Two procedures to flag radio frequency interference in the UV plane

• Main Authors: Sekhar, Srikrishna, Athreya, Ramana
• Format: Journal Article
• Language: English
• Published: 08-05-2018
• Subjects: Physics - Instrumentation and Methods for Astrophysics
• DOI: 10.48550/arxiv.1711.00128

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.