The LOFAR radio environment

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.

Discovery of Carbon Radio Recombination Lines in M82

“…Typically, a few percent of the data were flagged due to RFI, consistent with a study of the LOFAR RFI environment (Offringa et al 2013). The calibrator data were then averaged to 16 channels and 6 s before calibration with the BlackBoard Selfcal software system (Pandey et al 2009).…”

Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR

“…Hence, these baselines are also flagged during the preprocessing step. Typically about 5% of visibilities are flagged at this stage (Offringa et al 2013).…”

Radio Frequency Interference

“…The spectral resolution of the data was 0.76 kHz. Offringa et al (2013) found that the interference occupancy was 1.8% for the lower band and 3.2% for the higher one. They concluded that these levels of narrowband interference should not significantly restrict astronomical observations, but that it is important that the frequency range of LOFAR remain free of broadband interference.…”

“…5.7.1), which is located in the Netherlands with long baseline extensions in other countries, covers the frequency ranges 10-80 and 110-240 MHz, thus avoiding the FM broadcast band. Discussions of the problem of radio interference in these frequency ranges are given by Boonstra and van der Tol (2005) and Offringa et al (2013). The latter provides a detailed examination of the radio environment in the 30-78 MHz and 115-163 MHz ranges.…”