Publications (77)91.27 Total impact

Conference Paper: Efficient Wideband Gain Modeling for Interferometric Imaging Arrays in Radio Astronomy
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ABSTRACT: Current and future interferometric imaging arrays in radio astronomy may be limited by the accuracy with which various directiondependent 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 baselinedependent. A newly proposed method, called Astacking, offers a solution through the use of a parametrized model in which basis functions account for the directiondependence and the expansion coefficients account for the baselinedependence. This approach provides a simple tradeoff between computational complexity and calculation accuracy. Herein the use of the Astacking method to model directiondependent effects over a very wide frequency band is investigated.The 2015 IEEE APS 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 directiondependent effects that are also baselinedependent, 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 transformbased implementation of the forward calculation (sky to visibility) in a manner similar to the Wstacking solution for noncoplanar baselines, and offers a potential improvement in computational efficiency in scenarios where the gridding operation in a convolutionbased approach to directiondependent 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:10.1051/00046361/201425492 · 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 noncoplanar 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 multispacecraft interferometers, widefield imaging with noncoplanar 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 radiobright compact objects embedded in a diffuse freefree absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by highresolution images at low frequencies where the effects of freefree 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 lowfrequency spectra of compact and diffuse emission in the central starburst region of M82 via highresolution radio imaging at low frequencies. The international LOFAR telescope was used to observe M82 at 110126MHz and 146162MHz. 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) baselines. 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 edgeon starforming disk.Astronomy and Astrophysics 11/2014; 574. DOI:10.1051/00046361/201425089 · 4.38 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The dualpolarized 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 fieldsofview (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 squareroot 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 multibeaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full International LOFAR array. Sources were preselected on the basis of 325 MHz arcminutescale 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 lowfrequency 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 nondispersive 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. 


Conference Paper: Towards a uniform evaluation of the science quality of SKA technology options: Polarimetrie aspects
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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 axisymmetric primefocus and offset Gregorian reflector systems; and the two feeds are the Elevenfeed (wideband) and a choked horn (octave band). To evaluate the imaging performance we employ endtoend 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 figuresofmerit 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, endtoend simulations, such as the one presented here, is that they address the highlevel science output from SKA as a whole rather than lowlevel 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 highprecision 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 crosspolarization ratio, is not suitable as a polarimetry FoM unless it is given in an appropriate coordinate system. This is because although the crosspolarization ratio is relevant for raw, uncalibrated polarimetry, in general it is not relevant to the quality of the polarimetry after polarimetric calibration. However, a crosspolarization 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 crosspolarization ratio the intrinsic crosspolarization 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(659):2058  2065. DOI:10.1109/TAP.2011.2123862 · 2.18 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recently, a serial implementation of the onebit auto and crosscorrelation 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):297307. 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 Intercosmos19 satellite to study a Travelling Ionospheric Disturbance (TID) that occurred in response to LargeScale 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. Intercosmos19 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 postmidnight local times. Flayer lifting reached similar to 200 km, Ne 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 SolarTerrestrial Physics 04/2011; 73(56):567577. DOI:10.1016/j.jastp.2010.11.014 · 1.47 Impact Factor 
Conference Paper: Reconfigurable FPGABased Unit for Singular Value Decomposition of Large m x n Matrices.
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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 FPGAbased hardware architecture for SVD computation on large m × n matrices utilizing Hestenes approach and oneside 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 FPGAbased 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 largescale 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 highlatitude convection maps, data from 4 DMSP satellites and networks of groundbased magnetometers. The propagation of the LS AGWs caused quasiperiodic 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 largescale 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 quasiperiodic 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 groundbased sounding for studies of weak quasiperiodic variations in the dayside subauroral ionosphere related to LS AGW.Journal of Atmospheric and SolarTerrestrial Physics 06/2010; 72(91072):653661. 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 fieldsensing 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 OAMgenerating 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 autocorrelation of electron counts received by the PEACE electron experiment via an interexperiment link.12/2009: pages 7382; 
Conference Paper: Polarization Diversity for mboxSKA Widefield Polarimetry
Widefield Science and Technology for the mboxSKA  mboxSKADS Conference 2009; 11/2009 
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ABSTRACT: We derive a generalised van CittertZernike (vCZ) theorem for radio astronomy that is valid for partially polarized sources over an arbitrarily wide fieldofview (FoV). The classical vCZ theorem is the theoretical foundation of radio astronomical interferometry, and its application is the basis of interferometric imaging. Existing generalised vCZ 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 twodimensional electric field (Jones vector) formalism of the standard "Measurement Equation" of radio astronomical interferometry to the full threedimensional formalism developed in optical coherence theory. The resulting vCZ theorem enables allsky imaging in a single telescope pointing, and imaging using not only standard dualpolarized interferometers (that measure 2D electric fields), but also electric tripoles and electromagnetic vectorsensor interferometers. We show that the standard 2D Measurement Equation is easily obtained from our formalism in the case of dualpolarized antenna element interferometers. We find, however, that such dualpolarized 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 directiondependent effects will be important. Comment: To be published in MNRASMonthly Notices of the Royal Astronomical Society 11/2008; 395(3). DOI:10.1111/j.13652966.2009.14642.x · 5.11 Impact Factor
Publication Stats
818  Citations  
91.27  Total Impact Points  
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Institutions

20112015

Chalmers University of Technology
 • Department of Signals and Systems
 • Department of Earth and Space Sciences
Goeteborg, Västra Götaland, Sweden


20072011

University of Glasgow
 School of Physics and Astronomy
Glasgow, Scotland, United Kingdom


20042011

University of Sussex
 Biomedical Engineering Group
Brighton, England, United Kingdom


20002004

Swedish Institute of Space Physics
Kiruna, Norrbotten, Sweden
