Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.mrao.cam.ac.uk/projects/aavp/Abstracts.pdf Дата изменения: Tue Dec 7 02:35:44 2010 Дата индексирования: Tue Oct 2 04:00:42 2012 Кодировка: ISO8859-5 Поисковые слова: mercury program

Available Abstracts
6 December 2010

Wednesday 8 December
Steve Rawlings Alan Penny Oxford St Andrews SKA1 Science Requirements SETI searches with SKA Building on experience of using LOFAR for SETI searches, this talk will discuss the possibilities for SETI with SKA, and describe the design needs of SETI for an aperture array SKA. The wide frequency range and the narrow-frequency channels have hardware and software implications. The possibilities and demands for continuous all-sky searches will be covered. Jean-Pierre Macquart ICRAR Transients Science at Low Frequencies The wide field of view and/or multi-beaming capabilities of low frequency aperture arrays offer new opportunities to study the transient and variable radio sky. We will present the lessons learned from the recent implementation of a fast transients detector at the VLBA, V-FASTR and discuss how several novel aspects of this system port to applications at long wavelengths. We will also report on recent results and progress from the MWA 32 tile prototype, which has a transients science program in both the fast and slow domains. The small-D large-N design of the MWA, with steerable phasedarray antennas, make it an ideal transient science instrument. We also discuss several aspects of the physics of transients as they pertain to low frequency radio astronomy.


Tobia Carozzi

Onsala

Does the SKA really need to achieve its high polarimetric purity (and sensitivity) spec? As the SKA goes into its design verification phase, it is important to reconsider the instrument specifications and ask whether they can or need to be fulfilled. For the SKA AA, the specification for the polarization purity is a modest -20 dB. This may seem far from the requirements of SKA science drivers, which call for a polarimetric purity of between -30 to -40 dB, but this is because the specification is given in terms of raw polarization purity, that is, the cross-polarization prior to (polarimetric) calibration. This improvement is possible since one can expect that if the SKA AA polarimetric response (expressed as Jones or Mueller-like matrices) is known precisely, then any cross-polarization could be removed. But to what extent is this possible? Could SKA science be fulfilled at even lower degrees of raw polarization purity given that off-line calibration will often need to be done anyways. In this work we show that the raw cross-polarization ratio (i.e. the crosspolarization in a default coordinate system) has in fact little relevance to the final calibrated polarimetry. What does have direct relevance is the cross polarization with respect to the eigen-polarization modes of the antennas, which we call the intrinsic cross-polarization ratio IXR of the telescope. Crucially, IXR is found to be directly related to the total relative error in the Stokes parameters, and this in turn implies that polarization purity is inseparable from the overall telescope sensitivity. We therefore conclude that the polarization purity required by the SKA science drivers can be fulfilled even with a much lower cross-polarization ratio (given as IXR) than what is specified now, or alternatively, that the SKA sensitivity can be improved by improving the cross-polarization of the antennas.

Jan Noordam Parisa Noorishad

ASTRON Kapteyn Astronomical Institute

AA verification is essentially an EJones problem Efficient calibration method for phased arrays We aimed to have an efficient and computationally cheap calibration method for phased array stations. This is of importance and concern in new generation of radio telescopes. Redundancy calibration which is a linear one iteration method, seems well suited to this purpose. It requires some design considerations which will be pointed out in this talk. To understand its potentials and limitations and to do a proper error analysis, we use a general data model which has recently been introduced in the literature. We will present some results using HBA real and simulated data.

Stefan Wijnholds

ASTRON

A Model Based Approach to LOFAR Calibration and Imaging In this presentation, I show the strengths of a model based approach and its associated signal processing techniques in handling calibration and imaging problems. This will be illustrated by some examples based on actual LOFAR data. I will also demonstrate that such an approach becomes feasible in the LOFAR and SKA era, especially for processing of data from their respective cores. This has the important implication that advanced signal processing techniques can be used to tackle the challenging problems associated with dealing with time, frequency and direction dependent effects.


Thursday 9 December
Jaap D. Bregman ASTRON Performance aspects of sparse, dense, regular and irregular array configurations SKA1 will have 2 segments, dishes working above 0.3 GHz and optimum performance for 0.5 - 3.0 GHz and aperture arrays that cover 70 - 450 MHz. Although some overlap in frequency range is implied, the optimum performance for the aperture arrays as defined by the intended science applications concentrates at the edges of the band. This conflicts with performance of most practical systems that have their best performance in the centre of the band. EMBRACE and APERTIF array development has indicated that further receiver development could realize system temperature of less than current 45 K above the sky noise for properly matched antennas in an array. However, there are fundamental limitations in the effective area of an array depending on its wavelength and its element pitch. Especially the behaviour of the "effective characteristics" of an element depends on its environment being sparsely or densely populated on a regular or on an irregular grid. These aspects together have serious impact on the configuration of a station array and on the choice of an antenna element suitable for array application not only from electromagnetic but also from mechanical aspect. This is especially true when we want to maximize the effective area of a station array over a certain wavelength range within a given budget. We show how these aspects have been balanced in the design of LOFAR and we show some possible options for SKA including the number of station beams that are needed to provide a sensible total Field of View and a beam pattern that allows to reach the theoretical thermal noise in a synthesis image over the main beam of a phased array station. Nima Razavi-Ghods Cavendish Analysis of low frequency phased array The proposed SKA telescope will rely heavily on aperture phased arrays in the sub 1 GHz frequency band. This will likely consist of a mid-band frequency array (AA-mid) system working from 400 MHz to 1.4 GHz as well as a low frequency array (AA-lo) operating nominally from 70 MHz to 450 MHz. For both regimes, several different antenna array configurations are being studied for inclusion within the SKA programme. It is desirable for the SKA AA-lo to operate using a single collector technology and thus a single antenna array geometry. The question of the array geometry is therefore a critical one, since this, along with the antenna weighting scheme, defines the sidelobe level and profile, all of which contribute to the antenna temperature and sensitivity of the array. In this paper we focus on analysing the real sky contribution to system temperature as well as sensitivity for various low frequency antenna array configurations composed of 10,000 elements. Our simulations are carried out by assuming observations of three cold regions above and below the galactic plane over a period of 4.5 hours to evaluate the effects of side-lobes and grating lobes passing over very bright regions. We evaluate several regular as well as irregular type antenna configurations each with different minimum inter-element separations ranging from 0.5ЮЛ to 2ЮЛ. We assume uniform excitation of antennas as well as Taylor and Dolph -Chebyshev weighting. Furthermore, we study two different gain elements to determine their effect on the sensitivity of these low frequency phased arrays.


Jader Monari

IRA-INAF

Optimisation of the AA-low system A block diagram of the Italian concept of AAlo, will be presented. Some crucial block as the antenna and LNA, the receiver, analogue optical links and digitization processes, are optimized to have both best ana logue and digital dynamic range, bandwidth and the sampling rate suitable to operate on the whole 70-450 MHz band. At the sub-station level, calibration, RFIs mitigation, squeezing of the digital word length and digital beamformer, will carefully handled from many points of view as costs, reliability, repeatability, performances and the energy requirements related with the green energy.

Jonathan Pritchard

HI Science through cosmic time HarvardSmithsonian Hydrogen offers a unique tracer of the key events tha t occur as the Universe Center for Astrophysics evolves, since it is ubiquitous throughout the Universe. Low frequency radio observations of the redshifted 21 cm line of neutral hydrogen will be key in developing our understanding of the first billion years of cosmic evolution, when the first stars and galaxies formed. This period lies beyond the current observational frontier presenting challenges to theory and observation. In this talk, I will describe the physics that governs the evolution of the 21 cm signal with a focus o n those aspects that SKA and its pathfinders might hope to observe. I will discuss what can be learnt about the first galaxies and reionization and explore some of the challenges and opportunities ahead for 21 cm experiments. Probing the very first galaxies with the SKA In this talk I will describe the use of the high redshift 21cm signal to probe the very first galaxies to appear in the Universe. Using fast large volume simulations of the pre-Reionization epoch we have shown that the Lyman alpha radiation emitted from these young galaxies make a strong contribution to the 21cm signal on large scales at z~20. With the current setup, SKA should be able to measure this signal, therefore making it probably the only telescope capable of giving us detailed information about the radiation emitted by the first stars and characterizing their host galaxies. I then discuss the use of the redshift space distortions as a way to further constrain the Lyman alpha signal and demonstrate that they can be used as a model independent way to extract this signature with the SKA.

Mario Santos

IST

Ger de Bruyn

ASTRON

We* will present the (very broad) science case and the technical performance demonstration capability of a compactish AAVP for the midfrequency range (300-1200 MHz). Among the science applications are: the study of baryonic oscillations, low surface brightness local HI imaging, HI intensity mapping, HI absorption, Galactic foreground polarimetry and searches for (ms-)pulsars and transients. Note that this presentation could be split in several smaller presentations.

Leon Koopmans

Kapteyn

The Epoch of Reionization and Phase 1


Matt Jarvis

Herfordshire

Probing the epoch of reionization with multi-wavelength data sets One of the key aims for aperture arrays will be to detect the 21cm transition of hydrogen within the epoch of reionization. However, more can be gained from such surveys if they are combined with data at other wavelengths that can detect directly the stars, galaxies and AGN which may be responsible for reionizing the Universe. I will present current and future plans to measu re the galaxy and AGN density within the EoR with near-infrared surveys.

Joe Lazio Saswata Bhaumik

JPL Manchester

Magnetospheric Emissions from Extrasolar Planets. Low Noise Amplifier for low frequency SKA The paper shows design work of Low Noise Amplifiers (LNA) suitable for low frequency SKA of Aperture Array Verification Program. Two hybrid LNAs based on commercially available passive and active components have been developed. They are 2-stage differential LNAs with differential input and single ended output. LNA1 is operational from 50MHz to 400MHz. The gain of LNA1 is above 25dB in the operational band. Noise temperature of the same is below 35Kelvin between 100MHz and 450MHz with 20Kelvin at 330MHz. The transistors used are enhancement mode pHEMTs of Avago Technology with 4V drain bias (at power supply unit) and 0.5V gate bias voltages. The drain currents are 21mA and 100mA in stages 1 and 2 respectively. The LNA has P1dBIN compression point of -23dBm at 0.4GHz. LNA2 has over 36.5dB gain from 70MHz to 450MHz. The noise temperature is below 35Kelvin in the band. The P1dBIN is -24dBm at 400MHz with 3V and 0.5V drain and gate bias voltages respectively. The transistors used for LNA2 are enhancement mode pHEMTs of Avago Technology with drain current for each stage is 30mA.

Shantanu Padhi & Aziz Jiwani

ICRAR

Dual-Polarized Conical Log-Spiral Antenna for SKA (AA-LO) Program The conical logarithmic spiral antenna belongs to the class of wideband and frequency-independent antenna category and exhibits consistent beamwidth over the frequency band, linear impedance behaviour, low cross -polarization and relative high gain. The benign beam forming patterns with relative high gain makes an ideal candidate for SKA (AA-LO) program. Conical log-spiral antenna has been used in radio astronomy primarily in low frequency range. This paper describes the design and development of compact dual polarized conical spiral antenna (contra-wound). A scale model (350-2250 MHz) is used for our prototype development. Numerical and experimental results will be presented. The sensitivity analysis (A/T) of conical spiral antenna in array configuration (4 x4) will also be presented.

Stephane Gauffre

Bordeaux

Fast Analog to Digital Converter developments at LAB in the AAVP context The Laboratoire d'Astrophysique de Bordeaux (LAB) has developed fast ADCs for more than ten years, mostly for radio receivers for sub -mm telescopes. The technical specifications for these ADCs are generally large bandwidth, high speed (few GS/s) and low resolution (few bits). More recently the use of digital electronics in radio receivers has brought more flexibility and capability, which implies challenging ADC design with more bits and less power consumption. This tends to meet the requirements for the SKA project (a few GS/s with 6/8 bits). The presentation will review the past and ongoing developments for fast ADCs designed at LAB, present the current trends for sub-mm observatories and sketch plans for future realizations, especially in the AAVP framework.


Sascha Schediwy

University of Oxford

The D-PAD Sparse Aperture Array In this talk I report on recent developments on the D -PAD aperture array. This instrument replicates many aspects of an SKA aperture array, yet it is small enough to facilitate dynamic reconfigurations of both software and hardware. This flexibility enables many of the key SKA system design parameters to be experimentally investigated. In combination with simulation tools like OSKAR, D-PAD will be able to deliver critical information which cannot be learned from larger, established instruments like LOFAR. Some crucial features of D-PAD include low noise-temperature receivers, wide processed signal bandwidth, fast analogue-to-digital converters for direct signal sampling, and a frequency range which is optimised for working in high radio power environments. The array consists of beamformed sparse antennas, correlated array stations, and a flexible and powerful digital processing engine. These features combine to enable investigations into the effects of grating lobes on imaging dynamic range, novel grating lobe mitigation techniques, innovative direct imaging corrector algorithms, millisecond transient searches, broadband dynamic solar radio spectra, a survey of bright continuum sources, and a neutral atomic hydrogen survey.

David Zhang

University of Manchester

Dense Aperture Array Antenna Design for SKA The latest design of high performance compact aperture array antenna s for SKA AA-hi frequency band will be introduced. The newly invented antenna structure based on a low cost 2-D planar structure will be presented. The proposed new design demostrates an unique performance to meet all aspects of the SKA specifications with its low loss differential or single ended feeding interfaces for LNA. The open and flexible interface from the antenna allows easy integration with LNAs to ensure on-shelf availability. The advantages of the new design compared with the conventional taper ed slot antenna will be addressed.

Giuseppe Virone

IEIIT-CNR

Antenna Design Study for AAlo 70-450 MHz A proper antenna design study has been carried out according to the AAlo specifications. First, a theoretical limit for the maximum gain of the array element has been determined on the basis of the required Sky Coverage. Besides providing a basic guideline for the subsequent antenna designs, such a number can be useful to obtain an upper limit for the Aeff/Tsys for a passive array element. Several radiating architectures have been investigated also considering the cost parameter. The log periodic array exhibits both good matching and radiation characteristics in the overall frequency band. However, its dual polarization implementation is not cost effective when the low cross polarization requirement is considered. For this reason, a dual-polarized Vivaldi configuration has been developed. It provides a suitable matching level from 70 to 450 MHz (and more). Moreover, the cross polarization at zenith is zero owing to the symmetry of the antenna. This antenna does not require a ground plane. At 450 MHz, its directivity at 45ТА is larger than 1.8 dB in both the E - and H-planes. A preliminary Aeff/Tsys evaluation of some of the developed antenna designs will be discussed in a subsequent presentation/poster given by Federico Perini (IRA-INAF, Italy).


Federico Perini

INAF

Aeff/Tsys evaluation for AAlo sensors. The AAlo design needs an accurate evaluation of the Aeff/Tsys figure of merit which depends on the different proposed antenna sensors and their relative array configuration. This calculation is a critical issue to estimate a plausible number of the antennas in order to reduce costs and the complexity of the overall AAlo array and system. In order to correctly estimate the Tsys, a proper evaluation of the antenna temperature contribution (Ta), which is function both of the brightness temperature distribution and of the antenna pattern, is required. For this aim we exploit a proprietary code, already used to the SKADS/BEST-1 system with promising results, to some antenna sensor under investigation for AAlo. As a first positive consequence a useful and simple tool to calculate the brightness temperature vs frequency and zenithal angle, based on the guideline adopted by the Antenna Task Force of the SKA consortium, is now available and free to be distributed in the AAVP community.

Kris Zarb Adami

Oxford

FFT Array

Friday10 December
Peter Hall ICRAR AA-lo: A Fresh Look Within the current AAVP plan ICRAR leads a work package focused on exploratory technology for AA-lo (70-450 MHz). In early 2009 I published a plan for a single wide-band element investigation, incorporating activities designed to look at the applicability of relatively high-gain antennas, lowpower receiver and signal transport electronics, and self -powered galvanically-isolated station elements; see also the overview in the SKADS Limelette volume. This contribution will outline some of the early investigations at ICRAR, including simulation and measurement of conical log spiral antennas and their derivatives. While ICRAR is using the spiral as an exemplar of a true frequency-independent antenna having benign terminal and radiation properties, our principal aim is to shed light on to the usefulness of generic wide-band element solutions, bearing in mind the array packing constraints which flow from such a design. Equally importantly, we are using the spiral antenna as an exemplar in modelling the performance - cost trade-offs flowing from the use of high-gain elements. Our work is highly collaborative with AAVP partners and this contribution will outline some of the innovative lines of investigations grown from such collaboration.


Eloy de Lera Acedo

Cambridge

Array antenna designs for the SKA-AAlo Next-generation radio telescopes, such as the SKA, are currently being designed using aperture arrays. One particular science objective is to study the formation of the earliest galaxies which ionized the cold neutral hydrogen pervading much of the Universe at that time. This point in the evolution of the Universe is called the epoch of re-ionization, and it occurred at a sufficiently early time that the signals from the intrinsic 21-cm line of neutral hydrogen are "redshifted" into the frequency range from 70 MHz to several hundred MHz when observed today. One part of the SKA design therefore calls for an ultra-wideband antenna array able to operate from 70 to 450 MHz. This is called the SKA AA-lo. There are clear cost savings if this frequency range can be covered using a single antenna element design. Cambridge University has been focussing on the design of such ultra-wideband elements (bow-tie-like elements), to cover the SKA-AAlo band. They are very low-profile elements with the necessary performance to deliver the high sensitivity required by the SKA while keeping a simple and easily deployable element. In this talk I will present the latest designs so as the testing of several single element prototypes and a scaled array prototype. Simulation techniques for large finite arrays will be discussed as well.

Kris Zarb Adami For Stelio Montebugnoli

INAF

Reconfigurable back end for low frequency array. In the frame of the SKA Aperture Array Verification Program (SKA -AAVP), the back end of the italian concept of the Aperture Array (AAlo) subsystem (75 450 MHz) is planned to exploit both the velocity and modularity of the reconfigurable FPGA (Field Programmable Gate Array) devices. These chips represent the state of the art of fast, software-reconfigurable and modular data processing engines. The IRA-INAF AAlo concept will be tested on the field by using a CASPER group (University of Berkeley) cluster of Roach boards GOps/W) connected with a wide band 8 bit A/D converter (900 MS/s). An efficient data processing concept, including calibration, radio interference handling, multiple adaptive beamforming etc..., will be configured on the Roach board(s) cluster in order to allow the debugging, evaluation and tuning of the processing algorithms.