Description of the step 3 of data processing (see bottom panel of Fig. 2). The black vertical line shows the separation between lane 0 and lane 1. The top panel compares the flat spectrum before (blue) and after (black) the mitigation process. The middle panel is a zoom on the black spectrum. The red dashed-dotted line shows the zero baseline. The vertical spikes are detections of Cα RRLs. The bottom panel shows the value of the rms computed in a sliding window along the spectrum. The holes in the spectrum (at 35 MHz, 73 MHz and 79 MHz) are flagged regions contaminated by unmanageable RFI pollution during this observing block.