A.R. Offringa et al., "The LOFAR radio environment", Astronomy & Astrophysics, V549, no. A11, pp1-15, January 2013
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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