A. van Ardenne

Extending the Field of View With Phased Array Techniques: Results of European SKA Research

The radio frequency window of the square kilometre array is planned to cover the wavelength regime from centimeters up to a few meters. For this range to be optimally covered, different antenna concepts are considered. At the lowest frequency range, up to a few gigahertz, it is expected that multibeam techniques will be used, increasing the effective field-of-view to a level that allows very efficient detailed and sensitive exploration of the complete sky. Although sparse narrow-band phased arrays are as old as radio astronomy, multioctave sparse and dense arrays now considered for the SKA require new low-noise design, signal processing, and calibration techniques. The successful implementation of these new array techniques has already been introduced for the use of phased array feeds upgrading existing telescopes: enhancing aperture efficiency as well as effective field-of-view. Especially the development of low-cost array antenna design will allow a cost-effective large-scale implementation for the SKA. This paper addresses these new capabilities, emphasizing the R&D work done in Europe and aims to provide insight into the status of enabling technologies and technical research on polarization, calibration, and side-lobe control that will unleash the potential of phased arrays for future growth of radio astronomy synthesis arrays.

The LOFAR radio environment

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 LOFARs 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.

Concepts of the Square Kilometre Array; toward the new generation radio telescopes

We present the concepts and their unique features presently being studied for the Square Kilometre Array expected to be operational between 2010 and 2015. A comparison is made between telescopes in operation to day and SKAs ambitious set of requirements is presented. The framework for coordinating the studies of various countries aiming to conclude on a concept choice in 5 years is introduced. As a desirable side effect, efforts relevant for SKA, will result in new albeit smaller telescopes coming into operation at earlier stages.

Redundancy calibration of phased-array stations

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.

Electronic Multi-Beam Radio Astronomy Concept: Embrace a Demonstrator for the European SKA Program

ASTRON has demonstrated the capabilities of a 4m2, dense phased array antenna (Bij de Vaate et al., 2002) for radio astronomy, as part of the Thousand Element Array project (ThEA). Although it proved the principle, a definitive answer related to the viability of the dense phased array approach for the SKA could not be given, due to the limited collecting area of the array considered. A larger demonstrator has therefore been defined, known as “Electronic Multi-Beam Radio Astronomy Concept“, EMBRACE, which will have an area of 625 m2, operate in the band 0.4–1.550 GHz and have at least two independent and steerable beams.With this collecting area EMBRACE can function as a radio astronomy instrument whose sensitivity is comparable to that of a 25-m diameter dish. The collecting area also represents a significant percentage area (∼10%) of an individual SKA “station.” This paper presents the plans for the realisation of the EMBRACE demonstrator.

Mid-frequency aperture arrays: the future of radio astronomy

Aperture array (AA) technology is at the forefront of new developments and discoveries in radio astronomy. Currently LOFAR is successfully demonstrating the capabilities of dense and sparse AAs at low frequencies. For the mid-frequencies, from 450 to 1450MHz, AAs still have to prove their scientific value with respect to the existing dish technology. Their large field-of-view and high flexibility puts them in an excellent position to do so. The Aperture Array Verification Program is dedicated to demonstrate the feasibility of AAs for science in general and SKA in particular. For the mid-frequency range this has lead to the development of EMBRACE, which has already demonstrated the enormous flexibility of AA systems by observing HI and a pulsar simultaneously. It also serves as a testbed to demonstrate the technological reliability and stability of AAs. The next step will put AA technology at a level where it can be used for cutting-edge science. In this paper we discuss the developments to move AA technology from an engineering activity to a fully science capable instrument. We present current results from EMBRACE, ongoing tests of the system, and plans for EMMA, the next step in mid-frequency AA technology.

Phased array technology for GPR antenna design for near subsurface exploration

The geophysical explorations for near surface investigations for buried objects are carried out by ground penetrating radar (GPR) using sensors operating at radio and microwave frequencies. It is well realized that focused beaming from an antenna into subsurface yields high resolution (with improved illumination of the target area) and results in better delineation of the subsurface targets. ASTRON (the radio astronomy research center of the Netherlands) has been engaged in developing a new generation of all-sky, full sensitive, multibeam radio telescopes for astronomical research at radio frequencies. Using a highly sensitive tapered slot THEA antenna, measurement beams on the sky are electronically formed and steered as desired. Experimental data show great promise in detecting moving objects in the sky. The existing know-how of focused beaming using phased array technology developed at ASTRON could be extended for better illumination of non-moving subsurface objects for GPR investigations. Stacking (adding) of the data gathered using both the beam-forming transmitting and receiving GPR antennas would result in reduced surface clutter effects and in enhanced detection of the near surface targets. In this paper, the concept of phased array technology in a bistatic mode of operation for GPR application is described.

Application of redundancy calibration to phased arrays and some limitations

In new phased array instruments, a fundamental question is whether the geometrically redundant baselines in a regularly arranged phased array are really redundant. Based on real and simulated data, we demonstrate that for a phased array station, a regular arrangement of station elements is necessary but not sufficient to satisfy redundancy calibration requirements. This is due to the electromagnetic interaction between closely spaced antenna elements which leads to non-identical beams and possibly correlated noise. Each of these deterministic effects introduces a bias on the estimated calibration results. Understanding the nature of these effects has helped us to determine the limits of applicability of redundancy calibration for a given phased array, which are demonstrated in this paper.

Activities for the square kilometer array (SKA) in Europe

The paper is an overview of the activities in research and development of the new generation radio telescope, the SKA, in Europe. It focuses on the aperture array technology for decameter and longer wavelengths, including relevant actions for the low frequency array (LOFAR) and focal plane arrays (FARADAY) covering the lower and higher frequency bands. We describe the concepts, specifications and the key parameters as well as application directions for radioastronomy for SKA, FARADAY and LOFAR.

The SKA New Instrumentation: Aperture Arrays

The radio frequency window of the Square Kilometre Array is planned to cover the wavelength regime from cm up to a few meters. For this range to be optimally covered, different antenna concepts are considered enabling many science cases. At the lowest frequency range, up to a few GHz, it is expected that multi-beam techniques will be used, increasing the effective field-of-view to a level that allows very efficient detailed and sensitive exploration of the complete sky. Although sparse narrow band phased arrays are as old as radio astronomy, multi-octave sparse and dense arrays now being considered for the SKA, requiring new low noise design, signal processing and calibration techniques. These new array techniques have already been successfully introduced as phased array feeds upgrading existing reflecting telescopes and for new telescopes to enhance the aperture efficiency as well as greatly increasing their field-of-view (van Ardenne et al., Proc IEEE 97(8):2009) by [1]. Aperture arrays use phased arrays without any additional reflectors; the phased array elements are small enough to see most of the sky intrinsically offering a large field of view.