| 1. |
- van Haarlem, M. P., et al.
(författare)
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LOFAR : The LOw-Frequency ARray
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 556, s. 1-53
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Tidskriftsartikel (refereegranskat)abstract
- LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10–240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR’s new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.
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| 2. |
- Offringa, A. R., et al.
(författare)
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The LOFAR radio environment
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 549
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Tidskriftsartikel (refereegranskat)abstract
- Aims. This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations.Methods. We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30-78 MHz with low-band antennas and 115-163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz/1 s resolution.Results. We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR's nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution.Conclusions. Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills.
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| 3. |
- De Gasperin, F., et al.
(författare)
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Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15″ with a high dynamic range and good image fidelity. Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source. Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.
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| 4. |
- Noorishad, P., et al.
(författare)
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Redundancy calibration of phased-array stations
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 545, s. A108 (article no.)-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We assess the benefits and limitations of using the redundant visibility information in regular phased-array systems to improve the quality of the calibration. Methods. Regular arrays offer the possibility of using redundant visibility information to constrain the calibration of the array independently of a sky model and a beam model of the station elements. This requires a regular arrangement of the configuration of array elements and identical beam patterns. Results. We revised a previously developed calibration method for phased-array stations using the redundant visibility information in the system and applied it successfully to a LOFAR station. The performance and limitations of the method were demonstrated by comparing its application to real and simulated data. The main limitation is the mutual coupling between the station elements, which leads to non-identical beams and stronger baseline-dependent noise. Comparing the variance in the estimated complex gains with the Cramer-Rao Bound indicates that redundancy is a stable and optimum method for calibrating the complex gains of the system. Conclusions. Our study shows that the use of the redundant visibility does improve the quality of the calibration in phased-array systems. In addition, it provides a powerful tool obtaining system diagnostics. Our results demonstrate that designing redundancy in both the station layout and the array configuration of future aperture arrays is strongly recommended. This is particularly true in the case of the Square Kilometre Array (SKA) with its dynamic range requirement that surpasses any existing array by an order of magnitude.
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| 5. |
- BarboSá, D., et al.
(författare)
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A sustainable approach to large ICT science based infrastructures: The case for radio astronomy
- 2014
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Ingår i: 2014 IEEE International Energy Conference, ENERGYCON 2014; Dubrovnik; Croatia; 13 May 2014 through 16 May 2014. - 9781479924493 ; , s. 668-674
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Konferensbidrag (refereegranskat)abstract
- Large sensor-based infrastructures for radio astronomy will be among the most intensive data-driven projects in the world, facing very high power demands. The geographically wide distribution of these infrastructures and their associated processing High Performance Computing (HPC) facilities require Green Information and Communications Technologies (ICT): a combination is needed of low power computing, power and byte efficient data storage, local data services, Smart Grid power management, and inclusion of Renewable Energies. Here we outline the major characteristics and innovation approaches to address power efficiency and long-term power sustainability for radio astronomy projects, focusing on Green ICT for science.
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| 6. |
- Ivashina, Marianna, 1975, et al.
(författare)
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An axi-symmetric segmented composite SKA dish design: Performance and production analysis
- 2011
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Ingår i: Asia-Pacific Microwave Conference, APMC 2011; Melbourne, VIC; 5 - 8 December 2011. - 9780858259744 ; , s. 1182-1185
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Konferensbidrag (refereegranskat)abstract
- 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
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| 7. |
- Liao, Wan-Chun, 1985, et al.
(författare)
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Analysis of the strut and feed blockage effects in radio telescopes with compact UWB feeds
- 2012
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Ingår i: 6th European Conference on Antennas and Propagation, EuCAP 2012; Prague; Czech Republic; 26 March 2012 through 30 March 2012. - 9781457709180 ; , s. 611-615
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Konferensbidrag (refereegranskat)abstract
- The international radio astronomy community is currently pursuing the development of a giant radio telescope known as the Square Kilometre Array (SKA). The SKA reference design consists of several wideband antenna technologies, including reflector antennas fed with novel multi-beam Phased Array Feeds (PAF) and/or wide band Single Pixel Feeds (SPFs) that can operate at frequencies from 1 to 10 GHz [1], [2]. The baseline of this design represents an array of several hundred to a few thousand reflector antennas of 15-m diameter and that will realize sensitivity of 10,000 m 2/K. During the past years, several different reflector and feed concepts have been proposed and examined, but only a small number of these design options (that have a sufficient level of maturity) will be built and tested in a set-up that is closely resembling the final SKA system [3]. These tests are aimed to evaluate the overall system performance as well as construction and operational costs. The final choices for the dish and feed evaluation tests might include: (i) off-set Gregorian and axi-symmetric reflector antennas and; (ii) an optimized octave corrugated horn and the single-pixel wideband feeds such as quad-ridged horn and Eleven antenna [2], [4].
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| 8. |
- van Ardenne, Arnold, 1948, et al.
(författare)
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A dense aperture array with photonic beamforming for radio astronomy
- 2014
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Ingår i: Proceedings - 2014 International Conference on Electromagnetics in Advanced Applications, ICEAA 2014. - 9781467357104 ; , s. 856-859
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Konferensbidrag (refereegranskat)abstract
- This paper addresses the design and technologies of photonic beamformers relevant for use in the context of the socalled dense aperture arrays for the Square Kilometre Array[1]. The photonic beamformer approach complements the more conventional albeit state-of-the art RF and digital beamforming approach presently projected and the work aims to identify relevant design and technology issues for future use. This paper addresses these and presents the photonic design approach on a 36 element demonstrator array for possible use with the SKA of a 4:1 frequency bandwidth, UHF-band array having millions of elements.
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| 9. |
- van Ardenne, Arnold, 1948
(författare)
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Connecting early VLBI to SKA capabilities
- 2012
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Ingår i: Proceedings of Science. - 1824-8039. ; 2012-April
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Konferensbidrag (refereegranskat)abstract
- Several technical developments of early VLBI are relevant and now common for pathfinder and other initiatives toward the SKA. While no surprise when wearing a general "progress" hat, direct connections have so far not been stipulated or pointed out explicitly. In this contribution, I will reflect on some of these aspects of early VLBI developments which lead to relevant developments for the SKA and to which as seen from Dutch VLBI and beyond, Richard played a crucial facilitating role as well as a key role to the SKA endeavours. As I was connected with VLBI instrumentation developments in the Netherlands from the beginning, no doubt a personal view may bias some of its perceived importance. Nonetheless there can be no misunderstanding regarding Richards crucial and energetic role directly and indirectly in connecting VLBI with space to achieve even higher resolution imaging, higher sensitivity observing through so called tied array telescopes with the Westerbork imaging synthesis array, to exploring wider frequency coverage with these telescopes and of course in setting up the organization and managerial frameworks toward VLBI and JIVE in Europe. Here I will go into some detail regarding relevant technical aspects of VLBI for the SKA with emphasis on aperture (phased) arrays leaving the "shaping up" frameworks for others to address.
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| 10. |
- van Ardenne, Arnold, 1948, et al.
(författare)
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SKA antenna systems; Outlook for non-astronomy applications
- 2012
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Ingår i: Proceedings of 6th European Conference on Antennas and Propagation, EuCAP 2012. - 9781457709180 ; , s. 1199-1203
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Konferensbidrag (refereegranskat)abstract
- The globally endorsed Square Kilometre Array project primarily aims to advance high sensitivity radio astronomy using a distributed collection of radio telescope stations spiraling outward from the core along three to five arms out to 3000km. This planned highly sensitive instrument covering a frequency range from 70MHz up to 10GHz will be used as wideband, high resolution, wide observing field interferometer of which the first phase will be realized this decade. With the SKA telescope capabilities and with the underlying technologies, there are many space related applications outside the immediate radio astronomy domain. Examples are tracking space debris, precision orbit determination, simultaneous deep space tracking of multiple spacecrafts, GNSS and other ground segment applications, such as search and rescue tracking. After a brief introduction to the SKA, this paper will explore these potential application areas using the SKA based on its underlying approaches in the antenna and receiving subsystems.
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