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.

Accepted for publication in A&A. 15 figures, 16 pages

References in corpus (1)

• Post-correlation radio frequency interference classification methods

Cited by in corpus (20)

• Improved upper limits on the 21-cm signal power spectrum of neutral hydrogen at $\mathbf{z}\approx 9.1$ from LOFAR
• The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR
• The Lockman Hole project: LOFAR observations and spectral index properties of low-frequency radio sources
• Initial LOFAR observations of Epoch of Reionization windows: II. Diffuse polarized emission in the ELAIS-N1 field
• Fast gain calibration in radio astronomy using alternating direction implicit methods: Analysis and applications
• LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra
• The impact of interference excision on 21-cm Epoch of Reionization power spectrum analyses
• Optimizing Sparse RFI Prediction using Deep Learning
• Compression of interferometric radio-astronomical data
• LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources
• Measuring cavity powers of active galactic nuclei in clusters using a hybrid X-ray-radio method -- A new window on feedback opened by subarcsecond LOFAR-VLBI observations
• Discovery of Carbon Radio Recombination Lines in M82
• Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz
• A survey of spatially and temporally resolved radio frequency interference in the FM band at the Murchison Radio-astronomy Observatory
• Millisecond Pulsar Scintillation Studies with LOFAR: Initial Results
• LOFAR detections of low-frequency radio recombination lines towards Cassiopeia A
• Observation and calibration strategies for large-scale multi-beam velocity-resolved mapping of the [CII] emission in the Orion molecular cloud
• Simulating Spectral Kurtosis Mitigation Against Realistic RFI Signals
• Searching for the largest bound atoms in space
• A Matched Survey for the Enigmatic Low Radio Frequency Transient ILT J225347+862146