[show abstract][hide abstract] ABSTRACT: Phased array antenna systems are a defining technology in many current and planned radio astronomical instruments. Phased array technology enables new observing modes due to their large field-of-view, multi-beaming capability and rapid response. The radio astronomical application provides interesting challenges such as stringent requirements on system temperature and system calibration. In this paper, we discuss the motivation for using phased array systems in radio astronomy and provide an overview of recent advances and results in this area.
IEEE International Symposium on Antennas and Propagation, Orlando (Fl.); 07/2013
[show abstract][hide abstract] 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
Astronomy and Astrophysics 05/2013; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Cassiopeia A was observed using the Low-Band Antennas of the LOw Frequency
ARray (LOFAR) with high spectral resolution. This allowed a search for radio
recombination lines (RRLs) along the line-of-sight to this source. Five
carbon-alpha RRLs were detected in absorption between 40 and 50 MHz with a
signal-to-noise ratio of > 5 from two independent LOFAR datasets. The derived
line velocities (v_LSR ~ -50 km/s) and integrated optical depths (~ 13 s^-1) of
the RRLs in our spectra, extracted over the whole supernova remnant, are
consistent within each LOFAR dataset and with those previously reported. For
the first time, we are able to extract spectra against the brightest hotspot of
the remnant at frequencies below 330 MHz. These spectra show significantly
higher (15-80 %) integrated optical depths, indicating that there is
small-scale angular structure on the order of ~1 pc in the absorbing gas
distribution over the face of the remnant. We also place an upper limit of 3 x
10^-4 on the peak optical depths of hydrogen and helium RRLs. These results
demonstrate that LOFAR has the desired spectral stability and sensitivity to
study faint recombination lines in the decameter band.
Astronomy and Astrophysics 02/2013; 551(L11). · 5.08 Impact Factor
[show abstract][hide abstract] 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.
Astronomy and Astrophysics 12/2012; 549(A11):1-16. · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: M87 is a giant elliptical galaxy located in the centre of the Virgo cluster,
which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity
is responsible for the extended (80 kpc) radio lobes that surround the galaxy.
The energy generated by matter falling onto the central black hole is ejected
and transferred to the intra-cluster medium via a relativistic jet and
morphologically complex systems of buoyant bubbles, which rise towards the
edges of the extended halo. Here we present the first observations made with
the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz.
Images of M87 were produced at low radio frequencies never explored before at
these high spatial resolution and dynamic range. To disentangle different
synchrotron models and place constraints on source magnetic field, age and
energetics, we also performed a detailed spectral analysis of M87 extended
radio-halo using these observations together with archival data. We do not find
any sign of new extended emissions; on the contrary the source appears well
confined by the high pressure of the intra-cluster medium. A continuous
injection of relativistic electrons is the model that best fits our data, and
provides a scenario in which the lobes are still supplied by fresh relativistic
particles from the active galactic nuclei. We suggest that the discrepancy
between the low-frequency radio-spectral slope in the core and in the halo
implies a strong adiabatic expansion of the plasma as soon as it leaves the
core area. The extended halo has an equipartition magnetic field strength of
~10 uG, which increases to ~13 uG in the zones where the particle flows are
more active. The continuous injection model for synchrotron ageing provides an
age for the halo of ~40 Myr, which in turn provides a jet kinetic power of
Astronomy and Astrophysics 10/2012; 547(A56):1-20. · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Abell 2256 is one of the best known examples of a galaxy cluster hosting
large-scale diffuse radio emission that is unrelated to individual galaxies. It
contains both a giant radio halo and a relic, as well as a number of head-tail
sources and smaller diffuse steep-spectrum radio sources. The origin of radio
halos and relics is still being debated, but over the last years it has become
clear that the presence of these radio sources is closely related to galaxy
cluster merger events. Here we present the results from the first LOFAR Low
band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our
knowledge, the image presented in this paper at 63 MHz is the deepest ever
obtained at frequencies below 100 MHz in general. Both the radio halo and the
giant relic are detected in the image at 63 MHz, and the diffuse radio emission
remains visible at frequencies as low as 20 MHz. The observations confirm the
presence of a previously claimed ultra-steep spectrum source to the west of the
cluster center with a spectral index of -2.3 \pm 0.4 between 63 and 153 MHz.
The steep spectrum suggests that this source is an old part of a head-tail
radio source in the cluster. For the radio relic we find an integrated spectral
index of -0.81 \pm 0.03, after removing the flux contribution from the other
sources. This is relatively flat which could indicate that the efficiency of
particle acceleration at the shock substantially changed in the last \sim 0.1
Gyr due to an increase of the shock Mach number. In an alternative scenario,
particles are re-accelerated by some mechanism in the downstream region of the
shock, resulting in the relatively flat integrated radio spectrum. In the radio
halo region we find indications of low-frequency spectral steepening which may
suggest that relativistic particles are accelerated in a rather inhomogeneous
Astronomy and Astrophysics 05/2012; 543(A2256):1-13. · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: When an ultra-high energy neutrino or cosmic ray strikes the Lunar surface a
radio-frequency pulse is emitted. We plan to use the LOFAR radio telescope to
detect these pulses. In this work we propose an e?cient trigger implementation
for LOFAR optimized for the observation of short radio pulses.
Nuclear Instruments and Methods 02/2012; 664(1):171-185.
[show abstract][hide abstract] ABSTRACT: A novel method to form multiple instantaneous beams on the sky with a reflector antenna is to employ a dense Phased Array Feed (PAF). This technology is currently being developed to greatly increase the survey speed of existing and future radio telescopes. This paper reviews the current state of PAF development projects for radio astronomy, the particular challenges and the potential for incorporation of PAFs into the ultimate radio survey instrument - the Square Kilometre Array.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
[show abstract][hide abstract] ABSTRACT: APERTIF (APERture Tile In Focus) is a Phased Array Feed (PAF) system that is being developed for the Westerbork Synthesis Radio Telescope (WSRT) to increase its survey speed with a factor 20. This paper presents an overview of APERTIF and measurement results that demonstrate the unique capabilities of PAFs in practice: Wide field of view (scan range), low system temperature, excellent illumination efficiency, synthesis imaging and a significant reduction of the reflector feed interaction.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
[show abstract][hide abstract] ABSTRACT: An optimal beamforming strategy is proposed for performing large-field surveys with dual-polarized phased-array-fed reflector antennas. This strategy uses signal-processing algorithms that maximize the beam sensitivity and the continuity of a field of view (FOV) that is formed by multiple closely overlapping beams. A mathematical framework and a newly developed numerical approach are described to analyze and optimize a phased array feed (PAF) system. The modeling approach has been applied to an experimental PAF system (APERTIF prototype) that is installed on the Westerbork Synthesis Radio Telescope. The resulting beam shapes, sensitivity, and polarization diversity characteristics (such as the beam orthogonality and the intrinsic cross-polarization ratio) are examined over a large FOV and frequency bandwidth. We consider weighting schemes to achieve a conjugate-field matched situation (max. received power), maximum signal-to-noise ratio (SNR), and a reduced SNR scenario but with constraints on the beam shape. The latter improves the rotational symmetry of the beam and reduces the sensitivity ripple, at a modest maximum sensitivity penalty. The obtained numerical results demonstrate a very good agreement with the measurements performed at the telescope.
IEEE Transactions on Antennas and Propagation 07/2011; · 2.33 Impact Factor
[show abstract][hide abstract] ABSTRACT: The low frequency array (LOFAR) is a phased array radio telescope that is currently being built in The Netherlands with extensions throughout Europe. It was officially opened on June 12, 2010 and is an important pathfinder for the square kilometre array. The Dutch LOFAR system will consist of 36 stations covering the 10-250 MHz frequency range. In this paper we discuss the sky noise limited design of the antenna system and present a novel technique to obtain the ratio of effective area and system temperature directly from the calibration results, despite the presence of multiple sources within the 2π sr field of view of the antennas. This ratio is the key sensitivity parameter for radio telescopes. The presented technique allows in situ performance evaluation using astronomical calibration sources, i.e., without the use of reference sources, a controlled environment or lab equipment. We use this technique to evaluate the performance of some of the already available LOFAR hardware and demonstrate that LOFAR has the desired sky noise dominated performance.
IEEE Transactions on Antennas and Propagation 07/2011; · 2.33 Impact Factor
[show abstract][hide abstract] ABSTRACT: A novel method to form multiple beams on the sky with a reflector antenna is to employ a dense phased array feed (PAF). This technology is currently being developed to greatly increase the survey speed of existing and future radio telescopes. For high quality radio astronomical images the beam patterns of the telescope must be stable over the duration of the observation, which is typically 12 hours for an East-West array. This is more complicated for a PAF than for a horn feed because the beams of a PAF are formed by adding the responses of multiple elements. Electronic gain variations of the receiver channels will lead to beam pattern variations. In contrast, the beam pattern of a horn feed is determined by mechanics only. First, the beam stability requirements are derived for the APERTIF system, a PAF system which will be installed on the Westerbork Synthesis Radio Telescope (WSRT). From these requirements the stability of the individual receiver channels is derived. Measurements during ~3 hours demonstrate that the beam pattern stability of the existing horn feeds is according to the expectations. Good news is that the beam pattern of the PAF system is demonstrated to be equally stable during the same observation period under stable weather conditions.
Antennas and Propagation (EUCAP), Proceedings of the 5th European Conference on; 05/2011
[show abstract][hide abstract] ABSTRACT: We assess the sensitivity and polarimetric performance of a polarimetric phased array feed (PAF) system in which the two sets of nominally orthogonally polarized elements are beamformed separately. Out simulations of an actual PAF system indicate that such bi-scalar instead of full-polarimetric beamforming results in about 4% sensitivity loss and an XPD of about 45 dB. Our measurements confirm the sensitivity loss, but indicate worse polarimetric performance than the simulations. We indicate how the performance of a PAF system with bi-scalar beamforming can be improved by beamforming of the cross-polarization signals and polarimetric corrections to the beamformer outputs.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 01/2011
[show abstract][hide abstract] ABSTRACT: Present day synthesis radio telescopes have limited survey capabilities because of field of view restrictions. A novel method to form multiple beams on the sky is to employ a phased array feed (PAF). In telescopes with small f/D ratios, it is the only way to form closely packed beams on the sky. An additional advantage of this technique is that a PAF allows optimizing the secondary beam in terms of sensitivity, sidelobes and polarization characteristics. APERTIF (“APERture Tile In Focus”) is a PAF system that is being developed for the Westerbork Synthesis Radio Telescope (WSRT) to increase its survey speed with a factor 20. This paper presents a system overview of APERTIF and measurement results that demonstrate the unique capabilities of PAFs in practice: Wide field of view (scan range), low system temperature, excellent illumination efficiency, synthesis imaging and a significant reduction of the reflector - feed interaction.
Phased Array Systems and Technology (ARRAY), 2010 IEEE International Symposium on; 11/2010
[show abstract][hide abstract] ABSTRACT: This paper describes a numerical approach for the analysis of a reflector antenna system which is fed by a Phased Array Feed. This approach takes mutual interaction effects into account between the antenna array and the low noise amplifiers in the evaluation of the system sensitivity and optimization of the beamformer weights, and can be used when several signal and noise sources are present on the sky, ground, and inside the system itself. The described methodology has been applied to a practical PAF (comprising 144 tapered slot antennas operating from 1 to 1.75 GHz) which is installed at a 25-m reflector antenna. Comparison of numerical and experimental results shows a good agreement.
Electromagnetics in Advanced Applications (ICEAA), 2010 International Conference on; 10/2010
[show abstract][hide abstract] ABSTRACT: We describe a Phased Array Feed (PAF) system, called Apertif, which will be
installed in the Westerbork Synthesis Radio Telescope (WSRT). The aim of
Apertif is, at frequencies from 1.0 to 1.7 GHz, to increase the instantaneous
field of view of the WSRT 8 deg^2 and its observing bandwidth to 300 MHz with
high spectral resolution. This system will turn the WSRT into an effective
survey telescope with scientific applications ranging from deep surveys of the
northern sky of HI and OH emission and polarised continuum to efficient
searches for pulsars and transients. We present results obtained with a
prototype PAF installed in one of the WSRT dishes. These results demonstrate
that at decimetre wavelengths PAFs have excellent performance and that even for
a single beam on the sky they outperform single feed radio dishes. PAFs turn
radio telescopes into very effective survey instruments. Apertif is now fully
funded and the community is invited to express their interest in using Apertif