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Binning sufficient channels from continuum-subtracted data, or accumulating very long integrations, will show evenly-spaced horizontal stripes (see Figs. 1 and 2). These artefacts are present in both wide-band and narrow band modes but not visible in a single channel of the narrow band observations.

The RFI is persistent but variable in magnitude over time. There is some evidence that it is stronger at lower elevations.

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How to deal with it

It is not yet possible to provide a general script that will work on all MeerKAT observations. We can in the meantime Below we explain how the MHONGOOSE and Fornax surveys have dealt with the issue. Note also that this flagging method will be was implemented in an upcoming a release of CARACal (scheduled for July 2022) and will then be in late 2022, through which it is available to the general user community. Please contact the helpdesk if you think your data products have been affected and you require further assistance.

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Input data: continuum subtracted visibility file (MS-file in casa format)

1. Split the visibility file per target scan (using mstransform)

   1. On each scan

   2. create a 2D-image from a large enough (~26 kHz) line-free band (using wsclean)

   3. FFT the 2D-image (using casatools.image.fft)

   4. compute the median absolute deviation (mad) of the amplitudes of the FFT visibilities (using the scipy.stats function median_abs_deviation)

   5. identify in the uv-plane the coordinates of the amplitudes in the FFT above, for example median+300*(mad)

   6. create a mask with the u,v coordinates of the visibilities to flag

   7. flag over the full bandwidth the visibilities within the mask (using casacore.tables)

2. Recombine the now flagged scans into a single stripe-free measurement set

You can now proceed with your imaging.

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