The LOFAR radio environment

¹ University of Groningen, Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: offringa@mso.anu.edu.au
² Mount Stromlo Observatory, RSAA, Cotter Road, Weston Creek, ACT 2611, Australia
³ ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁴ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching bei München, Germany
⁵ Center for Astrophysics and Space Astronomy, University of Colorado, 389 UCB, Boulder, Colorado 80309-0389, USA
⁶ AlbaNova University Center, Department of Astronomy, 106 91 Stockholm, Sweden
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Observatoire de Paris, 92195 Meudon, France
⁹ University of Amsterdam, Astronomical Institute Anton Pannekoek, PO Box 94249, 1090 GE Amsterdam, The Netherlands
¹⁰ Max-Planck Institute for Astrophysics, PO Box 20 24, 53010 Bonn, Germany
¹¹ SRON NetherlandsInstitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹² Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia
¹³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Royal Observatory Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹⁵ Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
¹⁶ Astrophysical Institute Potzdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁷ University of Southampton, University Road, Southampton SO17 1BJ, UK
¹⁸ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁹ Chalmers University of Technology, 412 96 Gothenburg, Sweden
²⁰ Ruhr-Uuniversity Bochum, Universitätsstrae 150, 44801 Bochum, Germany
²¹ Thüringer Landessternwarte, Tautenburg Observatory, Sternwarte 5, 07778 Tautenburg, Germany
²² Radboud University Nijmegen, Faculty of NWI, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²³ Centre national de la recherche scientifique, 3 rue Michel-Ange, 75794 Paris Cedex 16, France
²⁴ University of Manchester, Oxford Road, Manchester, M13 9PL, UK
²⁵ University of Oxford, Wellington Square, Oxford OX1 2JD, UK
²⁶ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
²⁷ Rhodes University, RATT, Dep. Physics and Electronics, PO Box 94, Grahamstown 6140, South Africa
²⁸ University of Bonn, Regina-Pacis-Weg 3, 53012 Bonn, Germany
²⁹ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 28 August 2012
Accepted: 28 September 2012

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.