[Show abstract][Hide abstract] ABSTRACT: Current and future interferometric imaging arrays
in radio astronomy may be limited by the accuracy with which
various direction-dependent effects, such as the antenna gain
patterns, can be corrected. Doing this in an efficient manner
during the calibration and imaging process is especially difficult
due to these effects often being also baseline-dependent. A newly
proposed method, called A-stacking, offers a solution through the
use of a parametrized model in which basis functions account for
the direction-dependence and the expansion coefficients account
for the baseline-dependence. This approach provides a simple
trade-off between computational complexity and calculation
accuracy. Herein the use of the A-stacking method to model
direction-dependent effects over a very wide frequency band is
The 2015 IEEE AP-S Symposium on Antennas and Propagation and URSI CNC/USNC Joint Meeting, Vancouver; 07/2015
[Show abstract][Hide abstract] ABSTRACT: A general framework is presented for modeling direction-dependent effects that are also baseline-dependent, as part of the calibration and imaging process. Within this framework such effects are represented as a parametric linear model in which basis functions account for direction dependence, whereas expansion coefficients account for the baseline dependence. This separation enables the use ofa multiple fast Fourier transform-based implementation of the forward calculation (sky to visibility) in a manner similar to the W-stacking solution for non-coplanar baselines, and offers a potential improvement in computational efficiency in scenarios where the gridding operation in a convolution-based approach to direction-dependent effects may be too costly. Two novel imaging approaches that are possible within this framework are also presented.
Astronomy and Astrophysics 05/2015; 577. DOI:10.1051/0004-6361/201425492 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: I present an exact and explicit solution to the scalar (Stokes flux
intensity) radio interferometer imaging equation on a spherical surface which
is valid also for non-coplanar interferometer configurations. This imaging
equation is comparable to $w$-term imaging algorithms, but by using a spherical
rather than a Cartesian formulation this term has no special significance. The
solution presented also allows direct identification of the scalar (spin 0
weighted) spherical harmonics on the sky. The method should be of interest for
future multi-spacecraft interferometers, wide-field imaging with non-coplanar
arrays, and CMB spherical harmonic measurements using interferometers.
Monthly Notices of the Royal Astronomical Society Letters 04/2015; 451(1). DOI:10.1093/mnrasl/slv052 · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nuclear starburst in the nearby galaxy M82 provides an excellent
laboratory for understanding the physics of star formation. This galaxy has
been extensively observed in the past, revealing tens of radio-bright compact
objects embedded in a diffuse free-free absorbing medium. Our understanding of
the structure and physics of this medium in M82 can be greatly improved by
high-resolution images at low frequencies where the effects of free-free
absorption are most prominent.
The aims of this study are, firstly, to demonstrate imaging using
international baselines of the Low Frequency Array (LOFAR), and secondly, to
constrain low-frequency spectra of compact and diffuse emission in the central
starburst region of M82 via high-resolution radio imaging at low frequencies.
The international LOFAR telescope was used to observe M82 at 110-126MHz and
146-162MHz. Images were obtained using standard techniques from very long
baseline interferometry. images were obtained at each frequency range: one only
using international baselines, and one only using the longest Dutch (remote)
The 154MHz image obtained using international baselines is a new imaging
record in terms of combined image resolution (0.3$"$) and sensitivity
($\sigma$=0.15mJy/beam) at low frequencies ($<327$MHz). We detected 16 objects
at 154MHz, six of these also at 118MHz. Four weaker but resolved features are
also found: a linear (50pc) filament and three other resolved objects, of which
two show a clear shell structure. We do not detect any emission from either
supernova 2008iz or from the radio transient source 43.78+59.3. The images
obtained using remote baselines show diffuse emission, associated with the
outflow in M82, with reduced brightness in the region of the edge-on
Astronomy and Astrophysics 11/2014; 574. DOI:10.1051/0004-6361/201425089 · 4.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The dual-polarized primary beams of imaging radio telescopes are generally not perfectly orthogonal nor have the same gain, the resulting polarimetric images are distorted, in other words, they exhibit instrumental polarization. This is often modeled in radio astronomy using a Jones matrix formalism, and the standard practice is to calibrate (i.e. determine the Jones matrices) using known point source, polarized calibrators. Using point source calibrators on the other hand can be difficult and is ineffective for wide fields-of-view (FoV). Often however, a large portion of the FoV of imaging radio telescopes consists of unpolarized background. In this paper I estimate Jones matrices over the FoV by assuming that most of the background image is unpolarized and then taking the square-root of the brightness matrices. Results from LOFAR LBA data are shown and are consistent with expectation. The usefulness of this particular method, compared to using polarized point source calibrators, is its simplicity and the fact that it can cover most of the FoV.
Astronomy and Computing 11/2014; DOI:10.1016/j.ascom.2014.11.002
[Show abstract][Hide abstract] ABSTRACT: Aims. An efficient means of locating calibrator sources for International
LOFAR is developed and used to determine the average density of usable
calibrator sources on the sky for subarcsecond observations at 140 MHz.
Methods. We used the multi-beaming capability of LOFAR to conduct a fast and
computationally inexpensive survey with the full International LOFAR array.
Sources were pre-selected on the basis of 325 MHz arcminute-scale flux density
using existing catalogues. By observing 30 different sources in each of the 12
sets of pointings per hour, we were able to inspect 630 sources in two hours to
determine if they possess a sufficiently bright compact component to be usable
as LOFAR delay calibrators. Results. Over 40% of the observed sources are
detected on multiple baselines between international stations and 86 are
classified as satisfactory calibrators. We show that a flat low-frequency
spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz.
We extrapolate from our sample to show that the density of calibrators on the
sky that are sufficiently bright to calibrate dispersive and non-dispersive
delays for the International LOFAR using existing methods is 1.0 per square
degree. Conclusions. The observed density of satisfactory delay calibrator
sources means that observations with International LOFAR should be possible at
virtually any point in the sky, provided that a fast and efficient search using
the methodology described here is conducted prior to the observation to
identify the best calibrator.
[Show abstract][Hide abstract] ABSTRACT: We discuss how to evaluate SKA technology options with regard to science output quality. In this work we will focus on polarimetry. We review the SKA specification for polarimetry and assess these requirements. In particular we will use as a illustrative case study a comparison of two dish types combined with two different feeds. The dish types we consider are optimized axi-symmetric prime-focus and offset Gregorian reflector systems; and the two feeds are the Eleven-feed (wideband) and a choked horn (octave band). To evaluate the imaging performance we employ end-to-end simulations in which given sky models are, in software, passed through a model of the telescope design according to its corresponding radio interferometrical measurement equation to produce simulated visibilities. The simulated visibilities are then used to generate simulated sky images. These simulated sky images are then compared to the input sky models and various figures-of-merit for the imaging performance are computed. A difficulty is the vast parameter space for observing modes and configurations that exists even when the technology is fixed. However one can fixed certain standard benchmark observation modes that can be applied across the board to the various technology options. The importance of standardized, end-to-end simulations, such as the one presented here, is that they address the high-level science output from SKA as a whole rather than low-level specifications of its individual parts.
Electromagnetics in Advanced Applications (ICEAA), 2012 International Conference on; 01/2012
[Show abstract][Hide abstract] ABSTRACT: Many modern radio applications, such as astronomy and remote sensing, require high-precision polarimetry. These applications put exacting demands on radio polarimeters (antenna systems that can measure the state of polarization of radio sources), and in order to assess their polarimetric performance, a figure of merit (FoM) would be desirable. Unfortunately, we find that the parameter commonly used for this purpose, the cross-polarization ratio, is not suitable as a polarimetry FoM unless it is given in an appropriate coordinate system. This is because although the cross-polarization ratio is relevant for raw, uncalibrated polarimetry, in general it is not relevant to the quality of the polarimetry after polarimetric calibration. However, a cross-polarization ratio can be constructed from invariants of the Jones matrix (the matrix that describes the polarimetric response of a polarimeter) that quantifies polarimetric performance even after calibration. We call this cross-polarization ratio the intrinsic cross-polarization ratio (IXR) and conclude that it is a fundamental FoM for polarimeters. We then extend the IXR concept from the Jones calculus to the Mueller calculus and also to interferometers, and we give numerical examples of these parameters applied to the Parkes radio telescope, the Westerbork synthesis radio telescope, and the Effelsberg telescope.
IEEE Transactions on Antennas and Propagation 07/2011; 59(6-59):2058 - 2065. DOI:10.1109/TAP.2011.2123862 · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, a serial implementation of the one-bit auto- and cross-correlation functions (ACF and CCF respectively) in a field programmable gate array (FPGA) has been developed, based on asynchronous delay elements and counters, known as the counterbased correlation. This paper proposes a method of parallelizing this otherwise serial process, offering significant improvements in the applicability of this approach to more types of ACF. Furthermore, the possibility of obtaining lag results from a parallel data sequence without first shifting the entire sequence has been realized, hence decreasing the number of clock cycles necessary for the calculation of the ACF. A synchronous design was preferred here for reasons of stability and portability, the technology of choice again being an FPGA. The advantages offered by the counterbased implementation in terms of device area usage and speed still apply. A practical implementation in the instrumentation of an upcoming space mission is also discussed.
Microprocessors and Microsystems 05/2011; 35(3):297-307. DOI:10.1016/j.micpro.2011.01.001 · 0.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We used bottomside ground observations and topside sounding data from the Intercosmos-19 satellite to study a Travelling Ionospheric Disturbance (TID) that occurred in response to Large-Scale Internal Gravity Wave (LSIGW) propagation during a substorm on November 30, 1979. We built a global scheme for the wavelike ionospheric variations during this medium substorm (AE(max) similar to 800 nT). The area where the TID was observed looks like a wedge since it covers the nighttime hours at subauroral latitudes but contracts to a similar to 02 h local sector at low latitudes. The ionospheric response is strongly asymmetric because the wedge area and the TID amplitude are larger in the winter hemisphere than in the summer hemisphere. Clear evidence was obtained indicating that the more powerful TID from the Northern (winter) hemisphere propagated across the equator into the low latitude Southern (summer) hemisphere. Intercosmos-19 observations show that the disturbance covers the entire thickness of the topside ionosphere, from h(m)F2 up to at least the 1000 km satellite altitude at post-midnight local times. F-layer lifting reached similar to 200 km, N-e increases in the topside ionosphere by up to a factor of similar to 1.9 and variations in N(m)F2 of both signs were observed. Assumptions are made concerning the reason for the IGW effect at high altitudes in the topside ionosphere. The relationship between TID parameters and source characteristics determined from a global network of magnetometers are studied. The role of the dayside cusp in the generation of the TID in the daytime ionosphere is discussed. The magnetospheric electric field effects are distinguished from IGW effects.
Journal of Atmospheric and Solar-Terrestrial Physics 04/2011; 73(5-6):567-577. DOI:10.1016/j.jastp.2010.11.014 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Singular value decomposition (SVD) allows the factorization of real or complex matrices providing quantitative information with fewer dimensions along which data points exhibit more variation. These days SVD computation is being used in numerous applications, and because of its importance, different approaches for SVD hardware computation have been proposed, however, their application is limited by the inherent SVD calculation complexity making it possible to analyze up to 8 x 8 matrices until now, complying certain constrains like symmetry. This paper presents a generic and novel FPGA-based hardware architecture for SVD computation on large m × n matrices utilizing Hestenes approach and one-side Jacobi rotations. Four different study cases (2 x 2, 8 x 7, 16 x 32, and 32 x 127 matrices) validate the performance of the FPGA-based computation unit reaching a maximum estimation error of 3.3718 % in the SVD estimation of a large matrix.
2011 International Conference on Reconfigurable Computing and FPGAs, ReConFig 2011, Cancun, Mexico, November 30 - December 2, 2011; 01/2011
[Show abstract][Hide abstract] ABSTRACT: Novel measurement and approximation methodologies for studying orbital angular momentum (OAM) modes in radio beams, i.e., electromagnetic beam modes having helical phase fronts, are presented. We show that OAM modes can be unambiguously determined by measuring two electric field components at one point, or one electric field component at two points.
Radio Science 08/2010; 45(4). DOI:10.1029/2009RS004299 · 1.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Variations in the dayside ionosphere parameters as a result of a large-scale acoustic gravity wave (LS AGW) were studied for the 17 February 1998 substorm using the super dual auroral radar network (SuperDARN) measurements. This event was characterised by a sharp rise in the AE index with a maximum of ∼900 nT. The source of the disturbance responsible for the LS AGW appears to have been located within the plasma convection throat and in the dayside cusp region. The location of the source was obtained from studies of a number of datasets including high-latitude convection maps, data from 4 DMSP satellites and networks of ground-based magnetometers. The propagation of the LS AGWs caused quasi-periodic variations in the skip distance (with an amplitude up to 220–260 km) of the ground backscatter measured by up to 6 SuperDARN radars, including Goose Bay and Kapuskasing, resulting in two large-scale travelling ionospheric disturbances (LS TIDs). The LS TIDs had wave periods of 1.5 and 2 h, a velocity of ∼400 m/s for both, and wavelengths of 2200 and 2900 km, respectively. These quasi-periodic variations were also present in the peak electron density and height of the F2 layer measured by the Goose Bay ionosonde. The numerical simulation of the inverse problem show good agreement between Goose Bay radar and Goose Bay ionosonde measurements. But these LS TIDs would be difficult to deduce from the ground based ionospheric station data alone, because hmF2 variations were 10–40 km only and fOF2 variations between 10% and 20%. The results demonstrate how important SuperDARN radars can be, and that this is a more powerful technique than routine ground-based sounding for studies of weak quasi-periodic variations in the dayside subauroral ionosphere related to LS AGW.
Journal of Atmospheric and Solar-Terrestrial Physics 06/2010; 72(9-10-72):653-661. DOI:10.1016/j.jastp.2010.02.018 · 1.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent discoveries concerning rotating (helical) phase fronts and orbital angular momentum (OAM) of laser beams are applied to radio frequencies and comprehensive simulations of a radio OAM system are performed. We find that with the use of vector field-sensing electric and magnetic triaxial antennas, it is possible to unambiguously estimate the OAM in radio beams by local measurements at a single point, assuming ideal (noiseless) conditions and that the beam axis is known. Furthermore, we show that conventional antenna pattern optimization methods can be applied to OAM-generating circular arrays to enhance their directivity.
IEEE Transactions on Antennas and Propagation 03/2010; DOI:10.1109/TAP.2009.2037701 · 2.18 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Digital Wave Processor (DWP) is the central control and data processing instrument for the Cluster Wave Experiment Consortium.
DWP products in the Cluster Active Archive (CAA) provide a mainly supporting function for the rest of the consortium. This
includes a time correction dataset which allows the standard timing accuracy of 2 ms to be improved to around 20 μs, and experiment
command and status datasets which show what commands have been sent to the experiments, and the resulting status. DWP also
contains a particle correlator experiment that computes the auto-correlation of electron counts received by the PEACE electron
experiment via an inter-experiment link.
[Show abstract][Hide abstract] ABSTRACT: We derive a generalised van Cittert-Zernike (vC-Z) theorem for radio astronomy that is valid for partially polarized sources over an arbitrarily wide field-of-view (FoV). The classical vC-Z theorem is the theoretical foundation of radio astronomical interferometry, and its application is the basis of interferometric imaging. Existing generalised vC-Z theorems in radio astronomy assume, however, either paraxiality (narrow FoV) or scalar (unpolarized) sources. Our theorem uses neither of these assumptions, which are seldom fulfilled in practice in radio astronomy, and treats the full electromagnetic field. To handle wide, partially polarized fields, we extend the two-dimensional electric field (Jones vector) formalism of the standard "Measurement Equation" of radio astronomical interferometry to the full three-dimensional formalism developed in optical coherence theory. The resulting vC-Z theorem enables all-sky imaging in a single telescope pointing, and imaging using not only standard dual-polarized interferometers (that measure 2-D electric fields), but also electric tripoles and electromagnetic vector-sensor interferometers. We show that the standard 2-D Measurement Equation is easily obtained from our formalism in the case of dual-polarized antenna element interferometers. We find, however, that such dual-polarized interferometers can have polarimetric aberrations at the edges of the FoV that are often correctable. Our theorem is particularly relevant to proposed and recently developed wide FoV interferometers such as LOFAR and SKA, for which direction-dependent effects will be important. Comment: To be published in MNRAS
Monthly Notices of the Royal Astronomical Society 11/2008; 395(3). DOI:10.1111/j.1365-2966.2009.14642.x · 5.11 Impact Factor