Australia Telescope National Facility

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Step 2: Rms-based flagging

In this step, Pieflag calculates the median of the standard deviation, or rms, in short (typically 2min to 3min) sections of reference channel data for each baseline and pointing. The median rms value, , is used as a measure of a typical rms to which the other channels are compared. If the rms in a section exceeds , with typically 3, then the entire section is flagged. The concept of badness is not used by this algorithm, because groups of points are considered, and not single points. This algorithm can find bad data even in the presence of a strong source signal, and therefore complements the amplitude-based flagging. However, terrestrial and solar interference can increase amplitude levels without much effect on the rms on short timescales, which then is unnoticed by this algorithm (but likely to be detected by the first).