survey. The Apertif imaging surveys use a
configuration, where 40 partially overlapping compound beams (CBs) are
simultaneously formed on the sky and arranged in an approximately rectangular
shape. Aims. This manuscript aims to characterise the response of the 40
Apertif CBs to create frequency-resolved, I, XX and YY polarization empirical
beam shapes. The measured CB maps can be used for image deconvolution, primary
beam correction and mosaicing of Apertif imaging data. Methods. We use drift
scan measurements to measure the response of each of the 40 CBs of Apertif. We
derive beam maps for all individual beams in I, XX and YY polarisation in 10 or
18 frequency bins over the same bandwidth as the Apertif imaging surveys. We
sample the main lobe of the beams and the side lobes up to a radius of 0.6
degrees from the beam centres. In addition, we derive beam maps for each
individual WSRT dish as well. Results. We present the frequency and time
dependence of the beam shapes and sizes. We compare the compound beam shapes
derived with the drift scan method to beam shapes derived with an independent
method using a Gaussian Process Regression comparison between the Apertif
continuum images and the NRAO VLA Sky Survey (NVSS) catalogue. We find a good
agreement between the beam shapes derived with the two independent methods.Comment: Accepted for publication by A&A, 14 pages, 15 figure
Macro-financial linkages and bank behaviour: evidence from the second-round effects of the global financial crisis on East Asia
This paper studies the link between macro-financial variability and bank behaviour, which justifies the second-round effects of the global financial crisis on East Asia. Following Gallego et al. (The impact of the global economic and financial crisis on Central Eastern and South Eastern Europe (CESEE) and Latin America, 2010), the second round effects are defined as the adverse feedback loop from the slumps in economic activities and sharp financial market deterioration, which may influence the financial performance of bank, inter alia via deteriorating credit quality, declining profitability and increasing problems in retaining necessary capitalization. Differentiating itself from other research, this study stresses adjustments in four dimensions of bank performance and behaviour: asset quality, profitability, capital adequacy, and lending behaviour, assuming that any change in a bank-specific characteristic is induced by endogenous adjustments of the others. The empirical results based on partial adjustment models and two-step system GMM estimation show that bankâs adjustment behaviour is subject to the variation in the macro-financial environment and the stress condition in the global financial market. There is no convincing evidence to support the effectiveness of policy rate cut to boots bank lending and to avoid a financial accelerator effect
Characterising the Apertif primary beam response
Context. Phased array feeds (PAFs) are multi-element receivers in the focal plane of a telescope that make it possible to simultaneously form multiple beams on the sky by combining the complex gains of the individual antenna elements. Recently, the Westerbork Synthesis Radio Telescope (WSRT) was upgraded with PAF receivers to carry out several observing programs, including two imaging surveys and a time-domain survey. The Apertif imaging surveys use a configuration of 40 partially overlapping compound beams (CBs) simultaneously formed on the sky and arranged in an approximately rectangular shape. Aims. This work is aimed at characterising the response of the 40 Apertif CBs to create frequency-resolved I, XX, and YY polarization empirical beam shapes. The measured CB maps can be used for the image deconvolution, primary beam correction, and mosaicking processes of Apertif imaging data. Methods. We used drift scan measurements to measure the response of each of the 40 Apertif CBs. We derived beam maps for all individual beams in I, XX, and YY polarisation in 10 or 18 frequency bins over the same bandwidth as the Apertif imaging surveys. We sampled the main lobe of the beams and the side lobes up to a radius of 0.6 degrees from the beam centres. In addition, we derived beam maps for each individual WSRT dish. Results. We present the frequency and time dependence of the beam shapes and sizes. We compared the compound beam shapes derived with the drift scan method to beam shapes derived with an independent method using a Gaussian Process Regression comparison between the Apertif continuum images and the NRAO VLA Sky Survey (NVSS) catalogue. We find a good agreement between the beam shapes derived with the two independent methods
Ownership competence
Ownership is fundamental to firm strategy, organization, and governance. Standard ownership conceptsâmainly derived from agency and incomplete contracting theoriesâfocus on its incentive effects. However, these concepts and theories neglect ownership's role as an instrument to match judgment about resource use and governance with the firm's evolving environment under uncertainty. We develop the concept of ownership competenceâthe skill with which ownership is used as an instrument to create valueâand decompose it into matching competence (what to own), governance competence (how to own), and timing competence (when to own). We describe how property rights of use, appropriation, and transfer relate to the three ownership competences and show how our theory offers a fresh perspective into the role of ownership for value generation
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