... Robust Bandpass Removal Traditionally, the bandpass that is present in single-dish 21 cm spectra is removed by observing in a "signal-reference" mode. ... The bandpass is removed by dividing the signal spectrum by the reference spectrum. ... Instead, a statistical estimate of the bandpass at the position and time that a particular spectrum--the target spectrum--was acquired is made from a set of earlier and later spectra observed by the same beam--these are the reference spectra. ...