The LOFAR radio environment

A. R. Offringa 1 A. G. de Bruyn 1 S. Zaroubi 1 G. van Diepen O. Martinez-Ruby P. Labropoulos M. A. Brentjens 2 B. Ciardi 3 S. Daiboo G. Harker V. Jelić 1 S. Kazemi 4 L. V. E. Koopmans G. Mellema V. N. Pandey 5 R. F. Pizzo 2 J. Schaye 6 H. Vedantham V. Veligatla S. J. Wijnholds 2 S. Yatawatta 7 P. Zarka 8 A. Alexov 2 J. Anderson 3 A. Asgekar 2 M. Avruch R. Beck 9 M. Bell 10 M. R. Bell 10 M. Bentum 2 G. Bernardi 11 P. Best 12 L. Birzan 6 A. Bonafede 13 F. Breitling 14 J. W. Broderick 10 M. Brüggen 13 H. Butcher 2 J. Conway M. de Vos R. J. Dettmar 15 J. Eisloeffel H. Falcke 2, 16, 17 R. Fender 10 W. Frieswijk 2 M. Gerbers 2 Jean-Mathias Grießmeier 2, 18 A. W. Gunst 2 T. E. Hassall 19 G. Heald 2 J. Hessels 20 M. Hoeft 21 A. Horneffer 3 A. Karastergiou 22 V. Kondratiev 2 Y. Koopman M. Kuniyoshi 16 G. Kuper 2 P. Maat 2 G. Mann 23 J. Mckean 2 H. Meulman 2 M. Mevius 2 J. D. Mol 24 R. Nijboer J. Noordam 2 M. Norden H. Paas 25 M. Pandey R. Pizzo 2 A. Polatidis 2 D. Rafferty 26 S. Rawlings 27 W. Reich 16 H. J. A. Röttgering 6 A. P. Schoenmakers 2 J. Sluman 2 O. Smirnov 28 C. Sobey 16 B. Stappers 19 M. Steinmetz 23 J. Swinbank 29 Michel Tagger 18 Y. Tang 2 C. Tasse 30, 31 A. van Ardenne W. van Cappellen 32 A. P. van Duin M. van Haarlem 2 J. van Leeuwen 20 R. J. van Weeren 2 R. Vermeulen 2 C. Vocks R. A. M. J. Wijers 29 M. Wise 29, 2 O. Wucknitz 33
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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A. R. Offringa, A. G. de Bruyn, S. Zaroubi, G. van Diepen, O. Martinez-Ruby, et al.. The LOFAR radio environment. Astronomy and Astrophysics - A&A, EDP Sciences, 2013, 549, 15 p. ⟨10.1051/0004-6361/201220293⟩. ⟨insu-01289809⟩

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