Two procedures to flag radio frequency interference in the UV plane

Abstract

We present two algorithms to identify and flag radio frequency interference (RFI) in radio interferometric imaging data. The first algorithm utilizes the redundancy of visibilities inside a UV cell in the visibility plane to identify corrupted data, while varying the detection threshold in accordance with the observed reduction in noise with radial UV distance. In the second algorithm, we propose a scheme to detect faint RFI in the visibility time-channel plane of baselines. The efficacy of identifying RFI in the residual visibilities is reduced by the presence of ripples due to inaccurate subtraction of the strongest sources. This can be due to several reasons including primary beam asymmetries and other direction dependent calibration errors. We eliminated these ripples by clipping the corresponding peaks in the associated Fourier plane. RFI was detected in the ripple-free time-channel plane but was flagged in the original visibilities. Application of these two algorithms to 5 different 150 MHz datasets from the GMRT resulted in a reduction in image noise of 20-50% throughout the field along with a reduction in systematics and a corresponding increase in the number of detected sources. However, on comparing the mean flux densities before and after flagging RFI we find a differential change with the fainter sources ($25\sigma <$ S $<100$ mJy) showing a change of -6% to +1% relative to the stronger sources (S $>$ 100 mJy). We are unable to explain this effect but it could be related to the CLEAN bias known for interferometers.

Accepted for publication in AJ

References in corpus (6)

• The GMRT 150 MHz All-sky Radio Survey: First Alternative Data Release TGSS ADR1
• The VLA Low-frequency Sky Survey
• Post-correlation radio frequency interference classification methods
• The MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) Survey
• Automated Editing of Radio Interferometer Data with Pieflag
• RFI detection by automated feature extraction and statistical analysis

Cited by in corpus (2)

• Absolving the SSINS of Precision Interferometric Radio Data: A New Technique for Mitigating Faint Radio Frequency Interference
• Wide-field GMRT imaging of X-shaped Radio-Galaxies: Spectral properties of 4C32.25 and 4C61.23